Role of root exudates on the sorption of arsenate by ferrihydrite

Iron oxy‐hydroxides in soil are known to have a large affinity for arsenate (As(V)) inorganic species. At the soil–root interface such mineral components are embedded by mucilaginous material that is secreted from continuously growing root cap cells. In order to determine the role of plant mucilages in As(V) sorption by iron oxy‐hydroxides, we layered a calcium (Ca)‐polygalacturonate network (CaPGA) on to amorphous iron (Fe) (III) hydroxide (ferrihydrite, Fh) particles. The scanning electron micrographs of the CaPGA network coating the ferrihydrite (Fh–CaPGA) show a regular structure with a honeycomb‐like pattern where interlacing fibrils form a porous system. The FT‐IR spectra of Fh–CaPGA suggest that CaPGA fibrils are retained by the surface Fe(III) nuclei of Fh through electrostatic interactions. The sorption experiments carried out at pH 4.3 and 5.8 indicated a smaller amount of As(V) sorbed by Fh–CaPGA than by Fh alone, being less after 3 and 24 hours of reaction by about 70 and 30%, respectively. The sorption of As(V) by Fh was also studied in the presence of caffeic acid (CAF), an important root exudate. Simultaneous sorption kinetics show that As(V) sorption by Fh is almost independent of CAF concentration, indicating a greater affinity of arsenate ions towards the Fh surfaces. However, the amount of As(V) sorbed by the Fh coated by CaPGA, in the presence of 0.25, 0.5 and 1.0 mm CAF, is markedly smaller by about 20, 27 and 40%, respectively, than that found in the As(V)–CAF‐Fh ternary systems. This is caused mainly by redox reactions involving CAF and the surface Fe(III) nuclei of Fh leading to the formation of CAF oxidation products which prevent As(V) sorption.     

Cd(II)与As(V)在一株土壤细菌Delftia sp.上的共吸附研究

在土壤中，重金属（如Cd、Pb）和类金属（As、Sb）的复合污染非常复杂，而目前，还未有研究关注Cd、As在土壤微生物界面的共吸附行为。本研究利用宏观批吸附实验、电位滴定及光谱学手段探讨Cd(II)-As(V)在一株土壤细菌（Delftia sp.）表面的共吸附特点和机制。结果表明：Delftia sp.细菌表面富含羧基、磷酰基、氨基和羟基等官能团，解离常数（pKa）分别为4.57、7.04、8.22和10.45；当pH<5时，Cd(II)在复合体系的吸附量大于单一体系，pH>5时，Cd(II)在复合和单一体系吸附量差异不大；在pH 2–9范围内，As(V)在复合体系的吸附量都小于单一体系；细菌表面的羧基和少量氨基参与了Cd、As在细菌表面的络合反应，且Cd、As会在这些位点发生竞争作用。本研究证实，土壤pH是影响Cd、As在微生物表面共吸附的重要因素.     

Effect of Cadmium(Ii) and Anion Type on the Ageing of Ferrihydrite and its Subsequent Leaching under Neutral and Alkaline Conditions

The effect of cadmium(II) on the transformation of ferrihydrite[with Cd(II):Fe(III) ratios ranging from 0 to 5 mole %] in neutral and alkaline media (pH 7-11), combined with the effects of electrolyte type (NO₃ ^-, Cl^-, and SO₄ ^-2), was investigated at 20 °C over a period of 1 yr. The presence of Cd(II) strongly retards the conversion of ferrihydrite into hematite and/or goethite at pH 7–10, with decreases in the rate of transformation dependent on the amountof Cd(II). At a Cd(II):Fe(III) mole ratio of 1%, the transformation rate is NO₃ ^- > Cl^- > SO₄ ^-2, which correlates with the relative affinitiesof the anions for the ferrihydrite surface. The presence of Cd(II) promotes hematite formation at pH 9 and 10, whereas atpH 11 goethite is almost the sole product. With increasinginitial Cd(II) concentrations, increasing incorporationof Cd(II) into the products is observed. For 5 mole %Cd(II), ～ 2.5 mole % of Cd(II) is included in thetransformation products, principally hematite, while at pH 11, with 1 mole % Cd(II), all of the Cd(II) incorporates into thegoethite lattice. Transmission electron micrographs show that the presence of Cd(II) leads to a reduction in size and promotesthe twinning of goethite crystals, and can result in ellipsoidal-shaped hematite crystals. Leachability of Cd(II) fromfresh and aged coprecipitated Cd(II)-ferrihyrdite is dependent onthe extent of transformation of the ferrihydrite, with 70–90% of the Cd(II) leachable from ferrihydrite, while goethite is ableto incorporate and remove more Cd(II) than hematite.     

Heavy metal sorption on soil minerals affected by the siderophore desferrioxamine B: the role of Fe(III) (hydr)oxides and dissolved Fe(III)

Phytoextraction of heavy metals from polluted soils has often been found to be limited by the bioavailability of the pollutants. Inorganic or organic ligands are occasionally used as complexing agents to enhance the mobility of the heavy metals. However, the opposite effect is also possible. We studied the influence of the hydroxamate siderophore desferrioxamine B (DFOB) on the sorption of Cu, Zn and Cd to clay minerals, with the emphasis on the role of dissolved Fe(III) and Fe(III) minerals. Depending on the surface charge of the minerals and on pH, sorption of heavy metals can be either enhanced or diminished. We show here that this effect of DFOB disappears if dissolved Fe(III) is added to suspensions of clay minerals in excess to DFOB. We found that the solid Fe(III) phases ferrihydrite and goethite did not impede the effect of DFOB on the sorption of heavy metal, however. Between pH 4 and 10, DFOB completely prevented Cu sorption on ferrihydrite. A strong mobilizing effect was also observed for Zn, but not for Cd. In presence of goethite, concentrations of dissolved Cu, Zn and Cd were enhanced only above approximately pH 5, 7 and 8, respectively. Below these pH values the binding of these metals to goethite was even stronger with than without DFOB. In the absence of heavy metals, DFOB‐promoted dissolution of ferrihydrite was much faster than that of goethite due to the larger surface area of ferrihydrite. In the alkaline pH range, where sorption of DFOB on the surfaces of the iron oxides was greater, dissolution of both minerals was reduced.     

Risk assessment,spatial distribution,and source identification of heavy metal(loid)s in paddy soils along the Zijiang River basin,in Hunan Province,China

Purpose

This study aimed to reveal spatial distribution of As, Cd, Cr, Cu, Mn, Ni, Pb, Sb, V, and Zn in paddy soils in the Zijiang River basin and to evaluate its pollution status and potential ecological risks, and thus to provide basic information for rational utilization of paddy soils in the study area.

Materials and methods

The heavy metal(loid) concentrations in one hundred and thirty-five paddy soil samples (these samples were collected from the top 0–20 cm layer) were measured by inductively coupled plasma-optical emission spectrometry. The spatial distribution characteristics of the heavy metal(loid)s were depicted by the Ordinary Kriging interpolation analysis. The contamination degree and potential ecological risks of the heavy metal(loid)s in paddy soils were assessed by Nemerow’s comprehensive index, geoaccumulation index, potential ecological risk factor, and potential ecological risk index. The potential sources of the heavy metal(loid)s were deduced by Pearson’s correlation analysis, hierarchical cluster analysis, and principal component analysis.

Results and discussion

The mean concentrations of the heavy metal(loid)s decreased in the order of Mn?>?V?≈?Zn?>?Cr?>?Ni?≈?Pb?>?Cu?≈?Sb?>?As?>?Cd. Except for Cd and Sb, the mean concentrations of As, Cr, Cu, Mn, Ni, Pb, V, and Zn were close to the background reference values. The concentration of Cd in 94.8% of samples exceeded the soil quality standard value (grade II, 5.5?<?pH?<?6.5, GB 15618–1995). According to the assessments of pollution and potential ecological risks for the heavy metal(loid)s, 45.2% and 46.7% of samples were severely polluted and moderately polluted, respectively. The potential sources analysis indicated that Cd, Sb, and Zn mainly originated from agricultural, mining, and smelting activities; As, Cu, and Pb mainly originated from agricultural activities, while coal combustion by-products was another major source of these heavy metal(loid)s in paddy soils near the thermal power plant in the southwest corner of the study area; Cr, V, Mn, and Ni mainly originated from natural source.

Conclusions

Cadmium and Sb are the main contaminants in paddy soils in the study area, and there are hot-spot pollution areas.

     

Competitive sorption of copper,nickel, and zinc by an Oxisol

Abstract

The competitive sorption of copper (Cu), nickel (Ni), and zinc (Zn) in an Oxisol suspension was investigated using the Sheindrof‐Rebhun‐Sheituch (SRS) equation. Experimental data were obtained for a system containing these cations as single components, binary, and ternary mixtures. The associated competitive coefficients a_ij can be viewed as a way to quantify competitive interaction. The competition coefficients obtained for the cations indicated that the sorption of Ni and Zn by the soil was significantly inhibited by the presence of Cu, while the inhibition exerted in the reverse situation was milder. After a _ij ‐values were substituted in the SRS equation, the sorption values predicated by this equation for the ternary solute mixture Cu‐Ni‐Zn, were compared to values determined experimentally. The SRS equation modeled sorption successfully for the range of solute concentrations used.     

Comparative Assessment of Al(III) and Cd(II) Biosorption onto Turbinaria conoides in Single and Binary Systems

The present work investigated the ability of inactive brown seaweed, Turbinaria conoides, to biosorb aluminum(III) and cadmium(II) ions in both single and binary systems. Initial experiments were undertaken to determine the influence of pH and biosorption isotherms of each metallic ion. Owing to the presence of carboxylic groups, T. conoides exhibited high uptake capacity towards Al(III) and Cd(II) through ion-exchange mechanism. In the case of Al(III), T. conoides exhibited maximum biosorption at pH 4 with a capacity of 2.37?mmol/g, whereas the highest Cd(II) biosorption occurred at pH 5 with a capacity of 0.96?mmol/g. For both metal ions, T. conoides exhibited fast kinetics. Several models were used to describe isotherm (Langmuir, Freundlich, Redlich-Peterson, and Toth) and kinetic (pseudo-first and pseudo-second order) data. Desorption and reuse of T. conoides biomass in three repeated cycles was successful with 0.1?M HCl as elutant. In binary systems, the presence of Cd(II) severely affected Al(III) uptake by T. conoides. Compared to single-solute systems, Al(III) uptake was reduced to 56% compared to only 27% for Cd(II). Based on the model parameters regressed from the respective monometal systems, multicomponent Langmuir and Freundlich models were used to predict binary (Al + Cd) system of which the multicomponent Freundlich model was able to describe with good accuracy.     

Sorption behavior of selenium and antimony in soils as a function of phosphate ion concentration

Abstract

Both selenium (Se) and antimony (Sb) are major soil and water pollutants. Their sorption behavior in a soil–plant system was studied. Soil–soil solution distribution coefficients (K _ds) for Se and Sb were measured, using a radiotracer, as an indicator of their sorption levels. Both Se and Sb behave as oxoanions (SeO^2? ₄, H₂PO^? ₄ and SO^2? ₄) in soil; thus, the effects of concentrations of two major oxoanions (SeO^2? ₄ and SeO^2? ₃) on Se and Sb sorption were also examined. The K _d values for Se for Japanese soils significantly correlated with the K _d values for Sb (n = 141). The K _ds of both Se and Sb similarly decreased with increasing SbO^? ₃ concentration. These results indicated that the sorption of Se and Sb was similarly controlled by a ligand-exchange mechanism such as phosphate sorption in soil. However, an increase in the concentration of SeO^2? ₃ did not decrease the K _ds of Se and Sb. Furthermore, the ligand-exchangeable fractions of stable Se and Sb in major Japanese soils were determined by extraction with 0.1 mol L^?1 Na₂HPO₄ solution. For both Se and Sb, the phosphate-extractable fractions were 10-fold higher for Se and fivefold higher for Sb than their water-soluble fractions. Although the total Se and Sb amounts in soils were the same, their ligand-exchangeable fractions were different. Approximately 0.9–12% of total Se and 0.2–1.3% of total Sb were extracted by the phosphate solution. These findings suggested that Se was more likely to be mobilized by the addition of phosphate than Sb. The effect of plant-available phosphate in the soil and the phosphate sorption capacity of soil on Se and Sb availabilities for plants were also examined using a pot experiment with soybean plants. The experimental results suggested that a high content of available phosphate and/or low phosphate sorption capacity of soil increased both Se and Sb availabilities to the plant. However, the results also suggested that the soil Se availability to the plant was higher than that of Sb even though the soil total Se and Sb amounts were the same.     

Cadmium sorption behavior of natural and synthetic zeolites

Abstract

The sorptive properties of zeolites (a natural zeolite and 3 synthetic zeolites) for cadmium (Cd) were investigated with the intention of selecting suitable materials, for use as amendments in contaminated soils, to reduce the uptake of Cd by field crops. The Cd sorption data were well described by the Langmuir and the Freundlich equations and the coefficients values indicated significant sorptive capacities for Cd by these minerals. Synthetic zeolites have 100 to 500 times greater sorption capacities than the natural zeolite. In all cases, sorption increased with increasing pH and this relationship was linear over the pH range between 3 and 10 for the natural zeolite whereas for the synthetic zeolites a sharp increase was observed between pH 3 and 6. The critical pH above which maximum Cd sorption occurred on synthetic zeolites coincided with the pH where mineral dissolution ceases. The natural zeolite showed an inverse relationship between particle size (<0.15–2.0 mm) and Cd sorption. In desorption experiments with natural zeolite, 10 sequential extractions with 0.01 MNaC10₄ removed 19% Cd from the lowest sorption treatment and 44% of the metal from the highest level, showing that Cd was bound more strongly with decreasing concentrations. Desorption was greater with 0.01 MKC10₄than 0.01 MNaC10₄ especially in the first 5 extractions.     

Iron speciation in soils and soil aggregates by synchrotron-based X-ray microspectroscopy (XANES,μ-XANES)

Iron speciation in soils is still poorly understood. We have investigated inorganic and organic standard substances, diluted mixtures of common Fe minerals in soils (pyrite, ferrihydrite, goethite), soils in a forested watershed which constitute a toposequence with a hydrological gradient (Dystric Cambisol, Dystric Planosol, Rheic Histosol), and microsites of a dissected soil aggregate by X‐ray Absorption Near Edge Spectroscopy (XANES) at the iron K‐edge (7112 eV) to identify different Fe(II) and Fe(III) components. We calculated the pre‐edge peak centroid energy of all spectra and quantified the contribution of different organic and inorganic Fe‐bearing compounds by Linear Combination Fitting (LCF) conducted on the entire spectrum (E = 7085–7240 eV) and on the pre‐edge peak. Fe‐XANES conducted on organic and inorganic standards and on synthetic mixtures of pyrite, ferrihydrite and goethite showed that by calculating the pre‐edge peak centroid energy, the Fe(II)/Fe(III) ratio of different Fe‐bearing minerals (Fe sulphides, Fe oxyhydroxides) in mineral mixtures and soils can be quantified with reasonable accuracy. A more accurate quantification of the Fe(II)/Fe(III) ratio was possible with LCF conducted on the entire XANES spectrum. For the soil toposequence, an increased groundwater influence from the Cambisol to the Histosol was reflected in a larger contribution of Fe(II) compounds (Fe(II) silicate, Fe monosulphide, pyrite) and a smaller contribution of Fe(III) oxyhydroxides (ferrihydrite, goethite) to total iron both in the topsoil and the subsoil. In the organic topsoils, organically bonded Fe (33–45% of total Fe) was 100% Fe(III). For different microsites in the dissected aggregate, spatial resolution ofμ‐XANES revealed different proportions of Fe(II) and Fe(III) compounds. Fe K‐edge XANES andμ‐XANES allows an approximate quantification of Fe(II) and Fe(III) and different Fe compounds in soils and (sub)micron regions of soil sections, such as mottles, concretions, and rhizosphere regions, thus opening new perspectives in soil research.     

Sorption isotherms and kinetics of Sb(V) on several Chinese soils with different physicochemical properties

Purpose

Sorption of antimony on soils is the primary factor that influences its immobilization and migration in the environment. In the present study, the sorption of Sb(V) onto seven Chinese soils with different physicochemical properties was investigated for exploring the relationship between the sorption capacity of Sb(V) and the physicochemical properties of the soils.

Materials and methods

Sorption isotherms and kinetics experiments were performed to ascertain the sorption capacity and the kinetic rate, respectively. The relationship between the sorption capacity of Sb(V) and the physicochemical properties of the soils was analyzed by multiple linear regressions.

Results and discussion

The results showed that the sorption isotherms fitted with both the Langmuir and Freundlich equations very well (R ²?=?0.936–0.997), and the sorption kinetic of Sb(V) onto the seven Chinese soils followed a pseudo-second-order reaction. The maximum sorption capacity of Sb(V) on the soils ranged from 134 to 1,333 mg?kg^?1. Nearly 94 % of the variability in maximum sorption of Sb(V) modeled by Freundlich equation could be described by Fe_DCB (dithionite–citrate–bicarbonicum extractable), and nearly 98 % of the variability could be described by Fe_DCB and Al_DCB.

Conclusions

Multiple linear regressions can be successfully applied to analyzing the relationship between sorption capacity and soil properties. Fe_DCB and Al_DCB played important roles in Sb(V) sorption onto soils. It would be useful to understand the environmental behaviors of Sb and for the implementation of risk assessment management and remediation strategies of Sb.     

Relationship between Gu(II) sorption and active H+ sorption sites of soils

Sorption characteristics of Cu(PI) were investigated using six soils coPBected in Korea (JUF9 SUM, and HHM) and in Japan (HNG, TWD, and ISM). The Cu(IH) sorption amount increased with increasing initial Cu(II) concentration. The maximnm sorption amount of @u(PI) increased in the order of KHM< ISK< JUM < JUF < TWD < KNG, and was related to the pH and BZSE of soils. The H⁺ release curves due to Cu(II) sorption apparently were characterized by a two or three step pattern. The amount of H⁺ released due do Cu(II) sorption increased with the increase in the Cu(II) sorption amount. The amount of protons released per Cu(II) sorbed onto soils with a larger Gu(II) sorption amount tended to be smaller compared with soils with a smaller Cu(HHQ sorption amount. The W⁺ sorption amount of the original soils and those with Cu(II) sorption at the PZSE, which was referred to as σ_P (Sakurai et al. 1988: Soil Sci. Plant Nutr., 34, 171–182; 1996: Jpn. J. Soil Sci. Plant Nutr., 67, 32–39), was determined by the STPT method proposed by Sakurai et al. (4988: Soil Sci. Plant Nutp., 34, 171–182). The active H⁺ sorption sites of soils were used for Cu(II) sorption and their amount decreased after Cu(II) sorption because they were covered with Cu(II). Soils with a larger amount of active H⁺ sorption sites exhibited a higher aEamity to Cu(II) khan those with a smaller amount of active H⁺ sorption sites. The Cu(II) sorption created a positive charge in soils, causing the decrease in the amount of active H⁺ sorption sites.     

Sorption-desorption behaviors of heavy metals by biochar-compost amendment with different ratios in contaminated wetland soil

Purpose

Heavy metal pollution in soils has become a global environmental concern. The combination of biochar and compost has already been proved to be an attractive method in contaminated soil. The objective was to study the sorption-desorption characteristics of Cd, Cu, and Zn onto soil amended with combined biochar-compost.

Materials and methods

In this study, the soil was amended with combinations of biochar and compost with different ratios at 10% (w/w). To determine the sorption-desorption behaviors of heavy metals by biochar-compost amendment with different ratios, we determine the effects of different ratios on soil properties and use batch experiments to investigate sorption-desorption behaviors of Cd, Cu, and Zn.

Results and discussion

The results show that the Langmuir and Freundlich model can well describe the adsorption isotherm of Cd, Cu, and Zn in the soils with or without biochar-compost combinations. The incorporation of amendment combinations into soil significantly promotes the sorption affinity of soil on metals. The sorption capacity of Cd and Zn was improved as the compost percentage rose in biochar-compost more likely due to the increase of organic matter and available phosphorus, while that of Cu was stronger with 10 and 20% biochar addition in biochar-compost combinations likely as the result of the formation of new specific adsorption sites and the mobile Cu adsorption in compost after adding a certain amount of biochar in amendment mixtures. Additionally, a certain proportion of biochar applied into amendment mixtures could suppress desorption of Cd and Cu by pH change, and the Zn desorption rate gradually decreased as the compost ratio increased in amendment mixtures.

Conclusions

The results indicated that the various ratios between biochar and compost have a significant effect on sorption-desorption of metals in soil, which helps us consider the effective combination of biochar and compost in soil.

     

Interaction of Oxidation Products from Caffeic Acid with Fe(III) and Fe(II)

Abstract

Phenolic substances in the soil–plant system can be oxidized by metal ions, inorganic components, molecular oxygen as well as by phenoloxidases, giving rise to the formation of products of low or high molecular weight. Interactions of these products with iron, in both reduced and oxidized form, can affect the iron mobility in soil and rhizosphere, and thus its availability to plants. Here we report the results of a study on the complexing and reducing activity of the oxidation products from caffeic acid (CAF), obtained via electrochemical means, towards Fe(III) and Fe(II) in aqueous solution in the 3.0–6.0 pH range. The HPLC analysis of the filtered solutions after the CAF oxidation showed the formation of two main groups of products: (i) CAF oligomers formed through radicalic reactions which do not involve the double bond of the CAF lateral chain and (ii) products where this bond is involved. These oxidation products (COP) were found to interact with both Fe(III) and Fe(II) with formation of soluble and insoluble Fe(III)‐, and Fe(II)‐COP complexes. The COP were found to be able to reduce Fe(III) to Fe(II) mainly at pH < 4.0. A low redox activity was observed at pH ≥ 4.5 due to Fe(III) hydrolysis reactions as well as to the decrease in the redox potential of the Fe(III)/Fe(II) couple. Formation of hydroxy Fe(III)‐COP polymers occurs at pH > 3.5.     

Cadmium accumulation in Crassocephalum crepidioides (Benth.) S. Moore (Compositae) in heavy-metal polluted soils and Cd-added conditions in hydroponic and pot cultures

Abstract

We have identified Crassocephalum crepidioides (Benth.) S. Moore (Compositae) as a cadmium (Cd)-accumulator plant in a heavy-metal polluted environment. In soil polluted with Cd, 5.7–17.5 mg kg^?1 Cd, concentrations in the above-ground plant tissues were measured as 14.6–78.6 mg kg^?1 with transfer factors in the above-ground plant tissues (concentration in above-ground tissues/soil concentration) of 1.5–6.0. No other toxic heavy metals or plant micronutrients were found to have accumulated into the above-ground plant tissues. In a hydroponic culture with 1 µmol L^?1 Cd added to Hoagland's nutrient solution, Cd concentration in the above-ground plant tissues was 121.0 mg kg^?1, with a transfer factor of more than 1000. In a pot culture carried out for 9 weeks in a greenhouse, the highest Cd concentration in the above-ground plant tissues, 121.2 mg kg^?1, was found in a treatment with 5 mg kg^?1 Cd, whereas the highest Cd content in an above-ground plant tissue, 106.1 µg, was found in a treatment with 2 mg kg^?1 Cd. These results clearly showed that C. crepidioides is a Cd accumulator. In all samples, the Cd concentration in the above-ground plant tissues was higher than that in the roots. The results obtained in the present study show that this plant has a strong potential for use in phytoremediation in farm fields contaminated with Cd.     

Simple modelling of the sorption of iron‐cyanide complexes on ferrihydrite

The sorption of the iron‐cyanide complexes ferricyanide, [Fe(CN)₆]^3—, and ferrocyanide, [Fe(CN)₆]^4—, on ferrihydrite was investigated in batch experiments including the effects of pH (pH 3.5 to 8) and ionic strength (0.001 to 0.1 M). The pH‐dependent sorption data were evaluated with a model approach by Barrow (1999): c = a exp(bS)S/(S_max‐S), where c is the solution concentration; S is the sorbed amount; S_max is maximum sorption; b is a parameter; and a is a parameter at constant pH. Ferricyanide sorption was negatively affected by increasing ionic strength, ferrocyanide sorption not at all. More ferricyanide than ferrocyanide was sorbed in the acidic range. In the neutral range the opposite was true. Fitting the pH‐dependent sorption to the model resulted in a strong correlation for both iron‐cyanide complexes with a common sorption maximum of 1.6 μmol m^—2. Only little negative charge was conveyed to the ferrihydrite surface by sorption of iron‐cyanide complexes. The sorption of iron‐cyanide complexes on ferrihydrite is weaker than that on goethite, as a comparison of the model calculations shows. This may be caused by the lower relative amount of high‐affinity sites present on the ferrihydrite surface.     

Two-year fluctuation of Cd bioavailability in a remediated rice field under different water managements

Purpose

Based on two consecutive years of field-scale trials, under different water managements, we illustrated the persistence of remediation effect of palygorskite on a Cd-polluted rice field.

Materials and methods

The Cd uptake by a plant, pH and Cd chemical extractability, available P/K, and extractable Zn/Cu in paddy soils were used to evaluate the influence of palygorskite on Cd immobilization and soil fertility index.

Results and discussion

In contrast to the 1st year, at 0–1.5% palygorskite applied dose in soils, 0.025 M HCl–extractable Cd in continuous flooding reduced by 12.1–19.0%, and that in wetting irrigation increased by 10.9–18.9% in the 2nd year (p?<?0.05). The toxicity characteristic leaching procedure Cd reduction of 3.0–11.4% and increase of 8.9–12.0% were obtained under above-mentioned water managements (p?<?0.05). Compared with the 1st year, at different clay additional concentrations, grain Cd in continuous flooding reduced by 7.0–11.3%, and that in wetting irrigation increased by 6.5–10.8% in the 2nd year (p?<?0.05). Although trace elements in clay treated soils declined, they had no influence on the grain yield due to a minimum value higher than the critical value of 1.5 mg kg^?1 for Zn and 2.0 mg kg^?1 for Cu. The available P in continuous flooding took on a maximum increase of 8.2% in the 2nd year (p?<?0.05).

Conclusions

Two consecutive years of field-scale in situ demonstration tests revealed that continuous flooding was a preferable water management regime for Cd immobilization using palygorskite in the rice field. There were no remarkable differences in extractable Zn/Cu between 2 years.

     

Specific sorption of antimony (III) by the hydrous oxides of Mn,Fe, and Al

The hydrous oxides of Mn, Fe, and Al avidly sorbed Sb from μM Sb(OH), solutions, with uptake levelling off as initial Sb concentration increased. Capacity values decreased along the sequence MnOOH > Al(OH)₃ > FeOOH. The amount sorbed by each substrate decreased gradually at pH values > 6. Addition of 0.4M CH₃COONa to the aqueous phase (to minimise retention of weakly bound Sb) had little effect on MnOOH uptake capacity (～160 mmol, kg^?1 at pH < 7) but retention dropped rapidly at higher pH. With the other two substrates (pH 6–7) the calculated capacity values for specific Sb sorption were ～ 45 mmol kg^?1 FeOOH and ～ 33 mmol kg^? Al(OH)₃; about a third of the total capacity values. On these substrates specific Sb sorption tended to peak in the pH 7 to 8 region. The pH response pattern was modified using Sb tartrate sorbate solutions. Factors influencing Sb sorption included substrate surface charge, chemical form of Sb and surface interactions. Formation of a sparingly soluble metal coating was indicated by the uptake plateaus observed when increasing amounts of solid were added to Sb solutions containing acetate.     

不同pH、电解质浓度条件下草酸对针铁矿吸附Cd(II)的影响及机制

用吸附平衡法研究了不同草酸浓度、体系pH对针铁矿 (G)吸附Cd2+的影响与机制以及电解质 (KNO3)浓度对针铁矿、草酸化针铁矿 (G+40 )吸附Cd2+的影响差别及原因。结果表明 ,低浓度草酸 (1mmolL-1 )促进Cd2+的吸附 ;高浓度草酸 (1mmolL-1/sup )抑制Cd2+的吸附。已吸附在针铁矿表面的草酸对Cd2+ 吸附的影响与液相中草酸的影响不同 ,这主要与草酸引起的针铁矿表面电荷性质的变化、草酸在固液两相间的分配、草酸与Cd2+的配合作用和竞争作用有关。电解质 (KNO3)浓度对针铁矿和草酸化针铁矿吸附Cd2+的影响明显不同 ,随KNO3 浓度的提高 ,针铁矿的Cd吸附率由 44.5%增至 95%以上 ,而草酸化针铁矿吸附率由 29%降至6.2% ,这主要决定于二者的电荷零点 (PZC)和体系pH变化的不同。