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Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation-reduction potential (Eh) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing Eh stepwise from -300 to +300 mV using a sophisticated biogeochemical microcosm. At the Eh range from -300 to -100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., Desulfitobacterium spp.), acidification, and reductive dissolution of Fe(oxyhydr)oxides. At the Eh range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with Eh (-300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of Desulfitobacterium spp. decreased by 50 and 96%, respectively, at Eh of +200 mV as compared to Eh of -300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (Clostridium spp.) also decreased over 93% at Eh of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.

The reaction between Fe2+ and HClO constitutes a promising advanced oxidation process (AOP) for removing pollutants from wastewater, and •OH has been considered the dominant reactive oxidant despite limited evidence for this. Herein, we demonstrate that the Fe2+/HClO reaction enables the production of FeIVO2+ rather than •OH in acid medium, a finding that is strongly supported by multiple lines of evidence. Both X-ray absorption near-edge structure spectroscopic tests and Mossbauer spectroscopic tests confirmed the appearance of FeIVO2+ as the reactive intermediate in the reaction between Fe2+ and HClO. The determination of FeIVO2+ generation was also derived from the methyl phenyl sulfoxide (PMSO)-based probe experiments with respect to the formation of PMSO2 without •OH adducts and the density functional theory studies according to the lower energy barrier for producing FeIVO2+ compared with •OH. A dual-anode electrolytic system was established for the in situ generation of Fe2+ and HClO that allows the production of FeIVO2+. The system exhibits an enhanced capacity for oxidizing a model pollutant (e.g., phosphite) from industrial wastewater, making it an attractive and promising AOP for the abatement of aqueous contaminants.

Arsenic (As) redox-induced mobilisation and speciation in polluted gold mine sites in tropical climates largely remains unknown. Here, we investigated the impact of changes in soil redox potential (E

Lavrio is a Greek town with several abandoned Ag/Pb mines. In this study, 19 potentially toxic elements (PTEs) were measured in soil, weeds, and olives. Levels of seven of the studied PTEs in soil were highly elevated: Zn (56.2-58,726 mg kg

In this study, we applied coconut fiber-derived biochar (CFB) to a lead (Pb)-contaminated soil (at 600 mg kg−1) at 2% and 4% (w/w), in order to explore the efficacy and mechanism of CFB to immobilize Pb in soil during a 150-day incubation experiment. Our approach integrated various techniques namely micro-X-ray fluorescence, sequential extraction, X-ray absorption fine structure, scanning electron microscopy combined with energy dispersive X-ray spectroscopy to evaluate the Pb immobilization. Results indicated that the distribution of Pb in the studied soil was significantly affected by CFB application. The Pb content in organic matter bound fraction of the studied soil increased by 29.5% and 33.5% with 2% and 4% CFB, respectively, compared to control soil after 150-day of incubation. Lead-loaded humic acid (HA) and Pb3(PO4)2 were higher in the biochar-amended soil (2% CFB) as compared with the control soil. The CFB particles possibly offer more binding sites of PO43− and carboxylic functional groups than the binding sites of –FeO(OH), SiO32−, −Al2O3 and organic functional groups provided by the original soil particles alone (no biochar) for Pb. Overall, this study highlights that CFB can be a potential candidate to immobilize Pb for the restoration and remediation of Pb-contaminated soils.

Unraveling sorption mechanisms of lead (Pb) to silicon (Si)-rich biochar at molecular scale in aqueous solution are essential for the effective application of the biochars to the remediation of Pb and other metal(loid)s pollution in the environment. Thus, this study investigated the contributions of phytoliths and other compounds to the Pb sorption on Si-rich coconut fiber biochar (CFB500) and the corresponding sorption mechanisms using spectroscopic techniques, including the micro-X-ray fluorescence (μ-XRF), X-ray absorption fine structure (XAFS), scanning electron microscopy combined with energy dispersive X-ray spectroscopy, and X-ray diffraction. The μ-XRF and XAFS results showed that K, Ca, Cu, Mn, and Fe were released and significantly related to Pb in Pb-loaded CFB500; four major Pb species were formed with similar structures to lead carboxylate (e.g., Pb(C2H3O2)2), Pb3(PO4)2, PbSiO3, and PbCO3. On phytoliths in CFB500, Pb2+ ions were mainly sorbed on the sites of silicate with a structure similar to PbSiO3. The contribution of binding sites for Pb2+ sorption was ascribed to the outer-wall of carbon skeleton of CFB500, which was stronger than that provided by the mineral oxide aggregate and phytoliths on CFB500. Organic carbon functional groups, inorganic carbonates, silicates and phosphates on CFB500 mostly dominated the sorption sites for Pb2+. Our results suggest that CFB500 was a promising material for the remediation of Pb-contaminated aqueous environments (e.g., wastewater).

Agricultural lands adjacent to industrial activities are vulnerable due to the risk of trace elements (TEs) being accumulated into crops and subsequently humans. One such case concerns the industrial area of Volos, Greece, a suspected contaminated area which has never been studied. We measured Ag, Al, As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Sn, Tl, V, and Zn in soil and maize (leaves and grains) and assessed health risk of human exposure via soil ingestion and grain consumption. We found that the most highly enriched elements in soils were Tl (enrichment factor = 19), Se (17.68), Sb (14.81), As (7.89), Ni (6.91), Mo (5.22) and Cr (4.33); they all likely derived from anthropogenic activities and in particular from a nearby major steel factory, except for Ni which is known to be lithogenically elevated in that area. Synchrotron XANES spectra analysis revealed that As species were associated with ferrihydrite, and predominant species were As(V) (at ca. 85%) and As(III) (at ca. 15%). Although the total content of the studied elements was high, the ammonium bicarbonate-DTPA extractions recovered very low element concentrations, probably due to the fact that soil conditions decelerated solubility (i.e., soils were alkaline, clayey, and with high Fe oxides content). This was confirmed by the soil-to-grain transfer index, which was particularly low for all studied elements. In 5% of sampled grains concerning Cd, and in 40% concerning Pb, the European food-related regulation limits were surpassed. Health risk assessment showed a dramatically elevated risk for Tl via soil ingestion (hazard quotient, HQ = 2.399), a value that contributed 74% of the total risk. Similarly, concerning the grain consumption-related health risk, Tl was the predominant contributor (HQ = 0.128, contributing 40% of the total risk). Such elevated Tl risk which has rarely been reported previously, led to a considerably high hazard index (HI) well above the threshold of HI = 1. Cancer risk was below the 1 × 10−4 risk threshold for As and Pb. Our findings indicate that this study should be pivotal concerning similar industrially-affected agricultural soils of suspected contamination, since less-expected toxic elements such as Tl here may be primary contributors to health risk. Keywords: Heavy metals, Steel factory, Pollution indices, Corn

A better understanding of different retention mechanisms of potentially toxic elements (PTEs) by biochars during the remediation of contaminated sites is critically needed. In this study, different spectroscopic techniques including synchrotron-based micro-X-ray fluorescence (μ-XRF), X-ray absorption fine structure (XAFS), and near-edge XAFS spectroscopy (NEXAFS), were used to investigate the spatial distributions and retention mechanisms of lead (Pb) and copper (Cu) on phytolith-rich coconut-fiber biochar (CFB), and ammonia, nitric acid and hydrogen peroxide modified CFB (MCFB) (i.e., ACFB, NCFB and HCFB). The μ-XRF analyses indicated that sorption sites on ACFB and NCFB were more efficient compared to those on CFB and HCFB to bind Pb/Cu. XAFS analyses revealed that the percentage of Pb species as Pb(C

Sustainable engineering and management of hydromorphic arable soils need deep knowledge about the redox-mediated interactions between nutrients and soil colloids. Consequently, we examined the redox-mediated interactions of P with metal oxides and organic carbon (OC) in toe-, mid-, and upper-slope arable soils under dynamic redox changes using geochemical (biogeochemical microcosm), spectroscopic (XANES), and molecular (quantum chemical calculations (QCC)) approaches. We controlled the redox potential (EH) in two directions i.e., 1) slowly oxidizing direction (SOD; EH increased from −286 to +564 mV); and 2) slowly reducing direction (SRD; EH decreased from +564 to −148 mV). In the SOD of all soils, P, Fe2+ and OC mobilized at EH ≤ 200 mV, due to the pH decrease from 7.2 to 4.1 and dissolution of Fe-oxyhydroxides/carbonates, as indicated by the decrease of Fe–P and Ca–P determined by P–K-edge-XANES. At EH > 200 mV, P immobilized due to the strong P binding with Fe3+ as suggested by QCC. In the SRD of mid-slope-soil, P immobilized with decreasing EH, due to pH increase and P retention by aromatic carbon and/or precipitation by carbonates, as supported by increase of organic-P and Ca–P. These findings help for management of P in arable soils.

Iron (Fe)-modified biochar, a renewable amendment that synthetizes the functions of biochar and Fe materials, demonstrates a potential to remediate arsenic (As)-contaminated soils. However, the effectiveness of Fe-based biochar to immobilize As in paddy soils under varying redox conditions (Eh) has not been quantified. We tested the capability of the raw (RBC) and Fe-impregnated (FeBC) biochars to immobilize As in a paddy soil under various Eh conditions (from -400 to +300 mV) using a biogeochemical microcosm system. In the control, As was mobilized (686.2–1535.8 μg L-1) under reducing conditions and immobilized (61.6–71.1 μg L-1) under oxidizing conditions. Application of FeBC immobilized As at Eh +200 mV was caused by the increase of pH after RBC application. Amendment of the Fe-modified biochar can be a suitable approach for alleviating the environmental risk of As under reducing conditions in paddy soils.

Under anoxic conditions, aqueous Fe(II) (Fe(II)aq)-induced recrystallization of iron (oxyhydr)oxides changes the speciation and geochemical cycle of trace elements in environments. Oxidation of trace element, i.e., As(III), driven by Fe(II)aq-iron (oxyhydr)oxides interactions under anoxic condition was observed previously, but the oxidative species and involved mechanisms are remained unknown. In the present study, we explored the formed oxidative intermediates during Fe(II)aq-induced recrystallization of goethite under anoxic conditions. The methyl phenyl sulfoxide-based probe experiment suggested the featured oxidation by Fe(IV) species in Fe(II)aq-goethite system. Both the Mossbauer spectra and X-ray absorption near edge structure spectroscopic evidenced the generation and quenching of Fe(IV) intermediate. It was proved that the interfacial electron exchange between Fe(II)aq and Fe(III) of goethite initiated the generation of Fe(IV). After transferring electrons to goethite, Fe(II)aq was transformed to labile Fe(III), which was then transformed to Fe(IV) via a proton-coupled electron transfer process. This highly reactive transient Fe(IV) could quickly react with reductive species, i.e. Fe(II) or As(III). Considering the ubiquitous occurrence of Fe(II)-iron (oxyhydr)oxides reactions under anoxic conditions, our findings are expected to provide new insight into the anoxic oxidative transformation processes of matters in non-surface environments on earth.

Contamination of paddy soils by arsenic (As) is of great concern for human health and the environment. The impact of animal-derived biochar on As mobilization under fluctuating redox conditions in paddy soils has not been studied. Consequently, we investigated the effects of pig carcass-derived biochar (PB) on As (im)mobilization in a contaminated paddy soil under controlled redox potential (Eh) using a biogeochemical microcosm-setup. The addition of PB decreased the concentration of dissolved As at Eh = +100 and +200 mV by 38.7% and 35.4%, respectively (compared to the control), because of the co-precipitation of As with Fe-Mn oxides and the complexation between As and aromatic organic molecules. However, under reducing conditions (Eh = -300 mV), PB increased the dissolved As by 13.5% through promoting reduction and decomposition of As-bearing Fe minerals (e.g., ferrihydrite-As, Fe-humic-As). Under oxidizing conditions (Eh = +250 mV), PB increased the dissolved As by 317.6%, due to the associated increase of pH. We conclude that As mobilization in PB-treated paddy soils is highly affected by Eh. PB can be used to reduce the environmental risk of As under moderately reducing conditions, but it may increase the risk under highly reducing and oxidizing conditions in paddy soils.

The increasing use of indium in high-tech industries has inevitably caused its release into the environment. However, knowledge of its environmental fate has been very limited so far. This study investigates the indium uptake and accumulation by two staple crops, rice (Oryza sativa L.) and wheat (Triticum aestivum L.), and evaluates potential risks associated with their consumption. Rice and wheat were grown on three kinds of soil, including acidic soils spiked with a high indium concentration (1.0 mmol kg-1), which is considered the worst-case scenario, because high soil acidity promotes indium bioavailability. The results revealed that a large portion of soil indium was associated with iron hydroxides, even in acidic soils. Indium precipitates in soils resulted in relatively low availability at the plant root site. Most absorbed indium accumulated at the roots, with only a tiny portion reaching the grains. The corresponding Hazard Quotient indicated no adverse effects on human health. Due to the low translocation of indium from soil to grain, the consumption of rice and wheat grains harvested from indium-contaminated soils may pose an insignificant risk to human health. Further field studies are necessary to better elucidate the risks associated with consuming crops grown in indium-contaminated soils.

Molybdenum (Mo) is an emerging contaminant in the environment. To assess the mobility and availability of Mo in soils, this study investigated the effect of soil properties on the sorption and desorption of Mo in soils. The Mo K-edge X-ray absorption near edge structure (XANES) of the soils after Mo sorption showed that sorbed molybdate was the predominant species, with Fe/Al-molybdate and Ca-molybdate being the minor components in soils with low and high pH levels, respectively. Although acidic soils exhibited higher Mo sorptivity, they exhibited partial reversibility of Mo sorption, which may be attributed to the high solubility of Al-molybdate. Accordingly, the mobility of Mo may be relatively high in soils with a low pH, high exchangeable Al content, and high Fe-hydroxide crystallinity, such as Ultisols and Oxisols. At higher pHs, the sorption irreversibility of molybdate were enhanced due to the formation of Ca-molybdate precipitate. The results of this study indicated that sorption/desorption irreversibility and related mechanisms should be considered when evaluating the mobility and availability of Mo in soils.

Mobilization of phosphorus (P) in arable soils might be affected by groundwater fluctuations and the associated changes in redox potential (EH). However, the impact of systematic changes of EH on P mobilization in redoximorphic arable soils along a catena has not been studied so far. Therefore, we investigated P mobilization under different redox conditions in top- and sub-soil horizons of three groundwater affected arable soils along a slight slope (toe-, mid-, and upper-slope position) in Northern Germany using an automated biogeochemical microcosm system. The impact of pH, Al, Fe, Mn, and dissolved organic carbon (DOC) on P mobilization was also studied. The initial EH (+351 to +431 mV) and pH (6.5–7.0) decreased in all soil samples (EH = −280 mV; pH = 4.4) when creating a slurry. Thereafter, the pH increased to 7.1 and 6.4 with increasing EH in the mid-and toe-slope soil, respectively. Concentrations of dissolved P ranged between 20.8 mg L−1 under low EH in the toe slope topsoil and 0.69 mg L−1 under high EH in the toe- and mid-slop subsoil. Concentrations (mg L−1) of dissolved Fe (0.31–13.3) and DOC (92–2651) increased under low EH and decreased under high EH. The increase of P mobilization under low EH and pH in the soils might be due to the release of P via the reductive and acidic dissolution of Fe-(oxhydr)oxides and/or due to soil organic matter mineralization. The high mobilization of P under reducing conditions may increase its bioavailability; however, it may increase its loss in the soils, particularly in the toe slope profile.

Nanoscale lead (Pb) dioxide particles (nPbO2) are a newly identified corrosion product formed inside lead-bearing pipes or lead-containing faucets in drinking water distribution systems. These particles can release toxic lead ions, which cause drinking-water contamination and lead poisoning in humans, especially children. nPbO2 may be dislodged from pipes into drinking water; however, little research has focused on the bioavailability and toxicity of nPbO2in vivo. In this study, we used adult medaka fish (Oryzias latipes) as an animal model to investigate the uptake, lead dissolution, bioaccumulation and toxic effects of nanoscale [nPbO2] and microscale bulk [bPbO2] Pb dioxide and Pb(II)aq (Pb2+) in vivo upon acute to sub-chronic aqueous exposure. Both types of PbO2 particles were chemically stable in dechlorinated tap water with low water solubility. However, the X-ray absorption near-edge structure (XANES) analysis and analytical quantification of lead speciation showed that both nPbO2 and bPbO2 could be reductively dissolved into Pb(II)aq in both the intestine (major uptake route) and gills of the fish; thereby, enhancing hepatic Pb accumulation. The fish brains exhibited greater Pb accumulation and acetylcholinesterase inhibition with Pb(II)aq treatment than all the PbO2 treatments. The Pb content was greater in the gills, liver and brain with nPbO2 than bPbO2. This in vivo evidence implies the possibility of increased risk of exposure to Pb dissolution from PbO2 particles in the digestive system via drinking water, which can enhance the bioavailability of Pb uptake and toxicity in humans.

Gallium (Ga) is widely used in high-tech industries and is an emerging contaminant in the environment. This study aimed to determine Ga speciation in soils and Ga accumulation in rice plants (Oryza sativa L.) grown in three Ga-contaminated soils. The results showed that, among the soils, the acidic soil with a coarse texture had the highest soil Ga availability, which enhanced Ga uptake by rice roots. The Ga K-edge X-ray absorption near edge structure and sequential extraction results of the soils showed that the predominant species of Ga associated with iron hydroxides transformed to Ga(OH)3 precipitates, and the residue fraction increased with rice-growing time, resulting in lower Ga uptake by rice roots in the second half period of rice cultivation. A large fraction of Ga was accumulated in the rice roots, with only a small portion of Ga was transferred to the shoots and then to the rice grains. This study revealed that Ga speciation in soil-rice plant systems varied during rice cultivation and determined soil Ga availability to rice plants. Gallium accumulated in rice grains is distributed homogenously in the endosperm of the grains, suggesting a potential risk to public health via the intake of rice grains harvested from Ga-contaminated paddy fields.

Soil redox potential (EH) and pH are key parameters regulating the solubility and fate of phosphorus (P). However, the impact of soil acidification on the redox-induced mobilization and speciation of P in soils under a wide range of EH values has not been extensively studied. Here, we investigated the mobilization and speciation of P in an acidified agricultural soil at two different pH values (e.g., highly acidic soil; pH = 5.6 and slightly acidic soil; pH = 6.1) compared to the un-acidified soil (control soil; pH = 7.3) under a wide range of EH condition (+459 to -281 mV). The impacts of EH/pH-dependent changes of Fe-Mn oxides, and dissolved organic (DOC) and inorganic (DIC) carbon on P mobilization and speciation were also investigated using geochemical and spectroscopic (X-ray absorption near edge structure) techniques. The concentrations of dissolved P under anoxic conditions increased up to 69.3% in the highly acidic soil compared with the control soil. The decrease of the Fe-P fraction, the decrease of Ferrihydrite-Pads speciation, and the strong linear correlation between the dissolved P and Fe2+ (R2 > 0.85) supports the finding that enhanced P mobilization under anoxic conditions may be attributed to Fe reduction in the highly acidic soil. The concentration of dissolved Fe and P remained low until pH dropped below 6.35 for P and 6.28 for Fe, while a liner increase was found in dissolved Mn accompanying a general trend of pH decrease. This result suggests that the dissolution of reducible Mn under acidic soil conditions was an important factor for enhancing mobilization of dissolved P under anoxic conditions. This trend was due to the low amount of Mn, indirectly speeding up Fe reduction. These results can help to develop management practices to effectively mitigate P export and protect water resources from diffuse P pollution.

For the increasing need of stabilization both cationic and anionic metal(loid)s simultaneously, we newly developed a metal sorbent (FIXALL), consisting mainly of ferrihydrite and gypsum. The objectives of this study were to determine the molecular mechanisms of Pb and As stabilization in an aqueous system and to examine a simultaneous and long-term (up to 754 days) effect on Pb and As stabilization in an anthropogenically contaminated soil using the FIXALL sorbent. When the solution contained a low concentration of Pb (5 mg L −1 ), the mechanisms of Pb removal by FIXALL were based chiefly on the formation of inner-sphere surface complex with ferrihydrite. In the highly concentrated Pb solution (1200 mg L −1 ), contrarily, the removal of Pb by FIXALL was the direct consequence of the dissolution of gypsum and subsequent precipitation of PbSO 4 , which strengthens the drawback of low capability of ferrihydrite for Pb removal. Regardless of initial concentrations, the primary mechanism of FIXALL for As stabilization is attributed to the formation of inner-sphere surface complex with ferrihydrite. A contaminated soil study demonstrated that FIXALL could decrease the concentration of water soluble As and Pb simultaneously and continuously for 754 days without notable changes in their chemical species and soil pH.

Controlled drainage is considered as a soil management tool to improve water supply to crops and reduce nutrient losses from fields; however, its closure may affect phosphorus (P) mobilization in soil. To assess the P mobilization potential, three soil profiles with redoximorphic features were selected along a slight hill in Northern Germany. Soil samples from three depths of each profile were characterized for basic properties, total element content, oxalate- and dithionite-extractable pedogenic Al, Fe and Mn (hydr)oxides, P pools (sequential extraction), P species [PK-edge X-ray absorption near-edge structure (XANES) spectroscopy] and P sorption behavior. In topsoil (~ 10 cm depth), labile P (H2O-P + resin-P + NaHCO3-P) accounted for 26–32% of total P (Pt). PhosphorusK-edge XANES revealed that up to 49% of Ptwas bound to Al and/or Fe (hydr)oxides, but sequential fractionation indicated that > 30% of this P was occluded within sesquioxide aggregates. A low binding capacity for P was demonstrated by P sorption capacity and lowKfcoefficients (20–33 $${\text{mg}}^{{1 - n_{\text{f}} }} \,{\text{L}}^{{n_{\text{f}} }} \,{\text{kg}}^{ - 1}$$mg1-nfLnfkg-1) of the Freundlich equation. In the subsoil layers (~ 30 and ~ 65 cm depth), higher proportions of Al- and Fe-bound P along with other characteristics suggested that all profiles might be prone to P mobilization/leaching risk under reducing conditions even if the degree of P saturation (DPS) of a profile under oxic conditions was

The use of indium in semiconductor products has increased markedly in recent years. The release of indium into the ecosystem is inevitable. Under such circumstances, effective and accurate assessment of indium risk is important. An indispensable aspect of indium risk assessment is to understand the interactions of indium with plants, which are fundamental components of all ecosystems. Physiological responses of Arabidopsis thaliana exposed to indium were investigated by monitoring toxic effects, accumulation and speciation of indium in the plant. Indium can be taken up by plants and is accumulated mainly in roots. Limited indium root-to-shoot translocation occurs because of immobilization of indium in the root intercellular space and blockage of indium by the Casparian band in the endodermis. Indium caused stunted growth, oxidative stress, anthocyanization and unbalanced phosphorus nutrition. Indium jeopardizes phosphate uptake and translocation by inhibiting the accumulation of phosphate transporters PHOSPHATE TRANSPORTER1 (PHT1;1/4), responsible for phosphate uptake, and PHOSPHATE1 (PHO1), responsible for phosphate xylem loading. Organic acid secretion is stimulated by indium exposure. Secreted citrate could function as a potential detoxifier to lower indium uptake. Our findings provide insights into the potential fate and effects of indium in plants and will aid the evaluation of risks with indium contamination.

Biochar can reduce the mobility and availability of potentially toxic elements (PTEs) in soils and improve soil properties. However, immobilization efficiencies of biochar can be varied according to environmental conditions, such as pH and redox potential (Eh), especially for soils under flood-dry cycles. In the current study, biochar produced at 300 and 550 °C (referred as BC300 and BC550, respectively) and its feedstock (pine sawdust biomass, BM) were used to amend a lead (Pb)-contaminated soil under pre-defined redox windows (from −300 to +250 mV). Key features of the soil-solution were evaluated in detail, including pH, dissolved organic carbon, sulphate, and dissolved Al, Fe, and Mn. The BC550 reduced the amount of dissolved Pb and showed a different pattern of Eh-pH in the soil slurry compared with BM and BC300. This might be attributed to its higher alkalinity and surface area. The highest amount of dissolved Pb was found at slightly anoxic conditions (−100 to 0 mV) in CS (control soil), S&BM (soil amended with BM), and S&BC300 (soil amended with BC300), which could be associated with the dissolution of Fe/Mn oxides. Moreover, the fitting results of Pb X-ray absorption fine structure (XAFS) indicated that the proportion of Pb(CH3COO)2 was decreasing when changing from anoxic to oxic condition in S&BC300, while the Pb speciation pattern in soil was stable in S&BC550. These results suggested that BC550 is more suitable amendment for Pb immobilization than BM and BC300 in this study. In addition, biochar produced at higher temperatures can be more stable so it can be suitable for remediation of Pb-contaminated soils which are frequently flooded. Keywords: Immobilization/stabilization, Lead dissolution/mobility, Paddy soil, XANES spectroscopy, Black carbon

The objectives of this study were to investigate the amount of B incorporation into precipitated calcium carbonate (PCC) in the coprecipitation process, and to determine specific mineral phases (calcite or vaterite) and the mode of B coordination (trigonal or tetrahedral) in PCC under different pH and B concentrations. The amount of B incorporation into PCC increased in general with increasing aqueous B (Baq) concentrations in the pH range from 8 to 12. The B removal by PCC reached maximum (∼200 mmol kg−1) at pH 10 with Baq concentrations between 30 and 50 mM. The transformation of vaterite to calcite was promoted with increasing Baq at pH 8 and 10, whereas an excess concentration of aqueous (poly)borate anions (100 mM) inhibited crystal growth of calcite. As determined by B K-edge X-ray absorption fine structure spectroscopy, the coordination of B incorporated in PCC was preferentially tetrahedral (IVB, 55–70%) over trigonal (IIIB, 30–45%) at Baq

Intensive uses of agrochemicals and soil amendments often cause the elevation of Cu and Zn concentrations in vineyard (VY) and orchard soils. The concentration and speciation of Cu and Zn in the soils at millimeter resolution is critical to understanding the risk of transport of these metals via surface runoff and infiltration. The objective of this study was to investigate the concentration and chemical species of Zn and Cu in VY and persimmon (PS) soils at millimeter vertical resolution. The soils were collected with 5 mm increments down to 5 cm depth and with 5 cm increments down to 25 cm depth. The total concentration and chemical species of Zn and Cu were determined by total digestion and X-ray absorption fine structure (XAFS) spectroscopy, respectively. The Zn concentration of VY soil reached a maximum of 290 mg kg−1 at the uppermost layer of the profile (0.5–1.0 cm). The Cu concentration of VY soil reached a maximum of 201 mg kg−1 (10–15 cm). These Zn and Cu concentrations were greater than background levels. Zinc K-edge XAFS spectroscopy determined that the uppermost layer of VY soil (0–0.5 cm) contained 42% Zn associated with humus and lesser extent of Zn associated with gibbsite (37%) and kaolinite (21%). Zinc associated with humus was not observed in the VY soil profiles below 0.5 cm, whereas Zn associated with gibbsite and kaolinite contributed >83% of total Zn species. Copper K-edge XAFS spectroscopy determined the presence of Cu bonded with humus (40–67%) and Cu adsorbed on kaolinite (26–45%) in the entire soil profile. Our study found the remarkable variation of Cu and Zn concentration and speciation within several centimeters from the soil surface in vineyard and orchard landscapes.

Thallium (Tl) is a highly toxic environmental contaminant. To understand the mechanism controlling the mobility and availability of Tl in soil, the adsorption and desorption of Tl(I) were evaluated for six soils. The maximum Tl adsorption capacities of the soils ranged from 6.9 to 244.5 mmol kg−1. The corresponding Tl LIII-edge X-ray absorption near-edge structure (XANES) data revealed that illite, vermiculite, and smectite are the preferential soil constituents for adsorbing Tl(I). Tl(I) adsorption by the soils was reversible in the low Tl adsorption regime and exhibited various degrees of hysteresis as the amount of absorbed Tl increased. The Tl(I) adsorption and extent of adsorption reversibility were mainly dependent on the type and content of clay minerals and exchangeable cations in different soils. Despite high Tl(I) adsorption by the soils, the reversibility of Tl(I) adsorption may result in relatively high mobility and availability of Tl(I) in soils, leading to high environmental risk. Ignoring the reversibility of Tl adsorption could lead to an erroneous prediction of the mobility and availability of Tl in soils and a significant underestimation of the environmental risk posed by Tl in the environment.

Arsenic immobilization in acid mine drainage (AMD) is required prior to its discharge to safeguard aquatic organisms. Zero-valent aluminum (ZVAl) such as aluminum beverage cans (AlBC) was used to induce the oxidation of As(III) to As(V) and enhance the subsequent As removal from an artificially prepared AMD. While indiscernible As(III) oxidation was found in aerated ZVAl systems, the addition of 0.10–0.55 mM Fe(II) or Fe(III) into the AMD significantly promoted the As(V) production. Reactions between Fe(II) and H2O2, which was produced through an oxidative reaction of ZVAl with dissolved oxygen, generated OH radicals. Such OH radicals subsequently induced the As(III) oxidation. Over the course of the Fenton like reaction, ZVAl not only directly generated the H2O2, but indirectly enhanced the OH radical production by replenishing Fe(II). Arsenite oxidation in the aerated ZVAl/Fe and AlBC/Fe systems followed zero- and first-order kinetics. Differences in the kinetic reactions of ZVAl and AlBC with respect to As(III) oxidation were attributed to higher productive efficiency of the oxidant in the AlBC systems. After the completion of As(III) oxidation, As(V) could be removed simultaneously with Al(III) and Fe(III) by increasing solution’s pH to 6 to produce Al/Fe hydroxides as As(V) scavengers or to form Al/Fe/As co-precipitates.

This study investigated the effect of rice straw char (RSC) on the immobilization of Cr(VI) in soils. The Cr(VI) sorption experiments on the RSC and RSC-amended soils were conducted using the batch method. RSC exhibited Cr(VI) reduction capacity due to its black carbon content. The addition of RSC to the soils enhanced the overall Cr(VI) immobilization of the soils, which is primarily attributed to the Cr(VI) reduction capacity of RSC. The effects of RSC amendment on the Cr(VI) sorption of the soils increased with increasing RSC content in the soils and decreased with increasing pH or anion contents in the soil solutions. After Cr(VI) was sorbed by the soils, a portion of the Cr(VI) was converted to Cr(III) and the remainder was sorbed onto the soils. The presence of RSC in the soils decreased the portion of sorbed Cr(VI) in the soils and therefore lowered the potential remobilization of Cr(VI) from the soils. The results suggested that RSC amendment can be applied to develop a cost-effective meth...

An efficient method for removing and recovering molybdenum (Mo) from water was developed by using ZnFe2O4 nanoparticles. The Mo adsorption displayed a nonlinear isotherm that fitted well with the Langmuir isotherm, showing limited adsorption sites on the surface of ZnFe2O4. The adsorption of Mo(VI) was dependent on solution pH. With increasing pH, the build-up of negative charges of both adsorbent and adsorbate led to enhanced electric repulsion between them. The K-edge XANES spectra for the adsorbents collected after Mo adsorption revealed that Mo(VI) was the predominant oxidation state sorbed on ZnFe2O4, indicating that the reduction of Mo(VI) did not occur on ZnFe2O4. The different peak positions of k-space and R-space shown in K-edge EXAFS spectra demonstrated that the adsorbed Mo could be bound on the surface or be slipped in the vacancy position of the ZnFe2O4 crystal. Importantly, Mo could be efficiently adsorbed from photoelectric industrial wastewater and these adsorbed Mo anions were rapidly replaced by OH(-) ions, implying the potential for Mo removing and recovering in industrial wastewater.

Global concerns for the sustainability of agriculture have emphasized the need to reduce the use of mineral fertilizer. Although phosphorus (P) is accumulated in farmland soils due to the long-term application of fertilizer, most soil P is not readily available to plants. The chemical speciation of P in soils, which comprise heterogeneous microenvironments, cannot be evaluated with a high degree of specificity using only macroscopic analyses. In this study, we investigated the distribution and speciation of P accumulated in soils by using both macro- and microscopic techniques including chemical extraction, solution and solid-state 31P NMR, bulk- and micro- P K-edge X-ray absorption near edge structure (XANES), and electron probe microanalysis (EPMA). Soil samples were collected from a field in which cabbage was cultivated under three amendment treatments: i) mineral fertilizer (NPK), ii) mineral fertilizer and compost (NPK + compost), and iii) mineral fertilizer plus compost but without nitrogen fertilizer (PK + compost). Macro-scale analyses suggested that accumulated P was predominantly inorganic P and associated with Al-bearing minerals. The repeated application of compost to the soils increased the proportion of P associated with Ca which accounted for 17% in the NPK + compost plot and 40% in the PK + compost plot. At the microscale, hot spots of P were heterogeneously distributed, and P was associated with Fe and Ca in hot spots of the NPK + compost (pH 6) and PK + compost (pH 7) treated samples, respectively. Our results indicate that application of compost contributed to creating diverse microenvironments hosting P in these soils.

The aim of this study was to assess the soil contamination caused by potentially toxic elements (Al, As, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, V, and Zn) using various indices and the associated risk of human health for adults and children in selected soils from Germany (Calcic Luvisols, Tidalic Fluvisols, Haplic Gleysols, and Eutric Fluvisols) and Egypt (Haplic Calcisols, Sodic Fluvisols, and Eutric Fluvisols). Soil contamination degree has been assessed using indices such as contamination factor (CF), pollution load index (PLI), geo-accumulation index (Igeo), and enrichment factor. We also assessed the health risk for children and for male and female adults. Chromium, Cu, As, Mo, Ni, Se, and Zn in the German Fluvisols had high CF of >6, while in the Egyptian Fluvisols Se, Mo, As, and Al revealed a high CF. The PLI (1.1–5.2) was higher than unity in most soils (except for Tidalic Fluvisols), while the most important contributor was Se, followed by Mo and As in the Egyptian Fluvisols, and by Cr, Cu, and Zn in the German Fluvisols. The median value of hazard index (HI) for children in the studied soils indicated an elevated health risk (higher than one), especially in the German Fluvisols (HI = 4.0–29.0) and in the Egyptian Fluvisols (HI = 2.2–5.2). For adults, median HIs in all soils were lower than unity for both males and females. The key contributor to HI was As in the whole soil profiles, accounting for about 59% of the total HIs in all three person groupings. Our findings show that in the studied multi-element contaminated soils the risk for children’s health is higher than for adults; while mainly As (and Al, Cr, Cu, and Fe) contributed significantly to soil-derived health risk.

This work aims to study the pseudo-total content, geochemical fractions, and species of arsenic (As) in the bulk soil and in the coarse and fine particles of top soil and soil profiles collected from active and abandoned gold mine spoils in Ghana. The human health risk for adults (male and female) and children has been assessed. To achieve our aims, we collected 51 samples, characterized them, determined the total As content, and sequentially extracted the geochemical fractions of As including water-soluble and un-specifically bound As (FI); specific-sorbed/exchangeable As (FII); poorly (FIII)- and well-crystalline (IV) Fe oxide; and residual/sulphide fraction (FV). In selected samples, As species were determined using synchrotron-based X-ray absorption near edge structure (XANES). Pseudo-total As contents varied from 1807 to 8400 mg kg−1, with the extremes occurring at the abandoned mine spoil. Arsenic was almost 10-fold higher in the fine particles (

Coconut-fiber biochar (CFB) was applied at 3% (w/w) to two soils spiked with 250, 2500, 5000 mg kg−1 of lead (Pb), respectively, aiming to explore the effects of CFB and the significance of iron (Fe) plaque on rice roots on the accumulation and translocation of Pb in rice plants using micro-X-ray fluorescence and X-ray absorption spectroscopies. The CFB amendment resulted in a significant decrease in the EDTA-extractable Pb availability in the soils, which might be attributed to the increased amounts of Pb-loaded humic acid and Pb3(PO4)2 formed in the soils. Consequently, the addition of CFB caused a significant decrease in Pb concentrations of the brown rice harvested from the CFB-amended soils under all Pb levels by 14 %–47 %, as compared to those from the unamended soils. Therefore, CFB could be used as an immobilizing agent for Pb in contaminated soils. However, CFB application significantly inhibited the formation of Fe/Mn plaques on rice roots and reduced its interception effect on Pb uptake, which consequently increased the Pb translocation rate from root to shoot. Therefore, the increased translocation rate of Pb in rice plants by CFB should not be ignored when CFB is applied to remediate Pb-contaminated paddy soils.

Assessing the vanadium (V) fractionation and speciation to predict its bioavailability using a combined approach of geochemical extractions and micro-spectroscopic techniques is still not well studied. Therefore, we aimed to determine the bioavailability of V in rural soils using single extractants, sequential extraction procedure, and the X-ray absorption near edge structure (XANES) spectroscopy. We collected and characterized ninety four samples originated from horizons of seventeen soil profiles in Taiwan. We determined the total content of V and its geochemical fractions using the BCR sequential extraction procedure to predict its potential mobility. We also assessed the bioavailability of V in the soils using four availability indices i.e., CaCl2, HCl, ethylenediaminetetraacetic acid (EDTA), and NaHCO3 and related them to its uptake by Chinese cabbage (Brassica chinensis L.). Additionally, we determined the V speciation by vanadium K-edge XANES spectra. Moreover, we studied the elemental compositions of the soils using Electron Probe Micro Analysis (EPMA). Vanadium was mainly distributed in the residual fraction (81–98% of total V). Among the potential mobile fractions, V was mainly associated with Fe oxides, as identified by the BCR sequential extraction and EMPA. The XANES analysis indicated that V mainly existed in the soils as V(IV) and V(V). The EDTA and NaHCO3 extracted more V than CaCl2 and HCl, and both, particularly NaHCO3 were positively and significantly correlated with the total soil content and plant shoot concentrations of V; therefore NaHCO3 might be recommended as a bioavailability index for soil V. We hypothesize that the NaHCO3 may extract vanadate from soil surfaces and also vanadate transformed from vanadyl at alkaline pH during the extraction. The NaHCO3-extracted V can be predicted by a function of soil total V, CEC, and pH. Our results should be verified using different soils and plants in the future.

High contents of arsenic were detected in soils in Guandu plain, northwest Taiwan. To determine the sources and speciation of As in the soils, the depth profiles of soil properties, elemental composition and As speciation were investigated. The As concentrations in the soil profile ranged from 152 to 1222 mg kg−1, with the highest concentration at the depth of 70–80 cm. The As distribution was found to be positively correlated to Fe, Pb, and Ba. The As(V)-adsorbed ferrihydrite and scorodite were the predominant phases in the top layers (

Soil microorganisms play a crucial role in the bioremediation of pentachlorophenol (PCP)-contaminated soils. However, whether and how soil bacterial networks with keystone taxa affect PCP dechlorination is not well understood. The present study investigated the effects of citrate on soil bacterial networks mediating PCP dechlorination by direct and indirect transformation in iron-rich upland and paddy soils. The rates of PCP dechlorination and Fe(II) generation were accelerated by citrate addition, particularly in the paddy soils. Network analysis revealed that the topological properties of bacterial networks were changed by citrate addition; more modules and keystone taxa were significantly correlated with PCP dechlorination and Fe(II) generation in the networks. Random forest modeling indicated that Clostridiales was the most important bacterial order; it was significantly involved in both the direct and indirect pathways of PCP dechlorination. Citrate addition had less influence on the balance between the direct and indirect pathways of PCP dechlorination in the upland soils, whereas it enhanced biological PCP dechlorination more directly and efficiently in the paddy soils. Our results suggested that land-use type and citrate addition play a critical role in controlling the biogeochemical mechanisms of PCP dechlorination.

This chapter reviews studies on macroscale and X-ray absorption fine structures research techniques for nickel (Ni) speciation in soils. It begins with a review of macroscopic research to demonstrate the importance of chemical speciation as an approach for understanding the behavior of Ni, focusing on soil retention, reaction kinetics, and potential mobility. Ni present in soils can be associated with soil solid components through a variety of reactions, mainly precipitation, coprecipitation, adsorption, and ion exchange. The X-ray absorption spectroscopy analysis of soil Ni provides evidence that can be used to develop chemical speciation model studies to predict the bioavailability, transformations, and potential mobility of Ni in the terrestrial environment. Sequential extraction methods have been applied to study the effects of remediation methods on the mobility and bioavailability of Ni in contaminated soils by comparing the Ni fractionations before and after the remediation treatments.

Low-cost adsorbents, e.g., cow manure-based carbon, provide alternatives to remove veterinary antibiotic sulfamethazine (SMT) from contaminated water bodies. In this study, the chemical structures and compositions of cow manure (CM) carbonized at 400, 600, and 800 °C (CM400, CM600, and CM800) were examined using elemental analyzer (EA), Brunauer–Emmett–Teller, and spectroscopic techniques. Adsorptions of SMT on CM samples were conducted as functions of pH, hydrophobicity, and ionic strengths. Results of EA and spectroscopic analyses suggested that the raw CM and CM400 samples contained the highest amounts of O-containing groups and aliphatic domains. Amounts of such two chemical groups decreased as carbonization temperatures increased. The specific surface areas and total pore volumes of CM samples increased significantly when the carbonization temperatures exceeded 600 °C. SMT adsorption on CM samples could be described essentially by the pseudo-second-order kinetic, intra-particle diffusion, and Freundlich isotherm models. Low pH and ionic strength were favorable for SMT adsorption in CM samples, particularly for the CM800, because a strong π +–π electron donor–acceptor interaction (π +–π EDA) was formed between SMT and CM surfaces enriched with hydrophobic domains. Further, the high adsorption affinity of SMT to the CM600 and CM800 samples was attributed in part to their larger surface areas and total pore volumes. Generally, CM-based materials carbonized >600 °C showed relatively stable structures and exhibited strong aromatic properties. Moreover, maximum adsorption capacities of SMT on the CM800 sample (37–39 mM/kg) were significantly higher than those of other common adsorbents (0.02–35.93 mM/kg).

Copper is a trace element essential for living creatures, but copper content in soil should be controlled, as it is toxic. The physical-chemical-biological features of Cu in soil have a significant correlation with the Fe(II)/Cu(II) interaction in soil. Of significant interest to the current study is the effect of Fe(II)/Cu(II) interaction conducted on goethite under anaerobic conditions stimulated by HS01 (a dissimilatory iron reduction (DIR) microbial). The following four treatments were designed: HS01 with α-FeOOH and Cu(II) (T1), HS01 with α-FeOOH (T2), HS01 with Cu(II) (T3), and α-FeOOH with Cu(II) (T4). HS01 presents a negligible impact on copper species transformation (T3), whereas the presence of α-FeOOH significantly enhanced copper aging contributing to the DIR effect (T1). Moreover, the violent reaction between adsorbed Fe(II) and Cu(II) leads to the decreased concentration of the active Fe(II) species (T1), further inhibiting reactions between Fe(II) and iron (hydr)oxides and decelerating the phase transformation of iron (hydr)oxides (T1). From this study, the effects of the Fe(II)/Cu(II) interaction on goethite under anaerobic conditions by HS01 are presented in three aspects: (1) the accelerating effect of copper aging, (2) the reductive transformation of copper, and (3) the inhibition effect of the phase transformation of iron (hydr)oxides.

The mobility and bioavailability of cadmium (Cd) in soils are determined by physical and chemical properties and constituents of soils. This study investigated the effect of rice straw ash (RSA) on the Cd distribution in a contaminated rice paddy soil with submerging incubation. RSA was obtained from a simulated open-field burning. The adsorption of Cd by RSA at pH 4 was characterized by L-shaped nonlinear isotherm and was fully reversible, suggesting nonspecific surface adsorption. The adsorption increased with increasing pH due to increased negative charges of RSA and hydrolysis and precipitation of Cd ion. During submerging incubation, soluble Cd concentration in a Cd-contaminated rice paddy soil gradually increased with incubation time. The results of sequential extraction for the soil before and after submerging incubation showed that the portion of Cd associated with Fe- and Mn-oxides decreased and transferred to exchangeable and residual fractions. The presence of RSA in soils suppressed the release of Cd into the soil solutions. The results suggest that the occurrence of RSA in rice paddy soil can retard the movement of Cd in soil and thus lower the potential environmental risk of Cd in soil.

A copper ferrite generated from the sludge of printed circuit board (PCB) industry was investigated for its As(III) adsorption behavior. The adsorption of As(III) by this copper ferrite exhibited a L-shaped nonlinear isotherm and fitted well the Langmuir isotherm, implying limited binding sites and monolayer sorption occurred on the surface. The K-edge X-ray Absorption Near-Edge Structure (XANES) showed that the adsorbed As(III) could be oxidized to less toxic As(V) by copper ferrite. The maximum As adsorption capacity of the copper ferrite was 41.2 mg g −1 at pH 4.2 and decreased dramatically at higher pH (pH > 9.2) due to enhanced electrostatic repulsion between As(III) and the adsorbent surface. Desorption of As(III) using four different acid solutions demonstrated that the desorption rate decreased in the order of H 3 PO 4 > H 2 SO 4 > HCl > HNO 3 . These results demonstrate that the synthesized copper ferrite is an effective adsorbent for removing As(III) in aqueous solution. The new data obtained reveals informative knowledge for designing an arsenic removal technology in treating contaminated groundwater.

Although gallium (Ga) is a rare element, it is widely used in semiconductor devices. Ga contamination of the environment has been found in semiconductor-producing countries. Here, the physiological and molecular impacts of Ga in the model plant Arabidopsis thaliana were investigated in medium culture. The primary symptom of Ga toxicity is inhibition of root growth. The increased production of malondialdehyde (MDA) suggests that Ga stress could cause oxidative damage in plants. Roots were the main Ga accumulating sites. The distinctive Ga granules were deposited within the intercellular space in roots. The granules are Ga(OH)3 precipitation, which indicates immobilization or limited translocation of Ga in A. thaliana. Ga stress induces root secretion of organic acids such as citrate and malate. The expression of the transporters AtALMT and AtMATE, responsible for citrate and malate secretion, respectively, were elevated under Ga stress, so the secretion may play a role in the resistance. Indeed, supplying ...

Soil is a vital biological habitat, which is of primary importance in determining and regulating biological activity and biodiversity. Therefore, it is Earth’s most important resource in sustaining both belowground and aboveground biological activities. Biodiversity versus landscape diversity and land use practices in multifunctional landscapes have been addressed. Humans have so manipulated nature that few locations in the world remain without human influence, causing unforeseen changes in ecosystem continuously and biodiversity. Among the environmental compartments, about 90 % of environmental pollutants are bound with soil particles. The soil-bound pollutants may be released to the soil solution through physical, chemical and biological interfacial interactions and pose a threat to biodiversity and ecosystem integrity. These interfacial interactions are especially important in the rhizosphere, where the kinds and concentrations of biomolecules are different from the bulk soil because of intense biological activity. These biomolecules affect biogeochemical processes, soil microbial ecology, nutrient and contaminant dynamics, abiotic and biotic factors, and soil biodiversity through allelopathic interactions. Soil interfacial interactions under different pedogenic processes and anthropogenic activity in relation to belowground biodiversity and the impact on aboveground biodiversity, productivity and integrity should be an important and exciting area of science for years to come.

A novel fabricated copper ferrite was investigated for its As(V) removal in this study. The adsorption of As(V) by this recycled copper ferrite exhibited an L-shaped nonlinear isotherm, suggesting limited binding sites on the adsorbent surface. The As K -edge XANES indicated that the adsorbed As(V) on copper ferrite was not reduced to more toxic As(III) by Fe 2+ in the structure. The maximum As adsorption capacity of the copper ferrite was 45.66 mg g −1 at pH 3.7 and decreased with pH due to enhanced electrostatic repulsion between As(V) and the adsorbent surface. Desorption of As(V) using six different acid and salt solutions revealed that the desorption rate decreased in the order of H 3 PO 4 > Na 3 PO 4 > H 2 SO 4 > Na 2 SO 4 > HCl > HNO 3 . These results suggest that the recycled copper ferrite without surface modification is an effective adsorbent for removing As(V) from water, confirmed by effective removal of As(V) from contaminated groundwater. Furthermore, the novel fabricated copper ferrite can be used to recover rapidly in 20 s using a magnet. The information obtained in this work shows great potential for developing a cost-effective adsorbent for immobilization of arsenate using the recycled copper ferrite.

This study examined the change in the speciation of cadmium (Cd) in contaminated rice paddy soils with prolonged submergence. Three Changhua soils (CH1, CH2, and CH3) from central Taiwan and three Taoyuan red soils (TY1, TY2, and TY3) from northern Taiwan with different levels of heavy metal contamination were selected for the study. The Cd, Fe, Mn, soil pH, and Eh in soil solutions were determined as a function of submerging time. During submergence, the Fe and Mn concentration increased, while the SO4 2− concentration decreased. The concentrations of Cd immediately increased in the soil solutions after a short submerging time and then decreased with further submergence. The sequential extraction showed that the exchangeable fraction decreased and the oxide-bound fraction increased after submergence. According to preliminary calculations of the MinteqA2 program, sulfate cannot be reduced to sulfite or sulfide under the soil Eh and pH values observed for the soils after prolonged submergence. Thus, the observed decreases of sulfate concentration may result from sulfate reduction in the micro-environments, which cannot be accounted for by the thermodynamic calculation. The reduction of sulfate to sulfide may subsequently result in the formation of CdS precipitate, which attributes to the decreases of Cd concentrations in the soil solutions after prolonged submergence.

Humic substances are a major component of soil organic matter that influence the behavior and fate of heavy metals such as Cr(VI), a toxic and carcinogenic element. In the study, a repetitive extraction technique was used to fractionate humic acids (HAs) from a peat soil into three fractions (denoted as F1, F2, and F3), and the relative importance of O-containing aromatic and aliphatic domains in humic substances for scavenging Cr(VI) was addressed at pH 1. Spectroscopic analyses indicated that the concentrations of aromatic C and O-containing functional groups decreased with a progressive extraction as follows: F1 > F2 > F3. Cr(VI) removal by HA proceeded slowly, but it was enhanced when light was applied due to the production of efficient reductants, such as superoxide radical and H2O2, for Cr(VI). Higher aromatic- and O-containing F1 fraction exhibited a greater efficiency for Cr(VI) reduction (with a removal rate of ca. 2.89 mmol g−1 HA under illumination for 3 h). 13C NMR and FTIR spectra further demonstrated that the carboxyl groups were primarily responsible for Cr(VI) reduction. This study implied the mobility and fate of Cr(VI) would be greatly inhibited in the environments containing such organic groups.

Complete removal of heavy metal from complex heavy-metal wastewater (CHMW) requires advanced technology. This study investigated the feasibility of a multi-staged ferrite process (MSFP) for treating CHMW, containing Cd, Cu, Pb, Cr, Zn, Ag, Hg, Ni, Sn and Mn. Our experimental results showed that most of the supernatants after conventional single-step ferrite process could conform to the effluent standard of Environmental Protection Administration in Taiwan. However, the sludge could not satisfy the toxicity characteristic leaching procedure (TCLP) limits due to high Cd, Cu, and Pb concentrations. The performance of MSFP in removing heavy metals from wastewater was subsequently investigated and the parameters of three treating steps in MSFP were optimized under 70 °C and 90 °C at pH 9, and 80 °C at pH 10. After the three-staged procedures, all heavy metals in supernatant and sludge could fulfill the contamination levels regulated by law. In addition, the sludge generated from the MSFP was examined by XRD and forms a stable spinel structure, which could be effectively separated by external magnetic field.

This study investigated the reaction mechanisms of toxic hexavalent chromium (Cr(VI)) with polysaccharide biopolymers, including cellulose, hemicellulose and chitin, to understand their roles in determining the Cr(VI) removal efficiencies of biomaterials containing these polymers. The Cr(VI) removal rates and capacities at pH 2 followed the order of hemicellulose > chitin ≫ cellulose. The Cr(VI) removals by these biopolymers were primarily due to the reduction of Cr(VI) to Cr(III). The hydroxyl groups of these biomaterials were the predominant reactive sites for Cr(VI) reduction, and the oxidation of the hydroxyl groups by Cr(VI) led to the formation of carboxyl groups. In addition to the reactivity of functional groups toward Cr(VI), the accessibility of functional groups in different biopolymers may also determine the reduction rates of Cr(VI) by the biopolymers.

It has been suggested that lignocellulosic materials are cost-effective materials for remediation of toxic Cr(VI) pollution; however, inconsistent interpretations of the reaction mechanism are frequently found in the literature. To clarify the reaction mechanism, the reaction of Cr(VI) with individual components of lignocellulosic materials must be better understood. Thus, the purpose of this work was to investigate the reaction mechanism of Cr(VI) with cellulose, which is one of the main components of lignocellulosic materials. The Cr(VI) reaction was evaluated at pH 2 and the results showed that Cr(VI) could be reduced to Cr(III) by cellulose. As indicated by the FT-IR spectra, the hydroxyl groups in cellulose were the reactive sites for Cr(VI) reduction, giving rise to the formation of carboxyl groups. The resultant Cr(III) was either bound to cellulose or released into solution. As revealed by the Cr K-edge EXAFS of cellulose after its reaction with Cr(VI), Cr(III) was bound to cellulose by forming bidentate–mononuclear complexes with carboxyl groups, which resulted from the oxidation of hydroxyl groups. The first-order rate constant of Cr(VI) reaction with cellulose was determined to be 3.84 × 10 −7 s −1 at pH 2. The slow reduction of Cr(VI) by cellulose was attributed to the existence of H-bonding networks in cellulose, which restrict the accessibility of Cr(VI) to the hydroxyl groups in cellulose.