Massimo Pigna

Sorption/desorption of arsenate on/from Mg–Al layered double hydroxides: Influence of phosphate

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.

Sorption of arsenite and arsenate on ferrihydrite: Effect of organic and inorganic ligands

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).

Adsorption of heavy metals on mixed Fe-Al oxides in the absence or presence of organic ligands

A study was carried out on the adsorption of Co2+, Cu2+, Pb2+, and Zn2+ ions on mixed Fe-Al oxides inthe absence or presence of increasing concentrations of oxalate or tartrate. Mixed Fe-Al oxides were prepared by precipitating at pH 5.5 mixtures of Fe and Al ions at initial Fe/Al molar ratios (R) of 0, 1, 2, 4, 10 and ∞ (R0, R1, R2, R4, R10 and R∞).The oxides aged 7 days at 20 °C or 30 days at 50 °C showed different chemical composition and physico-chemical and mineralogical properties. All the mixed Fe-Al oxides showed presence of poorly crystalline materials (ferrihydrite) even after prolonged aging. The heavy metals wereselectively adsorbed on the oxides. For all the precipitates aged7 days at 20 °C, the selectivity sequence wasPb2+> Cu2+ > Zn2+ > Co2+, but the pH at which 50% ofeach cation was adsorbed (pH50) was different from sample tosample. It was found that usually the greater the amounts of Fe in Fe-Al gels the lower the pH50 for each metal, but the adsorption of a heavy metal was not linearly related to Fe content. The pH50 usually did not change significantly when the oxides were aged 30 days at 50 °C. Competitive adsorption of Cu and Zn on ferrihydrite (R∞) showed thatCu strongly prevented Zn adsorption even at an initial Zn/Cu molar ratio of 8, whereas Cu sorption was not inhibited. In thepresence of oxalate (OX) or tartrate (TR) (organic ligand/Pb molar ratio (rL) from 0 to 7) the quantities of Pb adsorbedon the Fe-Al oxides usually increased with increasing rL. The adsorption increase of Pb was particularly high on the oxidesricher in Fe (R4-R∞), but a significant increase was also observed on R0-R2 samples. The adsorption of Pb on the oxides hasbeen influenced not only by the presence and concentration of organic ligands but also by the sequence of addition of Pb and tartrate on the sorbents. It has been ascertained that on each oxide the greater amounts of Pb were adsorbed when tartrate wasadded before Pb and usually according to the following sequence: Tr before Pb > Pb before Tr > Pb + Tr > Pb.

Total arsenic, inorganic arsenic, and other elements concentrations in Italian rice grain varies with origin and type.

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.

EFFECTS OF PHOSPHORUS FERTILIZATION ON ARSENIC UPTAKE BY WHEAT GROWN IN POLLUTED SOILS

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.

Sorption of Cu, Pb and Cr on Na-montmorillonite: competition and effect of major elements.

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.

Effect of inorganic and organic ligands on the sorption/desorption of arsenate on/from Al-Mg and Fe-Mg layered double hydroxides.

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.

Factors Affecting the Formation, Nature, and Properties of Iron Precipitation Products at the Soil–Root Interface

Abstract 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 soluble complexes of Fe(II) or Fe(III) with organic ligands, usually present in the rhizosphere increases the solubility of Fe‐oxides. Mobilization of Fe from Fe oxides by siderophores is of great importance in natural systems. They can form stable Fe(III) complexes (pK up to 32) and thus mobilize Fe from Fe(III) compounds. These higher Fe concentrations are important for the supply of Fe to plant roots which excrete organic acids at the soil–root interface. Iron oxides adsorb a wide variety of organic and inorganic anions and cations, which include natural organics, nutrients, and xenobiotics. There is competition between anions and cations for the surfaces of Fe‐oxides. Root exudates suppress phosphate or sulfate adsorption on Fe‐oxides. This is a mechanism by which plant roots mobilize adsorbed phosphate and improve their phosphate supply. Anions adsorption on iron oxides modify their dispersion/flocculation behavior and thus their mobility in the soil system. That can increase or decrease the possibility of contact between Fe‐oxides and organics or organisms able to dissolve them.

Effect of particle size of drinking-water treatment residuals on the sorption of arsenic in the presence of competing ions

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<citrate<phosphate. It was also a function of As ion residence time on the WTR surfaces: the longer the residence time, the less effective were the competing ligands in As desorption.

Immobilization of acid phosphatase on uncalcined and calcined Mg/Al-CO3 layered double hydroxides

Acid phosphatase was immobilized on layered double hydroxides of uncalcined- and calcined-Mg/Al-CO(3) (Unc-LDH-CO(3), C-LDH-CO(3)) by the means of direct adsorption. Optimal pH and temperature for the activity of free and immobilized enzyme were exhibited at pH 5.5 and 37 degrees C. The Michaelis constant (K(m)) for free enzyme was 1.09 mmol mL(-1) while that for immobilized enzyme on Unc-LDH-CO(3) and C-LDH-CO(3) was increased to 1.22 and 1.19 mmol mL(-1), respectively, indicating the decreased affinity of substrate for immobilized enzymes. The residual activity of immobilized enzyme on Unc-LDH-CO(3) and C-LDH-CO(3) at optimal pH and temperature was 80% and 88%, respectively, suggesting that only little activity was lost during immobilization. The deactivation energy (E(d)) for free and immobilized enzyme on Unc-LDH-CO(3) and C-LDH-CO(3) was 65.44, 35.24 and 40.66 kJ mol(-1), respectively, indicating the improving of thermal stability of acid phosphatase after the immobilization on LDH-CO(3) especially the uncalcined form. Both chemical assays and isothermal titration calorimetry (ITC) observations implied that hydrolytic stability of acid phosphatase was promoted significantly after the immobilization on LDH-CO(3) especially the calcined form. Reusability investigation showed that more than 60% of the initial activity was remained after six reuses of immobilized enzyme on Unc-LDH-CO(3) and C-LDH-CO(3). A half-life (t(1/2)) of 10 days was calculated for free enzyme, 55 and 79 days for the immobilized enzyme on Unc-LDH-CO(3) and C-LDH-CO(3) when stored at 4 degrees C. Therefore, immobilization of acid phosphatase on Unc-LDH-CO(3) and C-LDH-CO(3) by direct adsorption is an effective means and would have promising potential for the practical application in agricultural production and environmental remediation.