Sorption of lead,copper, zinc,and cadmium by soils: effect of nitriloacetic acid on metal retention

Abstract

Laboratory experiments were carried out to evaluate lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) sorption‐desorption by three soils of contrasting characteristics. Talamanca (silt loam, montmorillonite, Calcic Haploxeralfs), Mazowe (clay, kaolinite, Rhodic Kandiustalf), and Realejos (sandy silt loam, allophane, Typic Hapludands). A second objective was to study the effect of nitriloacetic acid (NTA) on the sorption process. The Talamanca soil, which had a native pH of 6.4 and presented the highest effective cation exchange capacity (ECEC), sorbed more of each of the metal tested than did the other two soils. When the other two soils were compared metal sorption was also related to pH and ECEC. The very low sorption capacity showed by Realejos may be attributed to the low net surface negative charge density of this soil, arising from its allophanic nature. A common feature of the three soils was the relative strong sorption of both Pb and Cu relative to Cd and Zn with Pb showing the highest sorption levels. The selectivity sequences of metals retention were Pb>Cu>Zn>Cd for Talamanca soil, Pb>Cu>Zn≈Cd for Mazowe, and Pb>Cu>Cd>Zn for Realejos. Metal desorption values were low. The order of metal desorption (Cd≈Zn>Cu>Pb) was the same for the three soils studied. Quantitative differences observed in the extractability of the sorbed metals between the soils (Realejos>Mazowe>Talamanca) indicated that soil properties which enhanced metal sorption contributed at the same time to slow down the backward reaction. The addition of NTA to the soil suspension significantly depressed metal sorption by the three soils investigated. Compared with the free ligand system Pb, Cu, Zn, and Cd sorption in the presence of NTA decreased roughly 50%.     

Competitivity,selectivity, and heavy metals-induced alkaline cation displacement in soils

Abstract

The simultaneous incorporation of heavy metals into the soil is still a matter of great concern. Interaction (competitive sorption) between these metals and the soil solid phase may result in a deterioration of soil quality which relies basically on amounts of alkaline cations saturating soils sorptive complex. Results of this study indicate that Pb, Cu, C d, and Zn have induced solution pH decreases which were more intensive at highest metal loading rates. Partition parameters (Kd)-based sequences showed that Pb and Cu were more competitive than Cd and Zn and the overall selectivity sequence followed: Pb > Cu > Cd > Zn. Metal loadings and their competitive sorption have led to a strengthened displacement of alkaline cations (i.e. Ca²⁺, Mg²⁺, K⁺, Na⁺), especially of Ca²⁺ as a factor “stabilizing” soil sorptive complex. Such metals impact jointly with soils acidification are of great environmental concern since tremendous amounts of alkaline cations (especially Ca²⁺) may be potentially leached out, irrespective of the degree of soil contamination, as evidenced in the current study. High and positive ΔG values implied that the studied soils were characterized by generally low concentrations of exchangeable potassium which required high energy to get displaced (desorbed). Further studies on heavy metal uncontaminated or contaminated areas should be undertaken to provide with data which should be used for predictions on changes related to soil buffering capacity as impacted by heavy metal inputs.     

Comparative effectiveness of selected adsorbant materials as potential amendments for the remediation of lead-, copper- and zinc-contaminated soil

Bonemeal, coir, compost, green waste compost, peat and wood bark all potentially could be used as amendments to remediate heavy metal contamination in soils. Their ability to sorb Pb, Cu and Zn was evaluated in the laboratory, using metal solutions ranging from 0 to 5 mmol/L as contaminants. The effects on sorption of metal concentration, background salt concentration and metal competition were evaluated. Single metal sorption by the six amendments was significantly different at metal concentrations of 1.5–5 mmol/L, with green waste compost, coir, compost and wood bark having the highest capacities to adsorb Pb, Cu and Zn. Langmuir sorption maxima were approximately 87 mg Pb/g (coir and green waste compost), 30 mg Cu/g (compost and green waste compost) and 13 mg Zn/g (compost and green waste compost) (equivalent to approx. 0.5 mmol/g of Pb and Cu, and 0.2 mmol/g Zn), all in a background solution of 0.001 M Ca(NO₃)₂. A higher background salt concentration and a combination of all three metals led to significant reduction in the amounts of Pb, Cu and Zn sorbed by all the amendments tested. Competing heavy metal cations in solution decreased Pb sorption to about 50–60% of that from a solution containing Pb alone; Cu sorption was reduced to about 30–40%; the effect of competition on Zn sorption was variable. Overall, in both single metal and competitive sorption, the order of strength of binding was Pb>Cu>>Zn.     

Modelling trace metal extractability and solubility in French forest soils by using soil properties

Soil/solution partitioning of trace metals (TM: Cd, Co, Cr, Cu, Ni, Sb, Pb and Zn) has been investigated in six French forest sites that have been subjected to TM atmospheric inputs. Soil profiles have been sampled and analysed for major soil properties, and CaCl₂‐extractable and total metal content. Metal concentrations (expressed on a molar basis) in soil (total), in CaCl₂ extracts and soil solution collected monthly from fresh soil by centrifugation, were in the order: Cr > Zn > Ni > Cu > Pb > Co > Sb > Cd , Zn > Cu > Pb = Ni > Co > Cd > Cr and Zn > Ni > Cu > Pb > Co > Cr > Cd > Sb , respectively. Metal extractability and solubility were predicted by using soil properties. Soil pH was the most significant property in predicting metal partitioning, but TM behaviour differed between acid and non‐acid soils. TM extractability was predicted significantly by soil pH for pH < 6, and by soil pH and Fe content for all soil conditions. Total metal concentration in soil solution was predicted well by soil pH and organic carbon content for Cd, Co, Cr, Ni and Zn, by Fe content for Cu, Cr, Ni, Pb and Sb and total soil metal content for Cu, Cr, Ni, Pb and Sb, with a better prediction for acidic conditions (pH < 6). At more alkaline pH conditions, solute concentrations of Cu, Cr, Sb and Pb were larger than predicted by the pH relationship, as a consequence of association with Fe colloids and complexing with dissolved organic carbon. Metal speciation in soil solutions determined by WHAM‐VI indicated that free metal ion (FMI) concentration was significantly related to soil pH for all pH conditions. The FMI concentrations of Cu and Zn were well predicted by pH alone, Pb by pH and Fe content and Cd, Co and Ni by soil pH and organic carbon content. Differences between soluble total metal and FMI concentrations were particularly large for pH < 6. This should be taken into account for risk and critical load assessment in the case of terrestrial ecosystems.     

Competitive Effect of Copper,Zinc, Cadmium and Nickel on Ion Adsorption and Desorption by Soil Clays

This study evaluated the effect of competing copper, zinc, cadmium and nickel ions in 0.01 M Ca(NO₃)₂ on heavy metal sorption and desorption by soil clay fractions. Initial Cu addition levels varied from 99 mg kg^-1 to 900 mg kg^-1 and Zn, Cd and Ni levels were 94, 131 and 99 mg kg^-1, respectively. Sorption of Cu conformed to a Freundlich equation. The amounts of metals not displaced by successive 48 h desorption cycles with 0.01 M Ca(NO₃)₂ were considered ‘specifically adsorbed’. Total sorption of Zn and Cd generally decreased in the order: Vertisol > Gleyic Acrisol > Planosol clay. More than 70% of the copper was specifically sorbed. Specific sorption of Zn was depressed by competition with Cu in the three clays investigated. At surface coverages higher than 200 mg Cu per kg of soil clay, zinc sorption in the Planosol and Gleyic Acrisol clays took place at low affinity sites. The exchangeable component of sorbed cadmium accounted for >:60% of the sorption in the Vertisol clay, >70% in the Gleyic Acrisol clay and was almost 100% in the Planosol clay. Nickel was not retained by the Planosol and Gleyic Acrisol clays and was ionexchangeably adsorbed by the Vertisol clay. At the conditions studied, Ni and Cd remain a ready source of pollution hazard.     

复垦沉积池土壤中镍、铅、锌的吸附

The wastes used to amend soils sometimes have high concentrations of metals such as nickel (Ni), lead (Pb) and zinc (Zn). To determine the capacity of soils to retain these metals, the sorption capacities of different mine soils with and without reclamation treatments (tree vegetation and waste amendment) for Ni, Pb and Zn in individual and competitive situations were evaluated using the batch sorption technique. The untreated settling pond soil had low capacity for Ni, Pb and Zn retention. The site amended with wastes (sewage sludges and paper mill residues) increased the sorption capacity most, probably because of the higher concentrations of soil components with high retention capacity such as carbon and clay fraction. No significant competition was observed between metals in the competitive sorption experiment, indicating that the maximum of sorption was not achieved by adding 0.5 mmol L^-1 of metal. We can conclude that, despite the possible additions of Ni, Pb and Zn from wastes to degraded soils, sewage sludges and paper mill residues have a high sorption capacity that would prevent the metals from being in a mobile form.     

Pollution and Ecological Risk Assessment of Heavy Metals in Farmland Soils in Yanqi County,Xinjiang, Northwest China

A total of 50 farmland soil samples were collected from the Yanqi County, Xinjiang, China, and the concentrations of eight heavy metal elements (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn) were determined by standard methods. The spatial distribution, pollution level and ecological risk status of heavy metals were analyzed based on GIS technology, the Geo-accumulation Index (Igeo), the Pollution Load Index (PLI) and the Potential Ecological Risk Index (RI). Results indicated that: (1) The average contents of Cd, Cr, Ni, Pb, and Zn of farmland soils exceeded the background values of irrigation soils in Xinjiang by 1.5, 1.40, 1.33, 2.63, and 4.92 times, respectively. Cd showed a no-pollution level, Zn showed a partially moderate pollution level, Pb showed a slight pollution level, and Cr, Cu, As, Mn, and Ni showed no-pollution level, compared to the classification standard. The PLI values of heavy metal elements of farmland soils varied from 0.83 to 1.89, with an average value of 1.29, at the moderate pollution level. (2) The Individual Potential Ecological Risk Index for heavy metals in the study area was ranked in the order of: As > Ni > Cu > Cd > Pb > Cr> Zn. The RI values of heavy metals of farmland soils varied from 3.45 to 11.34, with an average value of 6.13, at the low ecological risk level. (3) Cu and Mn of farmland soils were mainly originated from the soil parent material and topography of the study area. As, Cd, Ni and Pb were mainly originated from human activities, and Cr and Zn may originated from both natural and anthropogenic factors in the study area.     

Cadmium soil sorption at low concentrations: V. Evidence of competition by other heavy metals

Sorption of Cd at low concentrations onto 12 Danish soils (coarse sands to sandy loams) was studied with respect to competitive effects of other heavy metals by means of laboratory batch experiments. Both a mixture of Ni, Co, and Zn and of Cr, Cu, and Pb effectively reduced the sorption of Cd onto the soils. The employed mixtures of competing heavy metals were considered to resemble moderately polluted conditions. Cadmium distribution coefficients were reduced 2 to 14 times due to competition, but at constant concentrations of competing heavy metals the shape of Cd isotherms was not affected. The effect of Ni, Co, and Zn, which like Cd is primarily governed in soil environments by sorption, was also studied individually. Apparently Zn, which is present in relatively higher concentrations than Ni and Co, accounts for most of the observed competition with Cd.     

Zeitschrift für Pflanzenernährung und Bodenkunde: Schwermetallgehalt von Wurzel und Sproßorganen der Rebe (Vitis vinifera L.) nach Düngung mit Müll-Klärschlammkompost

Heavy metal content of roots and shoots of vines (Vitis vinifera L.) after fertilization with garbage-sewage-sludge-compost The enrichment of Zn, Cu, Pb, Cd, Co, Ni and Cr from garbage-sewage-sludge-compost in vineyard soils, vines and must was studied in field-and pot-experiments. The following results were obtained: 1. In a field experiment, in which garbage-sewage-sludge-compost was applied, a marked soil enrichment of Zn, Cu, Pb, Cd and Cr was found. It was most evident at the 0–20 cm depth but also obvious at the 40–60 cm depth thus indicating downward migration. The soil was not enriched with Co and Ni. The heavy metal content of leaves, berries and must of riesling vines did not increase on the plots treated with garbage-sewage-sludge-compost. 2. In a pot trial, using an acid and an alkaline soil each mixed with garbage-sewage-sludge-compost, it was observed that only the uptake of Zn and Cu increased into the leaves, tendrils and wood of the riesling cuttings. In relation to the content of the substrate, the heavy metals were detected in the roots percentually in the following order: Cu, Cd > Zn > > Pb, Co, Ni, Cr The root contents were mostly substantially higher than those of the shoot. The migration from root to shoot decreased in the following percentual order: Zn > Cu > Cd, Pb 3. The heavy metal content decreased considerably from the roots to the upper plant organs. This was reflected in low concentrations of heavy metals in the vine must.     

Distribution of different fractions of cadmium,zinc, lead,and copper in unpolluted and polluted soils

McLaren and Crawford's method for fractionating soil Cu was modified, and used to fractionate soil Cd, Zn, Pb, and Cu in 38 soil samples from 11 soil profiles from industrially polluted and nearby unpolluted areas. Pollutant metals, especially Cd and Zn, were more soluble than the native soil metals. On average, approximately 45% of Cd was present in the CaCl₂ soluble (CA) fraction, whereas corresponding values for the other metals were below 10%. The percentages of each metal in the CA fraction followed the order Cd > Zn > Pb > Cu. The same order was observed for the acetic acid soluble (AAC) fraction. Approximately 30% of total Pb and Cu were present in the pyrophosphate soluble (PYR) fraction, and only 10% of total Cd and Zn. Approximately 20% of total Zn or Pb and 10% of Cd or Cu were present in the free oxide (OX) fraction. Only 20% of Cd and between 40–50% of the other 3 metals were present in the residual (RES) fraction. The results show that Cd is more labile than the other 3 metals.     

Mikrosondenuntersuchungen an unterschiedlich stark mit Schwermetallen belasteten Böden. 2. Gehalte an Schwermetallen und anderen Elementen in Huminstoffaggregationen,Streustoffen und Holzkohlepartikeln

Electron microprobe studies on soil samples with varying heavy metal contamination. 2. Contents of heavy metals and other elements in aggregations of humic substances, litter residues and charcoal particles EMA point analysis show that the organic matter constituents of heavy metal contaminated soils are highly enriched with heavy metals. The maximal trace element accumulation were for Cu up to 13,000 mg/kg, for Zn up to 48,000 mg/kg, for Cd up to 2,100 mg/kg and for Pb up to 193,000 mg/kg. The affinity for the accumulation of the different heavy metals in aggregations of humic substances can be described by the sequence Cu > Pb ? Cd > Zn ? Ni > Co. In very strongly acidified humic top soil horizons the Pb and Cd accumulation in the organic matter constituents is in competition with the accumulation in Fe and Mn oxides. The heavy metal contents (especially of Cu) of the organic matter are often correlated with the content of organically bound calcium. The EMA results also show that high heavy metal amounts occur in combination with Ca-accumulations in the epidermis and the outer bark parenchym of decayed roots. EMA point analysis of the interior of fungus sclerotias show that sclerotias can contain high amounts of heavy metals, in particular lead (up to 49,700 mg Pb/kg). From statistical results of EMA point analysis follows that lead and other heavy metals attached to humic substances are not only bound as metal organic complexes but also as organic metal phosphate complexes. Also charcoal particles of polluted soils contain high amounts of heay metals. The accumulation affinity is quite similar to that of humic substances.     

Heavy‐Metal Contamination of Soil and Vegetables in Wastewater‐Irrigated Agricultural Soil in a Suburban Area of Hanoi,Vietnam

The To Lich and Kim Nguu Rivers, laden with untreated waste from industrial sources, serve as sources of water for irrigating vegetable farms. The purposes of this study were to identify the impact of wastewater irrigation on the level of heavy metals in the soils and vegetables and to predict their potential mobility and bioavailability. Soil samples were collected from different distances from the canal. The average concentrations of the heavy metals in the soil were in the order zinc (Zn; 204 mg kg^?1) > copper (Cu; 196 mg kg^?1) > chromium (Cr; 175 mg kg^?1) > lead (Pb; 131 mg kg^?1) > nickel (Ni; 60 mg kg^?1) > cadmium (Cd; 4 mg kg^?1). The concentrations of all heavy metals in the study site were much greater than the background level in that area and exceeded the permissible levels of the Vietnamese standards for Cd, Cu, and Pb. The concentrations of Zn, Ni, and Pb in the surface soil decreased with distance from the canal. The results of selective sequential extraction indicated that dominant fractions were oxide, organic, and residual for Ni, Pb, and Zn; organic and oxide for Cr; oxide for Cd; and organic for Cu. Leaching tests for water and acid indicated that the ratio of leached metal concentration to total metal concentration in the soil decreased in the order of Cd > Ni > Cr > Pb > Cu > Zn and in the order of Cd > Ni > Cr > Zn > Cu > Pb for the ethylenediaminetetraaceitc acid (EDTA) treatment. The EDTA treatment gave greater leachability than other treatments for most metal types. By leaching with water and acid, all heavy metals were fully released from the exchangeable fraction, and some heavy metals were fully released from carbonate and oxide fractions. The concentrations of Cd, Cr, Cu, Ni, Pb, and Zn in the vegetables exceeded the Vietnamese standards. The transfer coefficients for the metals were in the order of Zn > Ni > Cu > Cd = Cr > Pb.     

Competitive sorption of cadmium and lead in acid soils of Central Spain

The bioavailability and ultimate fate of heavy metals in the environment are controlled by chemical sorption. To assess competitive sorption of Pb and Cd, batch equilibrium experiments (generating sorption isotherms) and kinetics sorption studies were performed using single and binary metal solutions in surface samples of four soils from central Spain. For comparisons between soils, as well as, single and binary metal solutions, soil chemical processes were characterized using the Langmuir equation, ionic strength, and an empirical power function for kinetic sorption. In addition, soil pH and clay mineralogy were used to explain observed sorption processes. Sorption isotherms were well described by the Langmuir equation and the sorption kinetics were well described by an empirical power function within the reaction times in this study. Soils with higher pH and clay content (characterized by having smectite) had the greatest sorption capacity as estimated by the maximum sorption parameter (Q) of the Langmuir equation. All soils exhibited greater sorption capacity for Pb than Cd and the presence of both metals reduced the tendency for either to be sorbed although Cd sorption was affected to a greater extent than that of Pb. The Langmuir binding strength parameter (k) was always greater for Pb than for Cd. However, these k values tended to increase as a result of the simultaneous presence of both metals that may indicate competition for sorption sites promoting the retention of both metals on more specific sorption sites. The kinetic experiments showed that Pb sorption is initially faster than Cd sorption from both single and binary solutions although the simultaneous presence of both metals affected the sorption of Cd at short times while only a minor effect was observed on Pb. The estimated exponents of the kinetic function were in all cases smaller for Pb than for Cd, likely due to diffusion processes into micropores or interlayer space of the clay minerals which occurs more readily for Cd than Pb. Finally, the overall sorption processes of Pb and Cd in the smectitic soil with the highest sorption capacity of the studied soils are slower than in the rest of the soils with a clay mineralogy dominated by kaolinite and illite, exhibiting these soils similar sorption rates. These results demonstrate a significant interaction between Pb and Cd sorption when both metals are present that depends on important soil properties such as the clay mineralogy.     

Response of the sorption behavior of Cu,Cd, and Zn to different soil management

This study investigated the effect of different farming practices over long time periods on the sorption‐desorption behavior of Cu, Cd, and Zn in soils. Various amendments in a long‐term field experiment over 44 y altered the chemical and physical properties of the soil. Adsorption isotherms obtained from batch sorption experiments with Cu, Cd, and Zn were well described by Freundlich equations for adsorption and desorption. The data showed that Cu was adsorbed in high amounts, followed by Zn and Cd. In most treatments, Cd ions were more weakly sorbed than Cu or Zn. Generally, adsorption coefficients K_F increased among the investigated farming practices in the following order: sewage sludge ≤ fallow < inorganic fertilizer without N ≈ green manure < peat < Ca(NO₃)₂ < animal manure ≤ grassland/extensive pasture. The impact of different soil management on the sorption properties of agricultural soils for trace metals was quantified. Results demonstrated that the soil pH was the main factor controlling the behavior of heavy metals in soil altered through management. Furthermore, the constants K_F and n of isotherms obtained from the experiments significantly correlated with the amount of solid and water‐soluble organic carbon (WSOC) in the soils. Higher soil pH and higher contents of soil organic carbon led to higher adsorption. Carboxyl and carbonyl groups as well as WSOC significantly influenced the sorption behavior of heavy metals in soils with similar mineral soil constituents.     

The transfer of heavy metals to barley plants from soils amended with sewage sludge with different heavy metal burdens

Purpose

Our main aim objective was to evaluate the transfer of Cd, Cr, Cu, Ni, Pb and Zn to barley (Hordeum vulgare) grown in various soils previously amended with two sewage sludges containing different concentrations of heavy metals. This allowed us to examine the transfer of heavv metals to barley roots and shoots and the occurrence of restriction mechanisms as function of soil type and for different heavy metal concentration scenarios.

Material and methods

A greenhouse experiment was performed to evaluate the transfer of heavy metals to barley grown in 36 agricultural soils from different parts of Spain previously amended with a single dose (equivalent to 50 t dry weight ha^?1) of two sewage sludges with contrasting levels of heavy metals (common and spiked sludge: CS and SS).

Results and discussion

In soils amended with CS, heavy metals were transferred to roots in the order (mean values of the bio-concentration ratio in roots, BCF_Roots, in brackets): Cu (2.4)?~?Ni (2.3)?>?Cd (2.1)?>?Zn (1.8)?>?Cr (0.7)?~?Pb (0.6); similar values were found for the soils amended with SS. The mean values of the soil-to-shoot ratio were: Cd (0.44)?~?Zn (0.39)?~?Cu (0.39)?>?Cr (0.20)?>?Ni (0.09)?>?Pb (0.01) for CS-amended soils; Zn (0.24)?>?Cu (0.15)?~?Cd (0.14)?>?Ni (0.05)?~?Cr (0.03)?>?Pb (0.006) for SS-amended soils. Heavy metals were transferred from roots to shoots in the following order (mean values of the ratio concentration of heavy metals in shoots to roots in brackets): Cr (0.33)?>?Zn (0.24)?~?Cd (0.22)?>?Cu (0.19)?>?Ni (0.04)?>?Pb (0.02) for CS-amended soils; Zn (0.14)?>?Cd (0.09)?~?Cu (0.08)?>?Cr (0.05)?>?Ni (0.02)?~?Pb (0.010) for SS-amended soils.

Conclusions

Soils weakly restricted the mobility of heavy metals to roots, plant physiology restricted the transfer of heavy metals from roots to shoots, observing further restriction at high heavy metal loadings, and the transfer of Cd, Cu and Zn from soils to shoots was greater than for Cr, Ni and Pb. Stepwise multiple linear regressions revealed that soils with high sand content allowed greater soil-plant transfer of Cr, Cu, Pb and Zn. For Cd and Ni, soils with low pH and soil organic C, respectively, posed the highest risk.     

Inter-relationships between soil properties and the uptake of cadmium,copper, lead and zinc from contaminated soils by radish (Raphanus sativus L.)

Radish was grown in 46 garden plots in England and Wales. Some of the gardens had been contaminated by heavy metals from lead mining. The soils were analysed for pH, organic content and cation exchange capacity; also for Cd, Cu, Pb, and Zn (total, organic bound, exchangeable and specifically sorbed). Acetic acid-soluble P and exchangeable K, Mg and Zn were also determined. Radish bulbs and leaves were analysed for heavy metals. The results were interpreted using linear and multiple linear correlation and regression analysis. Acetic acid satisfactorily predicted Cd uptake and Pb uptake was best predicted by total soil Pb. These regressions were not improved by including other soil properties. Zinc uptake was best modeled using exchangeable Zn and the predictive power of the regression was improved by including pH. However, the pH term was positive suggesting that raising soil pH would increase uptake. A poor relationship between total and exchangeable Zn was changed to a highly significant relationship by including cation exchange capacity and pH. The latter term was strongly negative. Uptake of Cu was not satisfactorily predicted.     

Effects of Chelating Compounds on Mobilization and Phytoextraction of Copper and Lead in Contaminated Soils

Six chelating compounds: ethylenediamine-tetraacetic acid (EDTA), ethylenediamine-N, N'-disuccinic acid (EDDS), tartaric acid, citric acid, glycine and histidine, were tested as potential agents to mobilize copper (Cu) and lead (Pb) from two soils polluted with the emissions from copper smelters. Copper was mobilized with the following efficiency: EDTA > citric and tartaric acids > histidine > EDDS and glycine, while Pb extractability followed the order: EDTA > EDDS >> tartaric and citric acid >> glycine and histidine. With respect to these results, EDTA and EDDS were chosen for a pot experiment on chelate-induced phytoextraction of Cu and Pb by maize (Zea mays). Chelates were applied at the rates of 0.2, 0.5, and 1.0 mmol kg^?1, and this experiment was carried out at two different watering regimes. Both EDTA and EDDS caused significant increase of Cu uptake from soils, but its concentrations in biomass were far below those required for efficient soil remediation. Lead uptake was only slightly affected by chelate application. Losses of Cu from soil by leaching were much higher than those caused by plant uptake.     

EVALUATION OF HEAVY METAL REMEDIATION USING MINERAL APATITE

The current study investigated the sorption and desorption of dissolved lead (Pb), cadmium (Cd) and zinc (Zn) from aqueous solutions and a contaminated soil by North Carolina mineral apatite. Aqueous solutions of Pb, Cd, and Zn were reacted with the apatite, followed by desorption experiments under a wide variety of pH conditions ranging from 3 to 12, including the extraction fluids used in the Toxicity Characteristic Leaching Procedure (TCLP) of the United States Environmental Protection Agency (US EPA). The sorption results showed that the apatite was very effective in retaining Pb and was moderately effective in attenuating Cd and Zn at pH 4–5. Approximately 100% of the Pb applied was removed from solutions, representing a capacity of 151 mg of Pb/g of apatite, while 49% of Cd and 29% of Zn added were attenuated, with removal capacities of 73 and 41 mg g^-1, respectively. The desorption experiments showed that the sorbed Pb stayed intact where only 14–23% and 7–14% of the sorbed Cd and Zn, respectively, were mobilized by the TCLP solutions. The apatite was also effective in removing dissolved Pb, Cd, and Zn leached from the contaminated soil using pH 3–12 solutions by 62.3–99.9, 20–97.9, and 28.6–98.7%, respectively. In particular, the apatite was able to reduce the metal concentrations in the TCLP-extracted soil leachates to below US EPA maximum allowable levels, suggesting that apatite could be used as a cost-effective option to remediating metal-contaminated soils, wastes, and/or water. The sorption mechanisms are variable in the reactions between the apatite and dissolved Pb, Cd, and Zn. The Pb removals primarily resulted from the dissolution of the apatite followed by the precipitation of hydroxyl fluoropyromorphite. Minor otavite precipitation was observed in the interaction of the apatite with aqueous Cd, but other sorption mechanisms, such as surface complexation, ion exchange, and the formation of amorphous solids, are primarily responsible for the removal of Zn and Cd.     

Heavy Metal Concentrations,Partitioning, and Storage in Slovak Forest and Arable Soils Along a Deposition Gradient

Along a heavy metal deposition gradient, caused by a Cu smelter, heavy metal concentrations, partitioning, and storage in forest and arable soils were examined. We sampled organic and mineral soil horizons (0—50 cm) at ten pairs of forest and arable sites derived from the same parent material. A-horizons were extracted with a seven-step sequence; O- and subsoil horizons were digested with strong acids (HNO₃/HClO₄). We found high concentrations of Cd (up to 17.38 mg kg^—1 in the O horizons/up to 2.44 mg kg^—1 in the A horizons), Cu (8437/415), Pb (3343/126), and Zn (1482/637) which decreased exponentially with distance from the smelter and with soil depth. The metal concentrations in the organic layers indicate that the average transport distance decreases in the order Cd > Zn > Pb > Cu. With regard to metal partitioning, NH₄NO₃- + NH₄OAc-extractable forms in the A horizons were most affected by the deposition being more pronounced under forest. In the uppermost 50 cm of the four soils nearest to the smelter two to four times higher Cd, Cu, Pb, and Zn storages were found in forest than in arable soils. At greater distance, the higher deposition onto forest soils due to the scavenging effect of the canopy obviously was compensated by stronger leaching.     

Mixed chelators of EDTA,GLDA, and citric acid as washing agent effectively remove Cd,Zn, Pb,and Cu from soils

Purpose

Soil washing with chelators is a viable treatment alternative for remediating multi-contaminated soils. The aim of this study was to investigate the removal efficiencies of Cd, Zn, Pb, and Cu in alkaline and acid multi-metal-contaminated soils by washing with the mixed chelators (MC).

Materials and methods

The batch experiments were carried out to evaluate the removal efficiencies of heavy metals in contaminated soils by the MC with different molar ratios of EDTA, GLDA, and citric acid, and evaluated the washing factors, including contact time, pH, MC concentration, and single and multiple washings at the same MC dose, on the removal efficiencies.

Results and discussion

Results showed that the removal efficiencies for Cd, Zn, Pb, and Cu by the MC (the molar ratio of EDTA, GLDA, and citric acid was 1:1:3) were as much as those of the only EDTA washing from both soil at the same application dose of total chelators; moreover, the application dose of EDTA decreased by 80%. For the alkaline-contaminated soil, the removal efficiencies of Cd, Zn, Pb, and Cu decreased with the increasing of the solution pH, which was opposite to acid-contaminated soil. This was attributed to that the metal-ligand complex could be obviously re-adsorbed on the soil surface sites, particularly in low pH values. The removal efficiencies of Cd, Zn, Pb, and Cu depended on MC concentration. A higher MC concentration led to a more effective removal of Cd, Zn, Pb, and Cu in alkaline-contaminated soil; however, their changes were slightly increased in acid-contaminated soil. At the same dose of MC, single washing with higher MC concentration might be favorable to remove heavy metals, moreover, with much less wastewater generation.

Conclusions

The MC (the molar ratio of EDTA, GLDA, and citric acid was 1:1:3) may be a useful, environmentally friendly, and cost-effective chelators to remediate heavily multi-metal-contaminated soil.