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The sulfate sorption was studied on Andisols with variable mineralogical composition and low organic matter content in presence and absence of phosphate or malate ligands at different pH. Crystalline clay mineralogy was similar for all the studied samples, except for the 2C horizon of the pedon 2, which did not show crystalline minerals. The soil samples showed content of allophane ranging from 16% to 42%. The sulfate adsorption decreased when descending the first 3 horizons (A1, 2A2 and 3Bw1) of the pedon 1. This behavior was attributed to the decreasing organic carbon content. At different pH sulfate sorption was drastically decreased by increasing the pH from 4.0 to 7.0, precisely by 80% for sample 5 to 100% for sample 1. Experiments on the competitive sorption of phosphate and sulfate on volcanic soils were carried out. Sulfate sorption was reduced even in the presence of low concentrations of phosphate ions (phosphate/sulfate molar ratio << 1). Sulfate sorption was influenced by many factors as the organic carbon content, pH, the mineralogical composition of soil samples and the order of sulfate and phosphate addition into the soils. Sulfate competes with phosphate particularly when added before phosphate and at low pH values (pH < 4.5). Phosphate more than malate affected the kinetics of sulfate sorption onto a volcanic soil containing a large amount of allophanic materials La adsorción de sulfato fue estudiada en Andisoles con una composición mineralógica variable y bajo contenido de materia orgánica en presencia y ausencia de fosfato o malato a diferentes pH. La mineralogía de las arcillas cristalinas fue similar para todos las muestras estudiadas, excepto para el horizonte 2C del pedon 2, la cual no mostró minerales cristalinos. Las muestras de suelo presentaron contenidos de alofán en el rango de 16 a 42 %. La adsorción de sulfato incrementó al descender los primeros 3 horizontes (A1, 2A2 and 3Bw1) del pedon 1. Este comportamiento fue atribuído a la disminución del contenido de carbono orgánico. A diferentes pH la adsorción de sulfato fue drásticamente disminuída el aumento del pH desde 4,0 a 7,0, desde un 80 % para la muestra 5 a 100 % para la muestra 1. La adsorción de sulfato fue reducida incluso en presencia de bajas concentraciones de iones fosfato (razón molar fosfato/sulfato 1). Experimentos de adsorción competitiva de fosfato y sulfatos fueron realizado en suelos volcánicos. La adsorción de sulfato fue influenciada por muchos factores, como el contenido de materia orgánica, pH, composición mineralógica de los suelos y el orden de adición de sulfato y fosfato en los suelos. Sulfato compite con fosfato particularmente cuando es adicionado antes que fosfato y a bajos valores de pH (pH 4,4). Fosfato más que malato afectó la cinética de la adsorción de sulfato en un suelo volcánico conteniendo una gran cantidad de materiales alofánicos

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.

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.

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 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.

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.

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.

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.