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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 sot-bed 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 arserate 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. (C) 2009 Elsevier Inc. All rights reserved.

This review provides insights on (1) the chemistry of arsenic (As) in the soil environment and factors (e.g., pH, presence, nature, and concentration of competing inorganic and organic ligands), which regulate the sorption/desorption of arsenate and arsenite (the most abundant As species in soils and waters) on/from soil components; (2) the chemical fractionation and speciation of As to identify species that are more mobile and phytoavailable; and finally (3) the uptake of As by selected edible plants and production techniques able to reduce its translocation in plant tissues.

The sorption of Cu and Pb added alone or in mixture in the absence or presence of low-molecular mass aliphatic acids (LMMAAs): oxalic, citric or glutamic acid onto a synthetic goethite at different pH values (from 3.0 to 5.5) was studied. Copper showed a higher affinity for goethite than lead both in absence or presence of LMMAAs. By increasing the initial organic ligand/metal molar ratio from 0 to 10, the adsorption of both the metals on the iron oxide initially increased and then decreased or remained constant. In the presence of oxalic acid the adsorption of Cu on the surfaces of goethite increased by increasing ratio up to 2–4 and then decreased. Citric acid showed a similar effect but the amounts of Cu fixed were lower. Glutamic acid had the lowest effect on the adsorption of this element onto goethite. Adsorption of Pb was strongly affected by citric acid. The amounts of Pb fixed onto the surfaces of goethite increased up to molar ratio 4 and then slightly decreased. On the contrary, oxalic and glut...

The competitive sorption of Cu, Cr and Pb on a ferrihydrite and two synthetic organomineral complexes, obtained by coprecipitating iron with different quantities of oxalate and tartrate were studied. Greater amounts (μmol/m2) of heavy metals were sorbed onto the organomineral complexes than ferrihydrite, the higher the carbon content in the samples the greater the sorption of heavy metals. Lead was sorbed much more and more tightly than Cr and Cu on the sorbents. In binary systems, the sorption and the inhibitive efficiency of one metal (M1) to the other (M2) were affected by the nature of the metal, initial M2/M1 molar ratio, addition sequence of M1 and M2, and residence time. Lead showed a much greater capacity to inhibit the sorption of the other two metals, whereas the opposite was true for Cu. In ternary systems, when equimolar amounts of each metal were added, the sorbents were covered with more than 50% by Pb alone. However, even in the presence of large excess of Pb and Cr, considerable extent of Cu sorption still occurred regardless of sorbent types, indicating that some sorption sites on each sorbent had a high affinity for Cu. The results of this study would contribute to further our current understanding on the mobility of multi heavy metals in soils and related environments.

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...

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.

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.

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.

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.