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Dihydrogenarsenate [H(2)AsO(4)(-), As(V)] or dichromate [Cr(2)O(7)(2-), Cr(VI)] at pH=4.0 showed to be sorbed on a Fe(OH)(x)-polymerin complex and ferrihydrite to a greater extent than on polymerin, the organic polymeric fraction of olive oil mill wastewater (OMW). In particular, the maximum amount (x(m)) of arsenate sorbed on Fe(OH)(x)-polymerin complex was similar to that on ferrihydrite (880.26 and 743.02 mmol kg(-1), respectively), and was much greater than that sorbed on polymerin (384.25 mmol kg(-1)). The sorption of dichromate was to a comparable extent on Fe(OH)(x)-polymerin complex and ferrihydrite (205.90 and 254.88 mmol kg(-1), respectively). Cr(III), a less toxic chromium form, mainly, and Cr(V) were indeed the effective forms sorbed on polymerin (200 mmol kg(-1)), as a consequence of the redox reaction of the strongly toxic Cr(VI) with the CH(2)OH groups of the polysaccharide moiety of this bio-sorbent, according to the data deriving from XPS and DRIFT analyses. The potential exploitation of the selected sorbents for the removal of As(V) or Cr(VI) from aqueous effluents is briefly discussed.

Use of layered double hydroxides (LDHs) in the environmental field is gaining popularity due to their potential to sorb toxic anions, attributed to their large surface area, high anion exchange capacity, and good thermal stability. In this study, four different LDHs (i.e., Cu-Al-, Mg-Al-, Mg-Fe-, and Zn-Al-LDH) were synthesized to select one or more efficient sorbents, capable of removing arsenite [As(III)] from contaminated waters. In particular, we studied the following: (1) X-ray diffraction patterns and specific surface area of the synthesized LDHs; (2) sorption isotherms of As(III) at pH 7.0; and (3) sorption of As(III) on LDHs, in the presence of inorganic anions [carbonate (CO3), chloride (Cl), fluoride (F), phosphate (PO4), sulfate (SO4)] commonly present in aquatic environments. The poorly crystalline LDHs (i.e., Cu-Al-LDH and Mg-Fe-LDH) sorbed greater amounts of As(III) than the well-crystalline LDHs (i.e, Mg-Al-LDH and Zn-Al-LDH). The efficiency of the competing anions at inhibiting As(III) sorption by the LDHs was Cl ≤ F < SO4 << CO3 << PO4, regardless of initial ligand/As(III) molar ratios (R) or LDH. Although Cu-Al-LDH sorbed lower amounts of As(III) than the Mg-Fe-LDH, it showed, surprisingly, a higher affinity for As(III). This surprising behavior puts this LDH in the forefront as a potential sorbent for the treatment of arsenic-contaminated waters.

Arsenic (As) co-precipitation is one of the major processes controlling As solubility in soils and waters. When As is co-precipitated with Al and Mg, the possible formation of layered double hydroxides (LDHs) and other nanocomposites can stabilize As in their structures thus making this toxic element less available. We investigated the nature and reactivity of Mg-Al-arsenate [As(V)] co-precipitated LDHs formed in solution affected by As concentration, pH, and aging. At the beginning of the co-precipitation process, poorly crystalline LDH and non-crystalline Al(Mg)-oxides form. Prolonged aging of the samples promotes crystallization of LDHs, evidenced by an increase in As K XANES intensities and XRD peak intensities. During aging Al- and/or Mg-oxides are likely transformed by dissolution/re-precipitation processes into more crystalline but still defective LDHs. Surface area, chemical composition, reactivity of the precipitates, and anion exchange properties of As(V) in the co-precipitates are influenced by pH, aging, and As concentration. This study demonstrates that (i) As(V) retards or inhibits the formation and transformation of LDHs and (ii) more As(V) is removed from solution if co-precipitated with Mg and Al than by sorption onto well crystallized LDHs.

This research studied the factors influencing the adsorption of phosphate and sulfate added alone or as a mixture on Andisols with different chemical, physico-chemical, and mineralogical properties and the desorption of sulfate (and phosphate) previously fixed on selected soil samples by increasing concentrations of phosphate (and sulfate). All the samples adsorbed amounts of phosphate 2–5 times greater than those of sulfate. By increasing pH, phosphate adsorption only slightly decreased, whereas sulfate retention decreased dramatically. At pH>5.5, sulfate sorption was usually very low or negligible. Allophanic materials increased the adsorption of the nutrients, whereas organic carbon and silicate prevented the adsorption of phosphate and especially of sulfate. When the anions were added as a mixture, sulfate adsorption was drastically decreased even in the presence of low amounts of phosphate (initial phosphate/sulfate molar ratio (R)≪1). A possible explanation of these findings is that phosphate adsorp...

Biomass-derived biochar is considered as a promising heavy metal adsorbent, due to abundance of polar functional groups, such as carboxylic, hydroxyl, and amino groups, which are available for heavy metal removal. The aims of this study were to evaluate the effectiveness of an orchard pruning-derived biochar in removing some heavy metals (through the evaluation of isotherms) and to study water dynamics at the solid-liquid interface as affected by heavy metal adsorption (through an innovative nuclear magnetic resonance (NMR) relaxometry approach). Both isotherms and NMR spectra revealed that Pb and Cr showed a good affinity for the biochar surface (Pb > Cr), while Cu was less affine. Accordingly, higher amounts of Pb and Cr were adsorbed by biochar as compared to those of Cu in the single systems. In binary systems (i.e., when two metals were applied simultaneously), Pb showed the highest inhibition of the adsorption of the other two metals, whereas the opposite was evidenced when Cu was used; the competitive adsorption was also strongly influenced by the metal residence time on biochar surface. In ternary systems (i.e., when all metals were applied simultaneously), even in the presence of high amounts of Pb and Cr, considerable adsorption of Cu occurred, indicating that some biochar adsorption sites were highly specific for a single metal.

Rice is comparatively efficient at assimilating inorganic arsenic (Asi), a class-one, non-threshold carcinogen, into its grain, being the dominant source of this element to mankind. Here it was investigated how the total arsenic (Ast) and Asi content of Italian rice grain sourced from market outlets varied by geographical origin and type. Total Cr, Cd Se, Mg, K, Zn, Ni were also quantified. Ast concentration on a variety basis ranged from means of 0.18 mg kg(-1) to 0.28 mg kg(-1), and from 0.11 mg kg(-1) to 0.28 mg kg(-1) by production region. For Asi concentration, means ranged from 0.08 mg kg(-1) to 0.11 mg kg(-1) by variety and 0.10 mg kg(-1) to 0.06 mg kg(-1) by region. There was significant geographical variation for both Ast and Asi; total Se and Ni concentration; while the total concentration of Zn, Cr, Ni and K were strongly influenced by the type of rice.

Arsenic pollution is currently a major health issue because As is toxic for human beings, animals, and plants. Knowledge of As mobility is therefore important to assess health risk. The sorption of arsenite and arsenate on metal oxides in the presence of various anionic ligands is closely linked to the mobility, bioavailability, and risk. It was reported that the sorption mechanisms and characteristics of arsenite and arsenate on Al-oxides were different from that on Fe-oxides. Previous work reports the sorption of arsenite and arsenate on Fe-oxides in the presence of ligands. Whereas there is few knowledge on the sorption of arsenite and arsenate by Al-oxides in the presence of ligands. Here, we studied the sorption of arsenite and arsenate on amorphous Al-oxide by batch experiments. We tested the effect of organic ligands: oxalate, malate, tartrate, citrate; and inorganic ligands: sulfate, phosphate, selenate, selenite. Results show that amorphous Al-oxide has more sorption affinity for arsenate than arsenite. The inhibition of As sorption by ligands at pH 6 is higher for arsenite than arsenate. For arsenite, the As sorption inhibition decreases in the order phosphate, citrate, malate, selenite, oxalate, tartrate, sulfate, and selenate. For arsenate, the As sorption inhibition decreases in the order phosphate, malate, citrate, selenite, tartrate, oxalate, sulfate, and selenate.

Layered double hydroxides (LDHs) show excellent capacity to sorb and exchange anions. Although arsenic (As) has a high affinity for sorbents containing Fe, only few studies have been carried out on the sorption of arsenite [As(III)] onto a Fe-based LDH. In this work we studied the sorption of As(III) onto a LDH containing Mg and Fe (Mg–Fe-LDH), as affected by: (i) pH; (ii) the presence of increasing concentrations of organic [citrate (CIT) and oxalate (OX)] and inorganic [phosphate (PO 4 ), selenite (SeO 3 ) and sulphate (SO 4 )] ligands, (iii) the effect of residence time on the desorption of As(III) by these ligands, and; (iv) the kinetics of desorption of As(III) by PO 4 . The As(III) sorption isotherms, carried out at different pHs, indicated a good affinity of the harmful oxyanion for the sorption sites of the Mg–Fe-LDH. This material was able to remove efficiently As(III) from contaminated solutions over a wide range of pH, but more in acidic than in alkaline systems. Competing ligands differently prevented the As(III) sorption on Mg–Fe-LDH, according to the following order: SO 4 3 4 . The desorption of As(III) by these anions decreased with increasing As(III) residence time on LDH surfaces. A comparison between the sorption/desorption of As(III) and As(V) on/from Mg–Fe-LDH in the presence or absence of anions highlights that less As(III) than As(V) is sorbed, whereas more As(III) than As(V) is desorbed by all the selected organic and inorganic ligands, but PO 4 .

Arsenite [As(III)] and arsenate [As(V)] sorption by Fe- and Al-based drinking-water treatment residuals (WTR) was studied as a function of particle size at different pHs, and in the presence of competing ligands, namely, phosphate, citrate, and oxalate. Both WTRs showed high affinity for As oxyanions. However, Al-WTR showed higher As(III) and As(V) sorption capacity than Fe-WTR because of their greater surface area. The effect of particle size on As sorption was pronounced on Fe-WTR, where the smaller fraction sorbed more As(III) and As(V) than the larger fractions, whereas relatively minor effects of particle size on As sorption was observed for Al-WTR. Arsenite sorption on both WTRs increased with increasing pH up to circum-neutral pHs and then decreased at higher pHs, whereas As(V) sorption decreased steadily with increasing pH. The capacity of competing ligands to inhibit sorption was greater for As(III) than As(V) on both WTRs (particularly on Al-WTR) following the sequence: oxalate.

We have studied the uptake and distribution of arsenic (As) and phosphorus (P) in roots, shoots and berries of tomato plants, grown on uncontaminated soil, irrigated with As-contaminated solutions at four concentrations (0, 0.5, 2 and 4 mg L -1 ), in presence or absence of P fertilization.The biomass of tomato plants decreased with increasing As concentration in irriga-tion water, especially tomato berries. In addition, the reduction of biomasses was significantly greater in plants non-fertilized with P. The beneficial effect generated with the P addition indicated that this nutrient played an important role in alleviating As toxicity in tomato plants. The higher the As concentration in irrigation water the higher the As concentration in plant tissues; most of the As absorbed by plants was accumulated in their roots. Phosphorus application has allowed to reduce As trans-location toward tomato berries, enhancing plant P status. These observations may be useful for certain areas of the World, in which As-contaminated waters are used for agricultural purposes.Keywords: tomato, arsenic, uptake, phosphorus fertilization, contamination.

The influence of compost on the growth of bean plants irrigated with As-contaminated waters and its influence on the mobility of As in the soils and the uptake of As (as NaAs III O 2 ) by plant components was studied at various compost application rates (3·10 4 and 6·10 4 kg ha −1 ) and at three As concentrations (1, 2 and 3 mg kg −1 ). The biomass and As and P concentrations of the roots, shoots and beans were determined at harvest time, as well as the chlorophyll content of the leaves and nonspecific and specifically bound As in the soil. The bean plants exposed to As showed typical phytotoxicity symptoms; no plants however died over the study. The biomass of the bean plants increased with the increasing amounts of compost added to the soil, attributed to the phytonutritive capacity of compost. Biomass decreased with increasing As concentrations, however, the reduction in the biomass was significantly lower with the addition of compost, indicating that the As phytotoxicity was alleviated by the compost. For the same As concentration, the As content of the roots, shoots and beans decreased with increasing compost added compared to the Control. This is due to partial immobilization of the As by the organic functional groups on the compost, either directly or through cation bridging. Most of the As adsorbed by the bean plants accumulated in the roots, while a scant allocation of As occurred in the beans. Hence, the addition of compost to soils could be used as an effective means to limit As accumulation in crops from As-contaminated waters.

Sorption of Cu and Cr in single and binary sorbate systems on goethite and goethite–Bacillus thuringiensis complex was studied. It was observed that the initial sorption rate and sorption capacity of heavy metals on goethite–B. thuringiensis complex were greater than that on goethite in single system at studied pH. The competitive ability of Cr in the sorption on both goethite and goethite–B. thuringiensis complex was stronger than that of Cu in binary system. Greater intensity of competitive sorption between heavy metals was found on goethite–B. thuringiensis complex than on goethite. The percentage of sorbed metals desorbed by Ca(NO3)2 from goethite was lower than that from goethite–B. thuringiensis complex, conforming the higher sorption affinity of heavy metals for goethite than that for goethite–B. thuringiensis complex. For each sorbed heavy metal, the desorption ratio was slightly higher from single sorption system than from binary sorption system, indicating the release of loosely sorbed metal and the retention of tightly bound cation during competitive sorption. The results obtained in this study are of significant importance for the assessment and remediation of multi-heavy metal pollution.

This paper describes the sorption of arsenate on Al-Mg and Fe-Mg layered double hydroxides as affected by pH and varying concentrations of inorganic and organic ligands, and the effect of residence time on the desorption of arsenate by ligands. The capacity of ligands to inhibit the fixation of arsenate followed the sequence: nitrate < nitrite < sulphate < selenite < tartrate < oxalate < phosphate on Al-Mg-LDH and nitrate < sulphate ≈ nitrite < tartrate < oxalate < selenite ≪ phosphate on Fe-Mg-LDH. The inhibition of arsenate sorption increased by increasing the initial ligand concentration and was greater on Al-Mg-LDH than on Fe-Mg-LDH. The longer the arsenate residence time on the LDH surfaces the less effective the competing ligands were in desorbing arsenate from sorbents. A greater percentage of arsenate was removed by phosphate from Al-Mg-LDH than from Fe-Mg-LDH, due to the higher affinity of arsenate for iron than aluminum.

We studied the sorption of As(III) and As(V) onto ferrihydrite as affected by pH, nature and concentration of organic [oxalic (OX), malic (MAL), tartaric (TAR), and citric (CIT) acid] and inorganic [phosphate (PO(4)), sulphate (SO(4)), selenate (SeO(4)) and selenite (SeO(3))] ligands, and the sequence of anion addition. The sorption capacity of As(III) was greater than that of As(V) in the range of pH 4.0-11.0. The capability of organic and inorganic ligands in preventing As sorption follows the sequence: SeO(4) ≈ SO(4) < OX < MAL ≈ TAR < CIT < SeO(3) ≪ PO(4). The efficiency of most of the competing ligands in preventing As(III) and As(V) sorption increased by decreasing pH, but PO(4) whose efficiency increased by increasing pH. In acidic systems all the competing ligands inhibited the sorption of As(III) more than As(V), but in alkaline environments As(III) and As(V) seem to be retained with the same strength on the Fe-oxide. Finally, the competing anions prevented As(III) and As(V) sorption more when added before than together or after As(III) or As(V).

The competitive sorption among Cu, Pb and Cr in ternary system on Na-montmorillonite at pH 3.5, 4.5 and 5.5 and at different heavy metal concentrations, and the effect of varying concentrations of Al, Fe, Ca and Mg on the sorption of heavy metals were studied. Competitive sorption of Cu, Pb and Cr in ternary system on montmorillonite followed the sequence of Cr ≫ Cu > Pb. Moreover, the competition was weakened by the increase of pH while was intensified by the increase of heavy metal concentration. The sorption of heavy metal on montmorillonite was inhibited by the presence of Ca and Mg, while Al and Fe showed different patterns in affecting heavy metal sorption. Aluminum and Fe generally inhibited the sorption of heavy metal when the pH and/or concentration of major elements were relatively low. However, promoting effects on heavy metal sorption by Al and Fe were found at relatively high pH and/or great concentration of major elements. The inhibition of major elements on heavy metal sorption generally followed the order of Al > Fe > Ca ≥ Mg, while Fe was more efficient than Al in promoting the sorption of heavy metals. These findings are of fundamental significance for evaluating the mobility of heavy metals in polluted environments.

In soil environments, sorption/desorption reactions as well as chemical complexation with inorganic and organic ligands and redox reactions, both biotic and abiotic, are of great importance in controlling their bioavailability, leaching and toxicity. These reactions are affected by many factors such as pH, nature of the sorbents, presence and concentration of organic and inorganic ligands, including humic and fulvic acid, root exudates, microbial metabolites and nutrients. In this review, we highlight the impact of physical, chemical, and biological interfacial interactions on bioavailability and mobility of metals and metalloids in soil. Special attention is devoted to: i) the sorption/desorption processes of metals and metalloids on/from soil components and soils; ii) their precipitation and reduction-oxidation reactions in solution and onto surfaces of soil components; iii) their chemical speciation, fractionation and bioavailability.

The role of phosphorus (P) application and arbuscular mycorrhizal fungi (AMF) on growth, arsenic (As) and P accumulation in lettuce plants growing in an As-polluted soil (total As 250 mg kg−1), was investigated. In particular, it was tested whether application of a commercial inoculum (CI), with (+P at 90 kg P ha−1) and without (−P at 0 kg P ha−1) P fertilizer, supported greater plant growth and provided more P, enhancing As tolerance, than indigenous fungi alone. The influence of these treatments on As and P availability in the rhizosphere and bulk soils was also investigated. Greenhouse pot experiments were established where plants were grown with and without commercial inoculum (+CI, −CI) in unsterilized conditions. Inoculation with commercial inoculum and P application together considerably increased plant biomass, by enhancing host plant P nutrition and lowering shoot and root As concentrations compared to plants inoculated only with native AMF. In the rhizosphere of +CI+P plants there was P soil depletion compared to −CI+P. The results evidenced that, with P addition, inoculation with commercial inoculum alleviated the toxicity of excessive As by improving P nutrition without increasing As concentrations in the plant, emphasizing the role of beneficial microbes and P fertilizer to improve soil fertility in As-contaminated soil.

In this study we have examinated the results of two experiments on the uptake and distribution of arsenic (As) in roots, shoots, and grain of wheat grown in As-polluted soils and in an unpolluted soil irrigated with As-contaminated water in absence or presence of phosphorus (P) fertilization. Arsenic concentrations in wheat samples of the two experiments are higher than those in the plants grown on uncontaminated soil. In the experiments showed in this work, it is highlighted the role of P fertilization in preventing As uptake and translocation in wheat plants. These findings could have important implications to reduce the potential risk posed to human health by As entering the food-chain.

In this study we have investigated the uptake and distribution of arsenic (As) and phosphate (Pi) in roots, shoots, and grain of wheat grown in an uncontaminated soil irrigated with solutions containing As at three different concentrations (0.5, 1 and 2 mg l−1) and in the presence or in the absence of P fertilization. Arsenic in irrigation water reduced plants growth and decreased grain yield. When Pi was not added (P−), plants were more greatly impacted compared to the plus Pi (P+) treatments. The differences in mean biomass between P− and P+ treatments at the higher As concentrations demonstrated the role of Pi in preventing As toxicity and growth inhibition. Arsenic concentrations in root, shoot and grain increased with increasing As concentration in irrigation water. It appears that P fertilization minimizes the translocation of As to the shoots and grain whilst enhancing P status of plant. The observation that P fertilization minimises the translocation of arsenic to the shoots and grain is interesting and may be useful for certain regions of the world that has high levels of As in groundwater or soils.

We have studied: (i) the sorption of arsenate on Mg-Al layered double hydroxides (LDHs) containing chloride (LDH-Cl) or carbonate (LDH-CO(3)) in the absence or presence of phosphate; (ii) the competitive sorption of arsenate and phosphate as affected by reaction time and pH and; (iii) the desorption of arsenate previously sorbed on the LDH by phosphate. The LDH samples were uncalcined (LDH-Cl-20 and LDH-CO(3)-20) or calcined at 450 degrees C (LDH-Cl-450 and LDH-CO(3)-450). More phosphate than arsenate was sorbed onto all the minerals but LDH-Cl-450 sorbed much lower amounts of both the ligands than LDH-Cl-20; vice versa LDH-CO(3)-450 showed a capacity to sorb arsenate and phosphate much greater than LDH-CO(3)-20. XRD analysis showed that arsenate was included into the layer spaces of LDH-Cl-20, but not in those of LDH-CO(3)-20. Competition in sorption between arsenate and phosphate was affected by pH, reaction time, surface coverage and sequence of addition of the ligands. Phosphate showed a greater affinity for LDHs than arsenate. The final arsenate sorbed/phosphate sorbed molar ratio increased with reaction time or by adding arsenate before phosphate, but decreased by increasing pH and by adding phosphate before arsenate. The effect of reaction time on the desorption of arsenate by phosphate was also studied.

Coprecipitation involving arsenic with aluminum or iron has been studied because this technique is considered particularly efficient for removal of this toxic element from polluted waters. Coprecipitation of arsenic with mixed iron-aluminum solutions has received scant attention. In this work we studied (i) the mineralogy, surface properties,and chemical composition of mixed iron-aluminum oxides formed at initial Fe/Al molar ratio of 1.0 in the absence or presence of arsenate [As/Fe+Al molar ratio (R) of 0, 0.01, or 0.1] and at pH 4.0, 7.0, and 10.0 and aged for 30 and 210 days at 50 degrees C and (ii) the removal of arsenate from the coprecipitates after addition of phosphate. The amounts of short-range ordered precipitates (ferrihydrite, aluminous ferrihydrite and/or poorly crystalline boehmite) were greater than those found in iron and aluminum systems (studied in previous works), due to the capacity of both aluminum and arsenate to retard or inhibit the transformation of the initially formed precipitates into well-crystallized oxides (gibbsite, bayerite, and hematite). As a consequence, the surface areas of the iron-aluminum oxides formed in the absence or presence of arsenate were usually much larger than those of aluminum or iron oxides formed under the same conditions. Arsenate was found to be associated mainly into short-range ordered materials. Chemical composition of all samples was affected by pH, initial R, and aging. Phosphate sorption was facilitated by the presence of short-range ordered materials, mainly those richer in aluminum, but was inhibited by arsenate present in the samples. The quantities of arsenate replaced by phosphate, expressed as percentages of its total amount present in the samples, were particularly low, ranging from 10% to 26%. A comparison of the desorption of arsenate by phosphate from aluminum-arsenate and iron-arsenate (studied in previous works) and iron-aluminum-arsenate coprecipitates evidenced that phosphate has a greater capacity to desorb arsenate from aluminum than iron sites.

A study was conducted to determine the solid phase distribution of copper in representative soils of Italy with wide differences in the chemical and physicochemical properties. Sites selected varie...

Arsenic mobilization in soils is mainly controlled by sorption/desorption processes, but arsenic also may be coprecipitated with aluminum and/or iron in natural environments. Although coprecipitation of arsenic with aluminum and iron oxides is an effective treatment process for arsenic removal from drinking water, the nature and reactivity of aluminum- or iron-arsenic coprecipitates has received little attention. We studied the mineralogy, chemical composition, and surface properties of aluminum-arsenate coprecipitates, as well as the sorption of phosphate on and the loss of arsenate from these precipitates. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0, or 10.0 and As/Al molar ratio (R) of 0, 0.01, or 0.1 and were aged 30 or 210 d at 50 degrees C. In the absence of arsenate, gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate, very poorly crystalline precipitates formed. Short-range ordered materials (mainly poorly crystalline boehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0, and 10.0 (R= 0.1) and did not transform into Al(OH)3 polymorphs even after prolonged aging. The surface properties and chemical composition of the aluminum precipitates were affected by the initial pH, R, and aging. Chemical dissolution of the samples by 6 mol L(-1) HCl and 0.2 mol L(-1) oxalic acid/ oxalate solution indicated that arsenate was present mainly in the short-range ordered precipitates. The sorption of phosphate onto the precipitates was influenced by the nature of the samples and the amounts of arsenate present in the precipitates. Large amounts of phosphate partially replaced arsenate only from the samples formed at R = 0.1. The quantities of arsenate desorbed from these coprecipitates by phosphate increased with increasing phosphate concentration, reaction time, and precipitate age butwere always lessthan 30% of the amounts of arsenate present in the materials and were particularly low (4%) from the sample prepared at pH 4.0. Arsenate appeared to be occluded within the network of short-range ordered materials and/or sorbed onto the external surfaces of the precipitates, but sorption on the external surfaces seemed to increase by increasing pH of sample preparation and aging. Furthermore, at pH 4.0 more than in neutral or alkaline systems the formation of aluminum arsenate precipitates seemed to be favored. Finally, we have observed that greater amounts of phosphate were sorbed on an aluminum-arsenate coprecipitate than on a preformed aluminum oxide equilibrated with arsenate under the same conditions (R = 0.1, pH 7.0). In contrast, the opposite occurred for arsenate desorption, which was attributed to the larger amounts of arsenate occluded in the coprecipitate.

A study on the individual sorption of Cr(III), Cu, and Zn on polymerin, the humic-acid-like fraction of olive mill wastewater, and its derivatives, K-polymerin and an Fe(OH)x-polymerin complex, showed that these heavy metals were strongly sorbed on polymerin and K-polymerin in the order Cr(III)CuZn. The sorption on Fe(OH)x-polymerin was to a lower extent compared with that of the other two sorbents, but to a higher extent compared with ferrihydride [Fe(OH)x]. Combined atomic absorption spectrometry and diffuse reflectance infrared Fourier transform spectroscopy analyses showed that the selected heavy metals were individually sorbed on polymerin by means of a cation exchange mechanism, which was consistent with the replacement of Ca, Mg, K, and H bound to the carboxylate groups of the biosorbent and the concomitant chelation of the heavy metals by the OH groups of polymerin polysaccharide component. In binary combination and equimolar ratio, Cu was sorbed by polymerin more selectively than Zn. In ternary combination and equimolar ratio, Cr(III), Cu, and Zn were sorbed by polymerin in the order Cr(III)CuZn. The sorbing capacity of Zn and Cu was strongly influenced by Cr(III), whereas the sorbing capacity of Cr(III) was not affected bythe presence of the other two metals. The overall sorbing capacity of the binary and ternary mixtures of the three metals on polymerin proved to be considerable and much greater than that on Fe(OH)x-polymerin. Simulated wastewaters contaminated with Cu and Zn were purified after three sorption cycles by polymerin renewed at each cycle, whereas those containing a mixture of Cr(III), Cu, and Zn showed residues of Zn after five cycles. We briefly discuss environmental and industrial advantages for a possible exploitation of polymerin.

The nature and morphology of Al(OH) 3 polymorphs obtained by precipitating Al and tartrate at pH 12.0 and at a tartrate/Al molar ratio ( R ) of 0.1 were studied by X-ray diffraction, transmission electron microscopy and electron diffraction. Co-crystallization of unusually long (up to 18 μm) ‘stick-shaped’ gibbsite with platy particles of both nordstrandite, mainly, and bayerite was observed. Face-to-face associations of plates nucleated perpendicularly on the gibbsite surfaces parallel to the c axis produced peculiar elongated ‘brush’-shaped particles. In a strongly alkaline environment the inhibiting effect of tartrate on Al(OH) 3 polymorph formation is reduced and production of co-crystallized phases with structural crystallographic similarities can be observed.

A number of iron oxides (hematite, goethite, lepidocrocite, maghemite, and magnetite) or short‐range ordered precipitates (ferrihydrite) may be found in soil environments, but in the rhizosphere the presence of organic ligands released by plants (exudates) or microorganisms promote the formation of ferrihydrite. Iron ions are liberated into soil solution by acidic weathering of minerals and then precipitated either locally or after translocation in soil environments. Humic and fulvic acids as well as organic substances produced by plants and microorganisms are involved in the weathering of primary minerals. Organic compounds play a very important role in the hydrolytic reactions of iron and on the formation, nature, surface properties, reactivity, and transformation of Fe oxides. Organic substances present in the rhizosphere interact with Fe promoting the formation of ferrihydrite and organo‐mineral complexes. The solubility of Fe precipitation products is usually low. However, the formation of s...

Metal oxides, noncrystalline or short-range ordered iron and aluminum hydroxides, poorly crystalline aluminosilicates, which are found within a wide range of soil orders, as well as organo-mineral complexes, are responsible for phosphate retention in soil environments. Strongly chelating organic acids produced by microorganisms or by plants (i.e., root exudates), as well as humic and fulvic acids, may strongly influence the adsorption of phosphate and its availability for plants. Maximum reduction in phosphate adsorption occurs when organic ligands are previously adsorbed on variable charge minerals or soils. The competitive adsorption of phosphate and organic ligands (e.g., oxalate, tartrate, malate, citrate) is influenced by pH, the nature of the ligands, and the nature of the surfaces of clay minerals and soils. Organic ligands may coprecipitate with OH-Al or OH-Fe species, forming organo-mineral complexes, which differ in chemical composition, surface properties, and reactivity toward phosphate. Nutrients and pollutants also compete with phosphate for the sorption sites of soil constituents. Sulfate inhibits phosphate adsorption or is not completely removed from the surfaces on which it is previously adsorbed only at low pH values. However, sulfate present in hydroxy-Al sulfate complexes is only partially removed even by large amounts of phosphate. Arsenate strongly competes with phosphate, but its efficiency in inhibiting phosphate adsorption is influenced by pH, concentration, order of anion addition, and nature of the surface of clay minerals and soils.