Cd Concentration In Soil Solution Research Articles

Prediction of soil solution concentrations and leaching losses of cadmium in agricultural soils.

Cadmium (Cd) leaching is often estimated in Cd balance models using the product of drainage water (precipitation excess) and soil solution Cd concentration. However, Cd concentrations are seldom available but rather predicted using empirical models. Despite the availability of empirical models that estimate soil solution Cd concentrations, they have limitations for use in New Zealand where total Cd in agricultural soils is low and organic matter (OM) high. This study derived a Freundlich-type model from desorption data and a soil-liquid partitioning coefficient (KD ) model based on sorption data to predict soil solution Cd concentrations from commonly measured soil parameters that could be used to calculate Cd leaching fluxes. Independent soil solution Cd concentrations and Cd fluxes measured in drainage water from field trials were used to validate the predictive capacity of the models. It was found that soil pH and OM content were the most important factors controlling soil solution Cd, along with total Cd. Both models explained 83% of the variation in measured soil solution Cd concentrations in an independent dataset. Comparisons between Cd fluxes predicted using the Freundlich-type model and measured fluxes were within 25% of each other at 6 of 19 field sites studied. However, physical and chemical nonequilibrium conditions in soils and uncertainty in measured values likely contributed to differences between predicted and measured Cd fluxes at other sites. To unravel the impact of nonequilibrium and soil physical conditions on Cd concentrations in drainage water, more data are required on Cd concentrations collected under field conditions. This will allow better validation of the approach used in Cd balance models to calculate Cd leaching from soils.

Simultaneous Immobilization of Arsenic, Lead, and Cadmium in Paddy Soils Using Two Iron-based Materials

Two iron-based materials, Fe-Ca composite (FeCa) and Fe-Mn binary oxide (FMBO), were applied to immobilize As, Pb, and Cd in heavy metal contaminated paddy soils. Seven kinds of paddy soil (tidal soil) contaminated by arsenic, lead and cadmium were collected from Shangyu, Shaoxing (SY), Foshan, Guangdong (FS), Shaoguan, Guangdong (SG), LiuYang, Hunan (LY), Ganzhou, Jiangxi (GZ), Dushan, Guizhou (DS), and Ma'anshan, Anhui (MAS). The effects of iron-based materials on the dynamic changes of As, Pb, and Cd concentration in soil solution, the stabilization efficacy of available As, Pb, and Cd in soil, and the effects of soil types and properties on stabilization efficacy were studied through soil incubation experiment. The results showed that the content of soil dissolved As, Pb, and Cd were lower in iron-based material treatments than in control throughout the incubation. The addition of two iron-based materials significantly reduced the availability of Cd, Pb, and As. Moreover, the stabilization efficiency of FeCa for As was higher than FMBO, but no significant difference was found in the stabilization efficiency of Pb and Cd between two materials. The stabilization efficiency of As, Pb, and Cd in FeCa treatments could be ordered as GZ > SG > DS and MAS; FS>SY, LY, and SG>MAS; SY, GZ, and DS>MAS, respectively. While the stabilization efficiency for As, Pb, and Cd in FMBO could be ordered as SY, LY, and GZ > DS > FS; FS > GZ > SY; DS > LY > MAS, respectively. In addition, the statistical results showed that the stabilization efficiencies of various soils under the treatment of iron-based materials were significantly correlated with sand content (negatively correlated for As), soil pH (positively correlated for Pb), and clay content (negatively correlated for Cd). In conclusion, the two iron-based materials evaluated in this study may be effective stabilization agents for remediating different types of arsenic-, lead-, and cadmium-contaminated soils.

Effects of Water Management on Soil Properties and Cd Behavior of Typical Paddy Soils

To explore the effects of water management mode on Cd environmental behavior in different parent-material-developed paddy soils, two parent-material-developed paddy soils (yellow clayey soil and granitic sandy soil) under three exogenous Cd levels (0.5, 2.0, and 5.0 mg·kg-1) with different water management modes (long-term flooding, moistening irrigation, and wet-dry rotation) were cultured in this study. The soil redox potential (Eh value), pH value, Cd concentration in soil solution, and Cd fractionation were also determined. The results showed that water management mode had different effects on the pH and Eh values of soils developed from different parent materials. The change rates of soil pH value were as follows:long-term flooding:-2.61% (yellow clayey soil), 2.25% (granitic sandy soil); alternation of dry and wet:-1.96% (yellow clayey soil); 0.52% (granitic sandy soil); wet irrigation:-4.08% (yellow clayey soil) and -0.52% (granitic sandy soil). The Eh value of the soils was negatively correlated with the pH value. The influence pattern of water management model on Cd mass concentration of soil solutions in two parent-material soils was consistent. The Cd mass concentration of soil solutions in granitic sandy soil was higher than that in yellow clayey soil. The mean values of Cd concentration were 1.03 μg·L-1 for yellow clayey soil and 1.07 μg·L-1 for granitic sandy soil. Water management mode had no significant effect on the proportions of organic bound Cd or Fe-Mn bound Cd in two different parent-material-developed soils. The long-term flooding mode promoted the transformation of exogenous Cd to residual Cd, and this promotion in yellow clayey soil was higher than that in granitic sandy soil. In conclusion, during the process of regulating soil Cd bioavailability through water management, the role of soil parent materials needs to be considered.

Effects of the addition and aging of humic acid-based amendments on the solubility of Cd in soil solution and its accumulation in rice

Humic substances can reduce mobility and bio-accessibility of Cd in soil and therefore inhibit its uptake by rice, which is a major source of human Cd intake. Yet, the effects of aging humic substances are not fully understood. A rice pot experiment was conducted to evaluate the effects of humic acid-based amendments on the mobility of Cd in soil solution and its uptake by rice when amendments were freshly added or aged for 130 d. The results showed that the aged and the unaged amendments generally decreased Cd concentration in soil solution, but the effect declined with time. Unaged HA-K (humic-potassium) reduced Cd concentration by 88% for the first sampling, but this dropped to 46% for the last sampling, compared to that of the control. All amendments, whether aged or not, reduced the content of Cd in rice seedlings, as well as in mature plants. Aged and unaged woody peat reduced the Cd content in seedlings by 79% and in grains by 70%, respectively. Aging of amendments caused lower pH and higher Cd concentration in the soil solution for all amendments and accordingly, the Cd content in rice seedlings or each part of mature plants in the aged group was higher than that of the unaged group. The applied amendments might reduce the solubility of Cd through the alteration in soil pH, and thus inhibit the uptake of Cd by rice, but the effects diminished with aging.

Correlations Between Different Extractable Cadmium Levels in Typical Soils and Cadmium Accumulation in Rice

Pot experiments were used to study the correlations between different extractable cadmium levels in typical soil and cadmium accumulation in rice. To analyze the pH, Cd in soil solution(SSE-Cd), TCLP extractable Cd level(TCLP-Cd), and Cd accumulation in rice at different growth stages of rice, we conducted pot experiments which selected the reddish clayey soil(developed from quaternary red clay parent materials) and purple paddy field(developed from purple sandy shale parent materials), meanwhile added with exogenous Cd with the gradients of 0, 0.5, 1, 2, 5, 10 mg·kg-1. The results showed that, during the rice growth period, the content of SSE-Cd in reddish clayey soil was in the range of 0 and 2.5 μg·L-1, and the average content was 0.57 μg·L-1; TCLP-Cd was in the range of 0 and 0.25 μg·L-1 with the average content of 0.10 mg·kg-1;The content of SSE-Cd in purple paddy field was in the range of 0 and 1.6 μg·L-1 with the average content of 0.48 μg·L-1; TCLP-Cd was in the range of 0 and 0.2 mg·kg-1, and the average content was 0.07 mg·kg-1. It showed that the cadmium concentrations in soil solution and the TCLP extractable Cd levels were both significantly reduced in two types of soil with the extension of rice growth period, and the content in reddish clayey soil was higher than that in purple paddy. The TCLP extractable Cd level was significantly positively correlated with Cd concentration in soil solution. The total Cd accumulation in rice plants gradually increased with increasing exogenous Cd concentration. There were significant positive correlations between Cd concentration in soil solution and Cd concentration in rice, Cd concentration in soil extracted by TCLP method and Cd concentration in rice and total Cd accumulation in rice plant. The soil environmental capacities of the two different parent materials varied greatly,and the safety threshold of Cd in purple paddy field was 2.06 times of that of reddish clayey soil. There were significant differences in Cd uptake and accumulation in different soils, so different measures may be needed to control Cd pollution in different parent materials. TCLP extractable Cd was more relevant with total Cd accumulation in rice, and had more extraction amount. Therefore, the TCLP method can more accurately evaluate the biological availability of soil Cd.

Cadmium removal from Cd-contaminated soils using some natural and synthetic chelates for enhancing phytoextraction

ABSTRACTThe influence of a natural (citric acid) and two synthetic chelates (HEDTA, EGTA) on Cd phytoextraction with radish was studied. The experiment was conducted with Cd treatments including 0 (control), 5, 20, 60 and 100 mg Cd kg−1 soil. When plants were fully developed, the designated HEDTA, EGTA and citric acid were applied to establish experimental treatments. Ten days after chelate applications, soil and plant samples were taken. Results indicated that synthetic chelates can considerably increase Cd concentration in soil solution compared with citric acid. Among the applied chelates, HEDTA provided more significant influences on soil solution. Cadmium concentration in both shoots and roots of radish was increased due to enhancing influence of chelate applications on Cd concentration in soil solution. Cadmium concentration in radish shoots was more than that of roots. This can be attributed to higher bioavailability and solubility of Cd in soil solution. In all treatments which received HEDTA, the cadmium concentration in both shoots and roots of radish was increased more than that of other applied chelates. Although HEDTA is more efficient in phytoextracting Cd than other chelates, due to durability of synthetic chelates toxic effects on soils, we recommend using larger amounts of light acids.

Simultaneous decrease of arsenic and cadmium in rice (Oryza sativa L.) plants cultivated under submerged field conditions by the application of iron-bearing materials

ABSTRACTThe Codex Alimentarius Commission has recently adopted maximum levels for inorganic arsenic (As; in 2014) and total cadmium (Cd; in 2006) in polished rice grains to maintain food safety and to decrease the risk to human health. As rice is a staple crop in Japan and monsoon Asian countries, reducing concentrations of As and Cd in rice is an urgent matter. In flooded conditions, Cd concentration in soil solution decreases whereas As concentration increases. Therefore, we aimed to evaluate the efficiency of iron-bearing materials to decrease As concentration in soil solution and rice (Oryza sativa L.) grain under submerged cultivation, while also considering Cd concentration. In experiments conducted in paddy fields in six regions, As concentrations in the soil solution during the cultivation period decreased in the following order: control (REF) > steel converter furnace slag (SCS) > non-crystalline iron hydroxide (FH) > zero-valent iron (ZVI). The concentrations of As in brown rice were in the same order, with ZVI achieving particularly strong reduction. Cadmium concentrations were low, probably owing to submerged cultivation conditions. Application of iron-bearing materials slightly and insignificantly reduced the yields of brown rice and straw. Application of these materials did not have a significant negative impact on the quality of rice. Our data indicate that the application of iron-bearing materials effectively reduces As concentrations in soil solution and rice grains without negative effects on yield and quality, with a particularly powerful effect of ZVI which is possibly explained by arsenic sulfide formation.

Zerovalent Iron with High Sulfur Content Enhances the Formation of Cadmium Sulfide in Reduced Paddy Soils

Core Ideas Sulfur impurities in ZVI affect Cd chemical speciation and solubility in reduced soils. Exchangeable Cd decreased in the ZVI‐treated soil with greater S content. The addition of ZVI with high levels of S enhanced the formation of CdS. The objective of this study was to investigate the role of S impurity in zerovalent iron (ZVI) on the chemical speciation and solubility of Cd in the reduced soils. Two types of ZVI with contrastingly different S levels (0.009 and 1.08%) were used to find how the solubility and speciation of Cd would be influenced by the S impurity in ZVIs. Synchrotron‐based X‐ray absorption fine structure (XAFS) spectroscopy was used to determine the relative proportion of CdS and other species in the Cd‐spiked soils amended with ZVIs with different S levels (hereafter low‐ and high‐S ZVIs). Microscale distribution and speciation of Cd at the interface between ZVI and soil particles were investigated using micro‐X‐ray fluorescence (μ‐XRF) and micro‐XAFS (μ‐XAFS) spectroscopy. The difference in S contents in ZVIs did not affect the soil solution Cd concentrations, but a significant decrease in exchangeable Cd was found in the soil with added high‐S ZVI. Linear combination fitting (LCF) on Cd K‐edge XAFS spectra of bulk soils determined up to 16% CdS in the reduced soils amended with ZVIs. The μ‐XRF map of the soil amended with high‐S ZVI showed that the spots with S accumulation were discretely distributed on the ZVI, and some S accumulating areas corresponded to Cd localization. The LCF on Cd K‐edge μ‐XAFS spectra for selected soil particles revealed that the proportion of CdS ranged from 20 to 87% (avg. 53 ± 22%) in the soil with low‐S ZVI and from 64 to 98% (avg. 84 ± 14%) in the soil with high‐S ZVI. A higher S content in ZVI, and probably in other amendments, decreases labile Cd fractions and enhances CdS formation in reduced soils.

Future trends in soil cadmium concentration under current cadmium fluxes to European agricultural soils

The gradual increase of soil cadmium concentrations in European soils during the 20th century has prompted environmental legislation to limit soil cadmium (Cd) accumulation. Mass balances (input–output) reflecting the period 1980–1995 predicted larger Cd inputs via phosphate (P) fertilizers and atmospheric deposition than outputs via crop uptake and leaching. This study updates the Cd mass balance for the agricultural top soils of EU-27+Norway (EU-27+1). Over the past 15years, the use of P fertilizers in the EU-27+1 has decreased by 40%. The current mean atmospheric deposition of Cd in EU is 0.35gCdha−1yr−1, this is strikingly smaller than values used in the previous EU mass balances (~3gCdha−1yr−1). Leaching of Cd was estimated with most recent data of soil solution Cd concentrations in 151 soils, which cover the range of European soil properties. No significant time trends were found in the data of net applications of Cd via manure, compost, sludge and lime, all being small sources of Cd at a large scale. Modelling of the future long-term changes in soil Cd concentrations in agricultural top soils under cereal or potato culture predicts soil Cd concentrations to decrease by 15% over the next 100years in an average scenario, with decreasing trends in some scenarios being more prevalent than increasing trends in other scenarios. These Cd balances have reverted from the general positive balances estimated 10 or more years ago. Uncertainty analysis suggests that leaching is the most uncertain relative to other fluxes.

Mobilization of cadmium by dissolved organic matter in the rhizosphere of hyperaccumulator Sedum alfredii

Pot experiments were conducted to investigate the role of dissolved organic matter (DOM) in the Cd speciation in the rhizosphere of hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of Sedum alfredii and its effects on Cd mobility. After growing HE S. alfredii, the rhizosphere soil solution pH of heavily polluted soil (HPS) and slightly polluted soil (SPS) was reduced by 0.49 and 0.40 units, respectively, due to enhanced DOC derived from root exudation. The total Cd concentration in soil solution decreased significantly but the decrease accounted for less than 1% of the total Cd uptake in the shoots of HE S. alfredii. Visual MINTEQ speciation predicted that Cd–DOM complexes were the dominant Cd species in soil solutions after the growth of S. alfredii for both soils, followed by the free metal Cd2+ species. However, Cd–DOM complexes fraction in the rhizosphere soil solution of HE S. alfredii (89.1% and 74.6% for HPS and SPS, respectively) were much greater than NHE S. alfredii (82.8% and 64.7% for HPS and SPS, respectively). Resin equilibration experiment results indicated that DOM from the rhizosphere (R-DOM) of both ecotypes of S. alfredii had the ability to form complexes with Cd, whereas the degree of complexation was significantly higher for HE-R-DOM (79–89%) than NHE-R-DOM (63–74%) in the undiluted sample. The addition of HE-R-DOM significantly (P<0.05) increased the solubility of four Cd minerals while NHE-R-DOM was not as effective at the same concentration. It was concluded that DOM in the rhizosphere of hyperaccumulating ecotype of S. alfredii could significantly increase Cd mobility through the formation of soluble DOM–metal complexes.

Shoot cadmium concentration of soil‐grown plants as related to their root properties

AbstractCadmium (Cd) is toxic to plants, animals, and humans. However, different plant species growing on the same soil may have very different shoot Cd concentrations depending on properties such as size of the root system, Cd net influx, shoot‐growth rate, Cd translocation from root to shoot, and the ability to affect Cd availability in the soil. To investigate possible reasons for different shoot Cd concentrations maize, sunflower, flax, and spinach were grown on an acid sandy soil (pH&lt;$&gt;_{{\rm{(CaCl}}_{\rm{2}} {\rm)}&lt;$&gt; 4.5, and Corg 2.8%) in a growth chamber with Cd additions as Cd(NO3)2 of none, 14, and 40 μmol (kg soil)–1 resulting in Cd soil‐solution concentrations of 0.04, 0.68, and 2.5 μM. Only the high Cd addition caused a significant growth reduction of flax and spinach. The shoot Cd concentration was up to 30 times higher in spinach than in maize; the other species were intermediate. Of the plant properties studied only the variation of the Cd net influx explained the differences in shoot Cd concentrations. This was due to a decreased (maize, sunflower) or increased (flax) Cd concentration in soil solution or more effective uptake kinetics (spinach).

A widely applicable model for predicting the cadmium concentration in soil solution

We developed a method for predicting the total cadmium (Cd) concentration [CdT] in soil solution. Disregarding minor ion pairs, [CdT] was approximated by . We assumed that Cd2+ activity (Cd2+) is dominated by the adsorption–desorption equilibrium, expressed stoichiometrically as . From this, we derived the balance equation We calculated (CdCl+) as the product of (Cd2+), chloride activity (Cl–), and a production constant. Our proposed prediction model was We conducted an experiment using six types of soil and manipulated the Cd content, pH, and chloride concentration [Cl−] to determine the effect on Cd concentration in the soil solution. The soil solution measurements produced rectilinear regression relationships between log(Cd2+) and log Q Cd – 0.6pH with slopes of 1.0 in all soil types, which validated the proposed Cd adsorption–desorption stoichiometry; the y-intercept of the regression lines represented log(K Cd/C Cd) and depended on the soil type. Furthermore, we were able to predict log(K Cd/C Cd) as a function of cation exchange capacity (CEC), and the activity coefficient (f Cd) as a function of electrical conductivity (EC) alone. Substituting these functions into the proposed prediction model, we obtained [CdT] = 0.288Q Cd10−0.6pH CEC −2.37{1 + 2.23EC 0.490 + 95.5[Cl−]}. The model provided a high goodness-of-fit (r 2 = 0.956) between the predicted and measured values over a concentration range of more than five-orders of magnitude. This prediction equation therefore appears to have wide applicability, suggesting that Cd concentration in soil solution can be reliably predicted from measurements of Cd content, pH, CEC, EC, and [Cl−].

Concentration of cadmium in durum wheat as affected by time, source and placement of nitrogen fertilization under reduced and conventional-tillage management

Cadmium concentration in durum (Triticum durum L.) grain may be influenced by fertilizer management. A three year field study was conducted at two locations in southwestern Manitoba, Canada, to determine the effect of source, timing and placement of N fertilizer on grain Cd concentration of durum wheat under reduced-tillage (RT) and conventional-tillage (CT) management. There was a significant year-to-year variation in grain yield and grain Cd concentration, indicating a strong effect of environment on phytoavailability of Cd. Soil type also had a dominating effect on Cd in durum grain. Grain Cd concentration and accumulation were always lower at the Newdale clay loam (CL) location than the Stockton fine sandy loam (FSL) location. Compared to CT, RT management decreased grain Cd concentration and accumulation. Application of N fertilizer significantly increased grain Cd concentration and decreased grain Zn concentration. The increase in grain Cd could be due to the increasing Cd concentration in soil solution, the improved crop growth, or a competitive interaction between Cd and Zn for binding sites in the soil system and for uptake sites in the roots, following the application of N fertilizer. Time, source and placement of N fertilization showed inconsistent effect on grain yield and grain Cd concentration. The effects when observed were minor compared to effects of year and soil type on grain Cd concentration. Application of anhydrous ammonia (NH3) generally resulted in higher Cd concentration in durum grain than application of same level of other N sources including urea ammonium nitrate (UAN), urea and ammonium nitrate (AN), probably due to greater N efficiency from the NH3 as compared to the other sources. Where differences due to placement occurred, banded application of fertilizers generally resulted in higher Cd concentration in durum grain than did dual-banded placement, indicating a higher fertilizer availability. Soil type, year and N fertilization greatly affected concentration of Cd in durum grain. Selection of a suitable source, timing and placement combination of N fertilization is important in optimizing crop yield and minimizing grain Cd concentration.

Determining cadmium critical concentrations in natural soils by assessing Collembola mortality, reproduction and growth

The toxicity of cadmium for the Collembola Folsomia candida was studied by determining the effects of increasing Cd concentrations on growth, survival and reproduction in three cultivated and forested soils with different pH (4.5–8.2) and organic matter content (1.6–16.5%). The Cd concentration in soil CaCl 2 exchangeable fraction, in soil solution and in Collembola body was determined. At similar total soil concentrations, the Cd concentration in soil solutions strongly decreased with increasing pH. Reproduction was the most sensitive parameter. Low organic matter content was a limiting factor for reproduction. Effect of Cd on reproduction was better described by soil or body concentrations than by soil solution concentration. Values of EC 50-Repro expressed on the basis of nominal soil concentration were 182, 111 and 107 μg g −1, respectively, for a carbonated cultivated soil (AU), an acid forested soil with high organic matter (EPC) and a circumneutral cultivated soil with low organic content (SV). Sensitivity to Cd was enhanced for low OM content and acidic pH. The effect of Cd on reproduction is not directly related to Cd concentration in soil solution for carbonated soil: a very low value is found for EC 50-Repro (0.17) based on soil solution for the soil with the highest pH (AU; pH=8.2). Chronic toxicity cannot be predicted on the basis of soluble fractions. Critical concentrations were 8×10 −5, 1.1, 0.3 μg mL −1, respectively, for AU, EPC and SV soils.

Cadmium Release in Soil Solutions at Low Moisture Content

Speciation of cadmium (Cd) was studied in four spiked agricultural soils at moisture content corresponding to 1.2 times field moisture capacity (FMC) and in the range from 1.2 FMC to soil–water 1∶10. Cadmium desorption isotherms were nonlinear in all soils, resulting in the decrease in Cd partition coefficient with loading. The Windermere Humic Aqueous Model (WHAM VI) was applied to predict Cd concentration in the solutions, and predicted values were compared with the measured ones. Based on total Cd content in soils, with concentrations of dissolved organic carbon (DOC), calcium (Ca), magnesium (Mg), and sodium (Na) and soil solution pH as the input variables, WHAM VI predicted Cd concentration in soil solutions with the root‐mean‐square error (RMSE) of log[Cd] RMSElog[Cd]=0.54 (n=37). Using total Cd content in soils, average concentrations of Ca and DOC in soil solutions, and soil pH instead of soil solution pH enabled prediction of Cd concentration in soil solution with RMSElog[Cd]=0.56. Calculation of Cd concentration as a function of moisture content resulted in RMSElog[Cd]=0.25 (n=20).

A Regional‐Scale Study on the Crop Uptake of Cadmium from Sandy Soils

Plant uptake is one of the major pathways by which cadmium (Cd) in soils enters the human food chain. This study was conducted to investigate the uptake of Cd by crops from soils within the wastewater irrigation area (WIA) of Braunschweig (Germany) and to develop a simple process-oriented model that is suited to predict Cd uptake at the regional scale. The sandy soils within the WIA (4300 ha) have received considerable loads of heavy metals by irrigation using municipal wastewater for up to 40 years. In 1998 and 1999, we sampled soil and plant material at 40 potato (Solanum tuberosum L.), 40 sugar beet (Beta vulgaris L.), and 32 winter wheat (Triticum aestivum L.) fields. In both years and for all three crops, we found close linear relationships between the Cd content of plant material and the Cd concentration in soil solution. For all three crops, we observed a trend of relatively increased Cd uptake in the year with the higher saturation deficit of the atmosphere. We interpret this to indicate that transpiration plays an important role in the Cd uptake of crops under the conditions of the WIA. In modeling the uptake of Cd by crops, we assume that uptake is proportional to mass flow, that is, the product of water transpired, Cd concentration in soil solution, and a plant-specific empirical parameter. The simulations agreed well with the observed Cd contents in crops. Our model explained between 66 and 87% of the observed variance.

Effect of chloride in soil solution on the plant availability of biosolid-borne cadmium.

Increasing chloride (Cl) concentration in soil solution has been shown to increase cadmium (Cd) concentration in soil solution and Cd uptake by plants, when grown in phosphate fertilizer- or biosolid-amended soils. However, previous experiments did not distinguish between the effect of Cl on biosolid-borne Cd compared with soil-borne Cd inherited from previous fertilizer history. A factorial pot experiment was conducted with biosolid application rates of 0, 20, 40, and 80 g biosolids kg(-1) and Cl concentration in soil solution ranging from 1 to 160 mM Cl. The Cd uptake of wheat (Triticum aestivum L. cv. Halberd) was measured and major cations and anions in soil solution were determined. Cadmium speciation in soil solution was calculated using GEOCHEM-PC. The Cd concentration in plant shoots and soil solution increased with biosolid application rates up to 40 g kg(-1), but decreased slightly in the 80 g kg(-1) biosolid treatment. Across biosolid application rates, the Cd concentration in soil solution and plant shoots was positively correlated with the Cl concentration in soil solution. This suggests that biosolid-borne Cd is also mobilized by chloride ligands in soil solution. The soil solution CdCl+ activity correlated best with the Cd uptake of plants, although little of the variation in plant Cd concentrations was explained by activity of CdCl+ in higher sludge treatments. It was concluded that chlorocomplexation of Cd increased the phytoavailability of biosolid-borne Cd to a similar degree as soil (fertilizer) Cd. There was a nonlinear increase in plant uptake and solubility of Cd in biosolid-amended soils, with highest plant Cd found at the 40 g kg(-1) rate of biosolid application, and higher rates (80 g kg(-1)) producing lower plant Cd uptake and lower Cd solubility in soil. This is postulated to be a result of Cd retention by CaCO3 formed as a result of the high alkalinity induced by biosolid application.

Immobilization and phytoavailability of cadmium in variable charge soils. III. Effect of biosolid compost addition

The effect of pH-increases due to Ca(OH)2 and KOH addition on the adsorption of cadmium (Cd) was examined in two soils which varied in their variable-charge components. The effect of Ca(OH)2 on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg−1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. The addition of Ca(OH)2 and KOH increased the soil pH, thereby increasing the adsorption of Cd, the effect being more pronounced in the soil dominated by variable charge components. There was a greater increase in Cd2+ adsorption in the KOH-treated than the Ca(OH)2-treated soil, which is attributed to the greater competition of Ca2+ for adsorption. Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Although addition of Ca(OH)2 effectively reduced Cd phytotoxicity, Cd uptake increased at the highest level, probably due to decreased Cd2+ adsorption resulting from increased Ca2+ competition. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of Ca(OH)2 decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fractions in soil. Since there was no direct evidence for CdCO3 or Cd(OH)2 precipitation in the variable charge soil used for the plant growth experiment, alleviation of phytotoxicity can be attributed primarily to immobilization of Cd by enhanced pH-induced increases in negative charge.

Content, Distribution, and Solubility of Cadmium in Arable and Forest Soils

Afforestation of former farmland decreases soil pH and thus increases the solubility of Cd in the soil, which may cause Cd leaching to streams and groundwater. The Cd concentration in soil and soil solution were determined in 11 pairs of Danish arable and forest soil profiles representing three different texture classes (sand, loamy sand, and sandy loam). The soil pH did not change or decrease with depth through the arable profiles, but did increase with depth in the forest profiles. Significantly higher Cd contents were found in the upper 30 cm of the arable soil compared with that of the forest soil. The total soil Cd concentrations correlated with the effective cation‐exchange capacity (ECEC), clay content, and organic matter content, but not the soil pH. The soil solution pH was unchanged or decreasing downwards through the arable profiles, but increasing with depth in the forest profiles. The soil solution concentration of Cd was significantly higher in the forest soils than in the arable soils. The Cd concentration in the soil solution decreased as pH increased. Both total soil and soil solution Cd concentrations were higher in the sandy loam soils compared with the loamy sand and sand soils. It is concluded that afforestation may lead to higher soil solution concentrations of Cd as decreasing pH and ECEC diminish Cd retention and reduces Cd concentrations in the forest topsoils.

REMEDIATION OF SANDY SOIL ARTIFICIALLY CONTAMINATED WITH CADMIUM USING A POLYACRYLATE POLYMER

Soil pollution with toxic metals has become an important social and environmental challenge. When the polluted area is large and the site poses little immediate risk to health or environment, in situ remediation technologies can be envisaged. Here we used a polyacrylate polymer to immobilize cadmium (Cd) in a sandy soil artificially contaminated with the metal. Growth of perennial ryegrass in pots with 9.5 kg of soil was stimulated in polymer-amended soil. Even in pots with the highest levels of Cd, growth was much less impaired than in pots without polymer. Shoot Cd concentrations were smaller in the plants cultivated in the amended soil. Because water soluble Cd was considerably reduced in the contaminated polymer-amended soil, the effect of the polymer on plant growth was attributed to the decrease of Cd concentration in soil solution.