Speciation of cadmium,copper, lead,and zinc in contaminated soils

Abstract

To investigate the activity of free cadmium (Cd²⁺), copper (Cu²⁺), lead (Pb²⁺), and zinc (Zn²⁺) ions and analyze their dependence on pH and other soil properties, ten contaminated soils were sampled and analyzed for total contents of Cd, Cu, Pb, and Zn (Cd_T, Cu_T, Pb_T, and Zn_T, respectively), 0.43 MHNO₃‐extractable Cd, Cu, Pb, and Zn (Cd_N, Cu_N, Pb_N, and Zn_N, respectively), pH, dissolved organic matter (DOC), cation exchange capacity (CEC), ammonium oxalate extractable aluminum (Al) and iron (Fe), and dissolved calcium [Ca²⁺]. The activity of free Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺ ions in soil solutions was determined using Donnan equilibrium/graphite furnace atomic absorption (DE/GFAA). The solubility of Cd in soils varied from 0.16 to 0.94 μg L^‐1, Cu from 3.43 to 7.42 μg L^‐1, Pb from 1.23 to 5.8 μg L^‐1, and Zn from 24.5 to 34.3 μg L^‐. In saturation soil extracts, the activity of free Cd²⁺ ions constituted 42 to 82% of the dissolved fraction, for Cu²⁺the range was 0.1 to 7.8%, for Pb²⁺ 0.1 to 5.1% and for Zn²⁺2 to 72%. The principal species of Cd, Cu, Pb, and Zn in the soil solution is free metal ions and hydrolyzed ions. Soil pH displayed a pronounced effect on the activity of free Cd²⁺, Cu^2t, Pb²⁺, and Zn²⁺ ions.     

A Comparison of Inorganic Solid Wastes as Adsorbents of Heavy Metal Cations in Aqueous Solution and Their Capacity for Desorption and Regeneration

The adsorption capacity of seven inorganic solid wastes [air-cooled blast furnace (BF) slag, water-quenched BF slag, steel furnace slag, coal fly ash, coal bottom ash, water treatment (alum) sludge and seawater-neutralized red mud] for Cd²⁺, Cu²⁺, Pb²⁺, Zn²⁺ and Cr³⁺ was determined at two metal concentrations (10 and 100 mg?L^?1) and three equilibrium pH values (4.0, 6.0 and 8.0) in batch adsorption experiments. All materials had the ability to remove metal cations from aqueous solution (fly and bottom ash were the least effective), their relative abilities were partially pH dependant and adsorption increased greatly with increasing pH. At equimolar concentrations of added metal, the magnitude of sorption at pH 6.0 followed the general order: Cr³⁺????Pb²⁺????Cu²⁺?>?Zn²⁺?=?Cd²⁺. The amounts of previously sorbed Pb and Cd desorbed in 0.01 M NaNO₃ electrolyte were very small, but those removed with 0.01 M HNO₃, and more particularly 0.10 M HNO₃, were substantial. Water treatment sludge was shown to maintain its Pb and Cd adsorption capability (pH 6.0) over eight successive cycles of adsorption/regeneration using 0.10 M HNO₃ as a regenerating agent. By contrast, for BF slag and red mud, there was a very pronounced decline in adsorption of both Pb and Cd after only one regeneration cycle. A comparison of Pb and Cd adsorption isotherms at pH 6.0 for untreated and acid-pre-treated materials confirmed that for water treatment sludge acid pre-treatment had no significant effect, but for BF slag and red mud, adsorption was greatly reduced. This was explained in terms of residual surface alkalinity being the key factor contributing to the high adsorption capability of the latter two materials, and acid pre-treatment results in neutralization of much of this alkalinity. It was concluded that acid is not a suitable regenerating agent for slags and red mud and that further research and development with water treatment sludge as a metal adsorbent are warranted.     

Heavy Metal Uptake by Rape

Abstract

In pot experiments, the effect of single and combined pollution of soil by lead (Pb), cadmium (Cd), and zinc (Zn) and uptake of heavy metals in Brassica napus L. were investigated. There were two main factors that affected the assimilation of Pb, Cd, and Zn by rape: (i) level of soil pollution by the particular element and (ii) the combined influence of Pb, Cd, and Zn. In general, with the increase of the concentrations of Pb, Cd, and Zn in the soil, there were increases in the concentrations of those elements in the roots, stems, and seeds. The main part of the Pb and Zn amounts taken up by the roots from the soil are fixed and accumulated in the roots, and small amounts of them move through the conductive system to the seeds. Cadmium moves relatively easily from root to stem and is accumulated in higher concentrations in the top of the plant.

There is a well‐expressed synergistic interaction between Pb²⁺ and Cd²⁺, as well as of Cd²⁺ and Zn²⁺. Zinc has a highly depressing effect on the assimilation of Cd²⁺, as does Pb²⁺ on the assimilation of Zn²⁺. The combined pollution by Pb, Cd, and Zn stimulated the assimilation of these elements by the roots and foliage and eliminated the effect of Zn²⁺ on Cd²⁺ and of Pb²⁺ on Zn²⁺.     

Comprehensive study of Pb (II) speciation in soil by X-ray absorption spectroscopy (XANES and EXAFS) and sequential fractionation

Purpose

The study is aimed at the analysis of the spatial–structural organization of Pb(II) in Chernozem soils and the relationship between the metal ion and the soil components using X-ray absorption spectroscopy and chemical extractive fractionation.

Materials and methods

In a model experiment, soil samples were artificially contaminated with elevated rates of Pb(NO₃)₂ and PbO (2000 and 10,000 mg kg^?1). The samples of mineral phases (bentonite, gibbsite, kaolinite, calcite, and hydromuscovite) were saturated with Pb²⁺ ions. The sequential fractionation of Pb in the soil was conducted by the Tessier method. X-ray absorption near-edge fine structure (XANES) spectra at the Pb L_III-edge (13.040 keV) were obtained on a Rigaku R-XAS Looper spectrometer. Extended X-ray absorption fine structure (EXAFS) L_III-edge Pb was measured at the Structural Materials Science beamline of the Kurchatov Center for Synchrotron Radiation.

Results and discussion

The results of successive extraction showed that Pb is associated with strongly bound organic substances, Fe and Mn (hydr)oxides, and carbonates. An increase in the portion of exchangeable fraction is observed under extreme loads. At the addition of Pb in the form of oxide and nitrate to the soil, the fractional compositions were similar, which indicates the good transformation of PbO in Chernozem. The features of XANES spectra indicate different orbital transitions in the electron shells of Pb²⁺ ions for monoxide (PbO) and soluble salt (Pb(NO₃)₂), which affect the ion properties and determine the individual structure of the coordination sphere. The analysis of XANES revealed that sorption of Pb in the soil samples and in the samples of mineral phases does not change its bond valence.

Conclusions

The increased degree of soil contamination with Pb is accompanied by decreasing the stable connection between metal and soil components. Lead ions in bentonite, kaolinite, hydromuscovite, gibbsite, and calcite are incorporated in the positions of the inner-sphere complex replacing some aluminum ions in the octahedral sites. This results in changes the Pb–O distances in Pb-bearing octahedrons. We may suggest that Pb²⁺ is also sorbed by dimer (Pb–Pb) silicate and/or aluminum groups. The structure of adsorbent surface plays the key role in the sorption of Pb²⁺ by mineral phases.

     

恒电荷土壤胶体对Cu2+ 、Pb2+ 的静电吸附与专性吸附特征

供试土壤胶体对Cu2 、Pb2 的吸附强度用pH50 值表示 ,其大小次序为 :土 >黄绵土、黑垆土 >黄褐土。离子强度实验和表面络合反应机制证明恒电荷土壤胶体对Cu2 、Pb2 的吸附含有专性吸附 ,n值可作为判断专性吸附与静电吸附比例的特征值 ,低pH值时 ,以水解 -络合吸附为主 ;高pH值时 ,以水解 -络合与沉淀吸附为主。静电吸附和专性吸附的比例与pH有关 ,各土壤胶体专性吸附百分数大小为 :黄褐土 >土 >黑垆土 >黄绵土。不同土壤胶体在同一介质中对Cu2 、Pb2 的固有络合常数logKintM 值及固有络合ΔG m 负值大小次序与吸附强度大小一致。在定pH定浓条件下 ,考虑离子之间的相互作用时 ,土壤胶体对重金属离子的吸附过程可用BDM等温式描述。供试土壤胶体对Cu2 、Pb2 专性吸附ΔG m 的大小与固有络合ΔG m 接近且大小次序也一致。     

Effects of Phosphorus Additions on Lead,Cadmium, and Zinc Bioavailabilities in a Metal-Contaminated Soil

The use of phosphorus (P) to reduce lead (Pb)bioavailability is being proposed as an alternative to excavationand disposal as a remedial technology for Pb-contaminated soilsin residential areas. The objective of this study was todetermine the influence of P sources and rates andCaCO₃additions on the bioavailabilities of Pb, cadmium (Cd), and zinc(Zn) in a contaminated soil material using plants, a sequentialextraction procedure, and ion activities in equilibrium solutionas indicators. A contaminated soil containing 370 mg kg^-1 Cd, 2800 mg kg^-1 Pb and 29100 mg kg^-1 Zn was amended ina factorial arrangement of CaCO₃ (0 or 2000 mg kg^-1) and P as rock phosphate or KH₂PO₄ at 0:1, 2:1 or 4:1P:Pb mole ratios. A pot study was conducted using sorghum-sudangrass (Sorghum bicolor L. Moench). The addition of P did not influence Pb concentrations in plant tissue and had little effect on Cd concentrations. An interaction between P source and level of P addition was found for Zn concentrations in plant tissue; concentrations increased with increasing amounts of P from KH₂PO₄ anddecreased with increasing amounts of P from rock phosphate. Sequential extraction results suggested a much greater reduction in Pb bioavailability from treatment withKH₂PO₄ than with rock phosphate and that P influencedthe fractionations of Cd and Zn. Activities of Cd²⁺,Pb²⁺, and Zn²⁺ in equilibrium solutions generally weredecreased by rock phosphate and increased by KH₂PO₄. Saturation indices suggested the addition ofKH₂PO₄shifted the soil equilibrium from octavite to hydroxypyromorphite, whereas solid-phase control of Cd²⁺ andZn²⁺ was not influenced by soil amendments. A soluble Psource was more effective in reducing Pb bioavailability thanrock phosphate but had variable effects on Cd and Znbioavailabilities.     

Effect of Sodium Chloride-induced Salinity on Phyto-availability and Speciation of Cd in Soil Solution

A greenhouse experiment with two levels of Cd (0.5 and 10 mg Cd kg^?1, in the form of CdCl₂), and five salinity levels of irrigation water (0, 8.6, 17.1, 34.2 and 68.4 mM NaCl) in triplicate was conducted to determine the effect of NaCl-induced salinity on the solubility and availability of Cd in clay loam and sandy calcareous soils. Corn seeds (Zea mays L.) were sown in pots. Forty-five days after planting, the shoots were harvested, and their Cd concentration was determined. The post-harvest electrical conductivity (EC_e), pH, and concentrations of cations and anions were determined in soil saturation paste extracts. Increasing irrigation water salinity resulted in significant increases in the total soluble Cd concentration in both studied soils. A positive correlation was found between the total soluble Cd and the chloride concentration in the soil solution.Solution speciation, calculated with MINEQL+ (a chemical equilibrium modeling system), predicted that Cd was present mainly as free Cd²⁺ ions followed by CdCl⁺ and $ {\text{CdSO}}^{0}_{4} $ in the soils irrigated with deionized water. However, Cd species in the soil solution were significantly altered by increasing chloride concentration, with Cd–chloro complexes becoming the dominant Cd species in the soil solution. Increasing the salinity level resulted in significant decreases in the shoot dry matter and increases in the shoot Cd concentration. Shoot Cd concentration was positively correlated with both the total Cd and Cd–chloro complexes in the soil solution.     

SOLUBILITY AND SORPTION OF CADMIUM IN SOILS AMENDED WITH SEWAGE SLUDGE

The mechanisms governing the retention and release of Cd in two soils, a loam and a loamy sand, pretreated with anaerobically digested sewage sludges or with chemical fertilizers, were studied using batch equilibration in 0.05 m Ca(NO₃)₂ solution containing up to 6 μg Cd/ml. Adsorption rather than precipitation as Cd₃(PO₄)₂ limited solution Cd²⁺ concentration. With the addition of 50 μg Cd/g, however, precipitation as CdCO₃ was likely at pH 7.6. Cadmium adsorption increased with increasing soil pH. The differences in Cd adsorption between different soil treatments were attributed mainly to the soil pH (6.9 to 7.9) induced by sludge application. About 82 to 92 per cent of adsorbed Cd was retained by cation exchange and complexing sites. Soils treated with sludge increased the amount of exchangeable Cd but reduced the amount of complexed Cd compared with the fertilized soil. Cadmium retention by cation exchange became more dominant as the amount of Cd in the soil was increased.     

Coefficients of association and activity of cadmium and lead ions in soil solutions

The thermodynamic calculations were made for the contents of Pb and Cd compounds in soil solutions and water extracts of the meadow-steppe and meadow solonetzes (Rostov oblast) based on their prescribed analytic concentrations of 3 and 5 μg/l, respectively. The solonetzes studied are characterized by their high carbonate content and high alkalinity. The activity of Pb²⁺ and Cd²⁺ was shown to be many times lower than their total concentration in the solution due to the association of these metals with carbonate and other anions and the formation of hydroxocomplexes CdOH⁺ and PbOH⁺. This fact is one of the reasons for the low input of Pb²⁺ and Cd²⁺ to plants growing on calcareous soils.     

Estimating Sorption Affinities of Heavy Metals on Humic Acid and Silica using a Constant Capacitance Model

The adsorption of heavy metals [cadmium (Cd ²⁺), cobalt (Co²⁺), nickel (Ni²⁺), zinc (Zn²⁺), and lead (Pb)] and calcium (Ca²⁺) on humic acid and silica were investigated to understand the adsorptive selectivities of heavy metals on the constituents of soil. The experiments for the adsorption of Cd and Pb were carried out in a 0.1 mol L^?1 (M) sodium nitrate (NaNO₃) background solution, whereas those for the other metals were done in a 0.1 M sodium chloride (NaCl) solution. The adsorptive affinities of the metal ions on the humic acid and silica were ranked by the intrinsic surface complexation constants [K _m ¹(int)] that were calculated approximately from the adsorptive data using a constant capacitance model. The log [K _m ¹(int)] values of the metals were in the order Zn²⁺(?2.29) > Cd²⁺(?2.41) > Co²⁺(?2.74) > Ni²⁺(?2.92) ?> Ca²⁺ (?3.33) for the humic acid and Zn²⁺(?4.23) > Cd²⁺(?4.49) > Ni²⁺(?4.51) ? Co²⁺ (?5.99) > Ca²⁺(?6.37) for silica.     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER. IV. ADSORPTION AND DESORPTION OF Pb2+, Cu2+, Cd2+, Zn2+ AND Ca2+ BY PEAT

The equilibria as well as the rates of adsorption and desorption of the ions Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺, and Ca²⁺ by soil organic matter were determined in batch experiments as a function of the amount of metal ions added to an aqueous suspension of HCl-washed peat. Simultaneous determination of the metal ions and hydrogen ions in the solution by atomic absorption spectrophotometry and pH-measurements showed that the adsorption of one divalent metal ion by peat was coupled with the release of two hydrogen ions. Since this equivalent ion-exchange process causes a corresponding increase of the electric conductivity of the solution, the rates of the adsorption and desorption processes were determined by an immersed conductivity electrode. The distribution coefficients show that the selective order for the metal adsorption by peat is Pb²⁺ > Cu²⁺ > Cd²⁺≌ Zn²⁺ > Ca²⁺ in the pH range of 3·5 to 4·5. The slope of -2, as observed in a double logarithmic plot of the distribution coefficients versus the total solution concentration confirms the equivalence of the ion-exchange process of divalent metal ions for monovalent H₃O⁺ -ions in peat. The absolute rates of adsorption, as well as the rates for the fractional attainment of the equilibrium, increase with increasing amounts of metal ions added. This behaviour is also observed for the subsequent desorption of the metal ions by H₃O⁺-ions. At a given amount of metal ions added, the absolute rates of adsorption decrease in the order Pb²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ > Ca²⁺, while the rates for the fractional attainment of the equilibrium decrease in the order Ca²⁺ > Zn²⁺≌ Cd²⁺ > Pb²⁺ > Cu²⁺. The half times for adsorption and desorption were in the range of 5 to 15 sec.     

Adsorptionsisothermen als Regelgrößen beim Transport von Schwermetallen in Böden

Adsorption isotherms as regulators controlling heavy metal transport in soils The adsorption and desorption of Pb²⁺ and Cd²⁺ from equilibrium solutions with heavy metal contents up to 5000 μg/1 were determined in bulk experiments for soil samples from an acid Braunerde developed in loess loam, taken from the humic surface layer of the mineral soil (0–10 cm) and from the subsoil fairly free from organic matter (30–40 cm). Pb and Cd in solutions were determined by flameless atomic absorption spectroscopy. Pb was more strongly retained in the solid phase than Cd, and higher amounts of heavy metals were retained in the humic surface soil than in the subsoil free of organic matter. In the case of Pb adsorption/desorption showed slight hysteresis in the subsoil. The quantity/intensity (Q/I) relationships found in the experiments could be described by the Freundlich equation. The Q/I-relationships were substituted in the general transport equation. With a simulation model the transport of Pb and Cd through the soil with vertical water flow was calculated by the use of the Continuous Simulation Programming Language (CSMP). Two different cases were considered: a small, continuous increase in the heavy metal input of the soil surface, and a high, instantaneous peak input. Simulation of the transport and distribution mechanisms induced by the inputs over a period of 10 years showed strong retention of lead in the surface layer and consequently a strong damping of the concentration peak in the soil solution. In contrast, cadmium is distributed more quickly over the whole profile, yet the concentration peak in the solution phase is, too, damped considerably in the surface layer by temporary retention in the solid phase. The results of the simulation runs are in accordance with the situation in real soils where often strong accumulation of Pb is found in the top soil, while Cd is accumulated only slightly.     

Enhanced Adsorption of Metal Ions Onto Polyethyleneimine-Impregnated Palm Shell Activated Carbon: Equilibrium Studies

In this study, palm shell activated carbon was impregnated with polyethyleneimine (PEI) and the effect of impregnation on batch adsorption of Ni²⁺, Cd²⁺or Pb²⁺ as well as the equilibrium behavior of adsorption of metal ions on PEI-impregnated AC were investigated. PEI impregnation evidently increased the single metal adsorption capacities of Ni²⁺ or Cd²⁺except for Pb²⁺, where its adsorption capacities were reduced by 16.67% and 19.55% for initial solution pH of 3 and 5 respectively. This suggested that PEI-impregnated AC could be used for selective separation of Pb²⁺ ions from other metal ions. The adsorption data of all the metal ions on both virgin and PEI-impregnated AC for both initial solution pH of 3 and 5 generally fitted the Langmuir and Redlich-Peterson isotherms considerably better than the Freundlich isotherm.     

Complexation of Cu2+ and Pb2+ by peat and humic acid

The binding of metal to humic substances is problematical. The approaches for studying metal binding to organic matter are briefly reviewed. Ion-selective electrodes (Cu²⁺ and Pb²⁺) were used to measure metal complexation by a whole peat and an extracted humic acid (HA) fraction. Scatchard plots and calculation of incremental formation constants were used to obtain values for the binding constants for the metals onto both peat and HA. Both the peat and the humic acid had a larger maximum binding capacity for Pb²⁺ than for Cu²⁺ (e.g. at pH = 5 HA gave 0·188 mmol Cu²⁺ g^?1 and 0·564 mmol Pb²⁺ g^?1: peat gave 0·111 mmol Cu²⁺ g^?1 and 0·391 mmol Pb²⁺ g^?1). Overall, the humic acid had a larger metal binding capacity, suggesting that extraction caused conformational or chemical changes. The binding constants (K₁) for Cu²⁺ increased with increasing pH in both peat and humic acid, and were larger in the peat at any given pH (e.g. at pH = 5 HA gave log K₁= 2·63, and peat gave log K¹= 4·47 for Cu²⁺). The values for Pb²⁺ showed little change with pH or between peat and humic acid (e.g. at pH = 5 HA gave log K¹= 3·03 and peat gave log K¹= 3·00 for Pb²⁺). In the peat, Cu²⁺ may be more able to bind in a 2:1 stoichiometric arrangement, resulting in greater stability but smaller binding capacity, whereas Pb²⁺ binds predominantly in a 1:1 arrangement, with more metal being bound less strongly. Whole peat is considered to be more appropriate than an extracted humic acid fraction for the study of heavy metal binding in organic soils, as this is the material with which metals introduced into an organic soil would interact under natural conditions.     

TiO2 photocatalytic degradation of tetracycline as affected by a series of environmental factors

Purpose

TiO₂ photocatalytic degradation of tetracycline (TC) in aqueous solution under UV irradiation was investigated as affected by different environmental factors, including cations, anions, organic acids, and surfactants.

Materials and methods

The solution of TC with TiO₂ was irradiated by medium mercury lamp. The concentrations of TC and metal ions were analyzed by HPLC and AAS, respectively. The degradation efficiency of TC was calculated based on TC disappearance.

Results and discussion

Photocatalysis was very effective for TC removal. The degradation efficiency of TC was significantly enhanced in the presence of Cu²⁺/Pb²⁺, SO₄ ^2?/Cl^?, and humic acid (HA) in the examined range, but did no change with Ni²⁺, Cd²⁺, or Zn²⁺. In addition, the results also showed that solution Cu²⁺ and Pb²⁺ ions could be reduced during the process, while Ni²⁺, Cd²⁺, and Zn²⁺ were still kept in the solution. However, tannic acid (TA), gallic acid (GA), citric acid (CA), salicylic acid (SA), hydroxypropyl-β-cyclodextrin (HPCD), polyoxyethylene lauryl ether (Brij35), or polyoxyethylenesorbitan monooleate (Tween80) significantly decreased the degradation efficiency of TC.

Conclusions

The photocatalytic approach could be successfully applied to remove TC, and environmental factors significantly influenced its degradation efficiency. It would be useful to understand the environmental behaviors of TC and for the implementation of remediation strategies of TC.     

Influence of pH on the release and chemical fractionation of heavy metals in sediment from a suburban drainage stream in an arid mine-based oasis

Purpose

The objectives of this study were to explore the influences of pH on the release of Cu, Zn, Cd, Pb, Ni, and Cr in sediments derived from the upstream, middle, and downstream reaches of Dongdagou stream in Gansu Province, Northwest China, and to examine the fractionation changes of heavy metals in the sediments after reaching their release equilibrium under different pH conditions.

Materials and methods

Sediment samples were obtained using a stainless steel grab sampler to collect the uppermost 10 cm of sediment from the channel bed. The pH-dependent release experiment was conducted in the solid-to-liquid ratio of 1:20 at different pH values (2, 4, 6, 8, 10, and 12) at room temperature. The total Cu, Zn, Cd, Pb, Ni, and Cr concentrations in the sediments were digested using an acid digestion mixture (HNO₃ + HF + HClO₄) in an open system. Metal fractionation of selected sediments was obtained using the Tessier sequential extraction procedure. Heavy metal concentrations in the samples were determined using atomic absorption spectrophotometry.

Results and discussion

The mean concentrations of heavy metals in sediments decreased in the following order: Zn (1676.67 mg kg^?1) > Pb (528.65 mg kg^?1) > Cu (391.34 mg kg^?1) > Cr (53.48 mg kg^?1) > Ni (34.27 mg kg^?1) > Cd (11.53 mg kg^?1). Overall, the solubility of Cu, Zn, Cd, Pb, and Ni decreased with increasing pH, and they were strongly released at pH 2. Moreover, the solubility of Cr increased with increasing pH, and its release was highest at pH 12. After reaching the release equilibrium of heavy metals under different pH conditions, the percentages of organic Cu, Zn, Cd, and Fe-Mn oxyhydroxide Pb decreased, compared to their initial fractions. The residual fractions of Ni and Cr were dominant, regardless of pH.

Conclusions

The average concentrations of Cu, Zn, Cd, and Pb in sediments were highly elevated compared with the soil background values in Gansu Province, China. The results of this pH-dependent release experiment showed that the release behaviors of Cu, Zn, Pb, and Cr followed an asymmetric V-shaped pattern, whereas Cd and Ni followed an irregular L-shaped pattern. The changes in the release of heavy metals in sediments were related to their redistribution between chemical fractionations.

     

Transfer of Cd, Pb, and Zn to water spinach from a polluted soil amended with lime and organic materials

Purpose

The transfer of heavy metals from soil to crops comprises several steps, including soil-to-root and subsequent root-to-shoot tranfer. The purpose of this study was to investigate the different steps of soil-to-crop transfer of Cd, Pb, and Zn.

Materials and methods

This study was carried out with a greenhouse pot experiment using a soil polluted with Cd, Pb, and Zn which was amended with rice straw, pig manure, sheep dung, or peat, with and without lime. Water spinach (Ipomoea aquatica) was used as the test crop and was grown after a season of rice cultivation.

Results and discussion

The results showed that all the amendments promoted the root-to-shoot transfer of Cd, Pb, and Zn. The soil-to-root transfer factors (TFs) of Pb and Zn tended to increase with increasing available Pb and Zn in the soils, while no clear relationship between the TF of Cd and available soil Cd was observed. The root-to-shoot TF of Cd, Pb, and Zn tended to decrease with increasing available amounts in the soils and were negatively correlated with the concentrations of the metals in the roots (r _Cd?=?0.820, r _Pb?=?0.789, r _Zn?=?0.769).

Conclusions

The soil-to-root transfer of Cd, Pb, and Zn was significantly different from the root-to-shoot transfer. The soil-to-root transfer was mainly influenced by the amount of available metal in soil, whereas the root-to-shoot transfer was mainly controlled by the concentrations of the metals in the root.     

ADSORPTION OF LEAD, COPPER, ZINC, COBALT, AND CADMIUM BY SOILS THAT DIFFER IN CATION-EXCHANGE MATERIALS

The adsorption of Zn²⁺, Pb²⁺, Cu²⁺, Co²⁺, and Cd²⁺ (M²⁺) by soils was measured at concentrations ranging from 10-⁷ to 10-² M in 10-³ to 10-² M CaCI₂. Exchange between Ca²⁺ and M²⁺, and solubility products [M²⁺][OH^?]² indicate that M²⁺ is not precipitated as hydroxide but is adsorbed on cation-exchange sites. The proportion of selective adsorption sites with specified values of the selectivity coefficient calculated using Ca as reference ion, increased in the order montmorillonite < humus, kaolinite, < allophane. imogolite < halloysite, iron oxides. Raising the soil pH by Ca-saturation increased both the amount and affinity of adsorption. Selectivity of adsorption increased in the order Mg, Ca < Cd, Co < Zn < Cu, Pb, and the selectivity coefficient varied from < 1 to > 10 000. The formation of the coordination complexes of heavy metal with deprotonoted OH and COOH groups as ligands is suggested as a possible mechanism of selective adsorption.     

Persistence of CaCl2 washing effect for amelioration of a Cd-contaminated paddy field soil

Abstract

Soil washing is one of the methods used to remediate soil contaminated with heavy metals, and when the contaminated elements have been effectively removed the washed soil can be used for agriculture. Soil washing was conducted using 0.5 mol L^?1 CaCl₂ solution at pH 4 as an extracting agent to remediate a paddy field soil contaminated with Cd. Dolomite powder was applied to neutralize the soil to the original pH 6.2. After CaCl₂ washing, the content of Cd extractable in 0.1 mol L^?1 HCl decreased from 2.4 to 0.8 mg kg^?1. Subsequently, a pot experiment was carried out to evaluate the effect of soil washing on Cd concentration in polished rice (Cd_pr) for three successive years. Using the washed soil, Cd_pr was ≤ 0.2 mg kg^?1 with and without a treatment that simulates midseason drainage, whereas it was > 0.5 mg kg^?1 in the unwashed soil with the midseason drainage treatment. The reasons for low Cd_pr growth in the washed soil were the low content of exchangeable Cd in the soil and the resultant high soil pH (> 7). To evaluate the effect of soil pH on Cd_pr in the fourth year, we adjusted soil pH to 5 with H₂SO₄ before transplanting rice seedlings. The Cd_pr in the washed soil with the midseason drainage treatment increased to 0.47 mg kg^?1, whereas it was less than 0.2 mg kg^?1 under continuous flooding. Thus, high pH or whole season flooding are important to keep Cd_pr at ≤ 0.2 mg kg^?1 even after soil washing. With the application of dolomite and other ordinary fertilizers, soil properties were little affected by the present soil washing procedure because the difference in rice yield between the washed and unwashed plots was not significant within each year.