Soil solution concentration of Cd and Zn canbe predicted with a CaCl2 soil extract

Risk assessment of heavy metals in soil requires an estimate of the concentrations in the soil solution. In spite of the numerous studies on the distribution of Cd and Zn in soil, few measurements of the distribution coefficient in situ, K_d, have been reported. We determined the K_d of soils contaminated with Cd and Zn by measuring metal concentrations in the soil and in the soil solution and attempted to predict them from other soil variables by regression. Soil pH explained most of the variation in logK_d (R² = 0.55 for Cd and 0.70 for Zn). Introducing organic carbon content or cation exchange capacity (CEC) as second explanatory variable improved the prediction (R² = 0.67 for Cd and 0.72 for Zn), but these regression models, however, left more than a factor of 10 of uncertainty in the predicted K_d. This large degree of uncertainty may partly be due to the variable degree of metal fixation in contaminated soils. The labile metal content was measured by isotopic dilution (E value). The E value ranged from 18 to 92% of the total metal content for Cd and from 5 to 68% for Zn. The prediction of K_d improved when metals in solution were assumed to be in equilibrium with the labile metal pool instead of the total metal pool. It seems necessary therefore to discriminate between ‘labile’ and ‘fixed’ pools to predict K_d for Cd and Zn in field contaminated soils accurately. Dilute salt extracts (e.g. 0.01 m CaCl₂) can mimic soil solution and are unlikely to extract metals from the fixed pool. Concentrations of Cd and Zn in the soil solution were predicted from the concentrations of Cd and Zn in a 0.01 m CaCl₂ extract. These predictions were better correlated with the observations for field contaminated soils than the predictions based on the regression equations relating logK_d to soil properties (pH, CEC and organic C).     

Field-based partition coefficients for trace elements in soil solutions

A total of 48 elements was detected in the soil solutions centrifuged from two acid sandy (humus-iron podzol) profiles from southern England. Concentrations ranged from mm for the major ions to mm for trace metals such as U and the rare earth elements. Field-based solid/solution partition coefficients, K_d, were determined by calculating the ratio of the amount of an element extracted by 0.43 m HNO₃ or a neutral salt (0.01 m CaCl₂ or 0.1 m Ba(NO₃)₂) to the concentration in the soil solution. These partition coefficients did not show the expected trend in selectivity. For example, Cd consistently had one of the highest K_d values, higher even than Cu. This was thought to be due in part to the nature of the K_d which reflects a balance between binding to the soil solids and to the dissolved organic carbon (DOC), which is present at relatively high concentrations (1–20 mm ) in the soil solutions. Because of the underlying functional similarity between metal binding by the solid and dissolved organic matter, the partition coefficient (and hence element mobility) will be relatively insensitive to changes in pH and metal-ion activity in the soil solution.     

Methods for determining labile cadmium and zinc in soil

Isotopically exchangeable cadmium and zinc (‘E values’) were measured on soils historically contaminated by sewage sludge and ones on zinc‐rich mine spoil. The E‐value assay involves determining the distribution of an added metal isotope, e.g. ¹⁰⁹Cd, between the solid and solution phases of a soil suspension. The E values for both metals were found to be robust to changes in the position of the metal solid?solution equilibrium, even though the concentration of dissolved metal varied substantially with electrolyte composition and soil:solution ratio. Concentration of labile metal was also invariant over isotope equilibration times of 2–6 days. The use of a submicron filtration procedure, in addition to centrifuging at 2200 g , proved unnecessary if 0.1 m Ca electrolyte was used to suspend the soils. The proportion of ‘fixed’ metal, in non‐labile forms, apparently increased with increasing pH, although there was considerable variation in both sets of contaminated soil. Zinc and cadmium in the sludged soils were similarly labile. Several possible methods for the measurement of chemically reactive metal were explored for comparison with E values, including single extraction with 1 m CaCl₂ and a ‘pool depletion’ (PD) method. The latter involves comparing solid?solution metal equilibria in two electrolytes with differing degrees of (solution) complex formation, 0.1 m Ca(NO₃)₂ and CaCl₂. Both the single extraction and the PD method gave good estimates of E value for Cd, although the single extraction was more consistent. Neither technique was a useful substitute for determining labile Zn, because of weak chloro‐complexation of Zn^2+. We therefore suggest that 1 m CaCl₂ extraction of Cd alone be used as an alternative to E values to avoid the inconvenience of isotopic dilution procedures.     

Influence of submergence and subsequent drainage on the partitioning and lability of added selenium fertilizers in a sulphur‐containing Fluvisol

We used an isotope dilution method to examine the time‐dependent changes in the partitioning and lability of selenium (Se) in a Sri Lankan rice soil after adding fertilizer with selenite (Se(IV)) and selenate (Se(VI)) (1 mg kg^?1) and incubation under anaerobic (submerged) (30 days) and subsequent aerobic (drained) conditions (7 days) in controlled reaction vessels mimicking rice paddy water management practices. The K_d (the ratio of sorbed ion to that in solution) values for Se(IV) were significantly (P≤ 0.001) larger than those for Se(VI) in all treatments and at all sampling times. The K_d values for Se(IV) and Se(VI) decreased significantly (P≤ 0.001) with time during the anaerobic and subsequent aerobic phases. Applied Se(IV) fertilizer was rapidly removed into non‐labile pools during the anaerobic phase (day 0 = 60% labile and day 14 = no labile Se), with no significant increase in the labile pool following short‐term aeration. The results suggest that the rapid decrease in Se(IV) lability may be caused by the strong non‐reversible (at least for 7 days) sorption of Se (IV). In contrast, applied Se(VI) fertilizer was 90% labile at 0 day and decreased during the anaerobic phase to 30% after 30 days. There was no significant change in the lability of Se(VI) following the short‐term aerobic phase following anaerobic conditions. These results indicate that Se(IV) would not be an effective pre‐planting fertilizer for rice production. Selenate is likely to be more effective, but losses to non‐labile forms during the submerged phase of rice production may mean that efficiency of pre‐planting Se(VI) fertilizer is also compromised.     

Partitioning of Trace Metals in Suspended Sediments from Huanghe and Changjiang Rivers in Eastern China

Assessing metal contamination of sediments requires knowledge of the geochemical partitioning of trace metals at the sediment-water interface. Under controlled laboratory conditions, sequential extraction was conducted to determine the associations of metals (Cd, Cr, and Zn) and radiotracers (¹⁰⁹Cd, ⁵¹Cr, and ⁶⁵Zn) with various geochemical phases and the different partitioning and mobility of metals for two types of surface sediments collected from the Huanghe and Changjiang Rivers in Eastern China. The residual phase was the major phase for stable metal binding, indicating that these sediments had little subjection to recent anthropogenic influences. Fe–Mn oxides were the next important binding phases for metals. The partitioning of metals in various geochemical phases as a function of the duration of the radiolabeling was also examined. Trace metals transferred among the different geochemical phases over the 30 days radiolabeling period, particularly between the carbonate and Fe–Mn oxides phases. The freshwater-sediment distribution coefficients (K _d) of three metals were investigated in batch experiments using the radiotracer technique. The decreasing K _d with increasing metal concentration(from 0.5 to 200 μg L^-1) may be explained by competitive adsorption. The metal K _d in sediments from the Changjiang River was greater than those from the Huanghe River, presumably because of the higher Fe/Mn and organic carbon contents in Changjiang River sediment. The K _d decreased with increasing total suspended solid load from 3 to 500 mg L^-1, and was Cr > Zn > Cd. For Cd and Zn, increasing the pH from 5 to 8 resulted in an increase in K _d due to the reduced H⁺ competition and increasing sorptionpotential. However, the K _d for Cr in the sediments from both rivers showed no relationship with pH, presumably becauseof the complexity of the Cr species and environmental behavior.     

Comparing concurrent in situ and batch‐extracted trace element concentrations

Different procedures to investigate dissolved trace element concentration at the transition from unsaturated to saturated zone in soils were compared by concurrent sampling of soil solution and solid soil material in this zone. The in situ sampled soil solution from the percolated water was used to measure in situ concentrations, while solid soil material was used to measure concentrations at two liquid–solid ratios using batch experiments on 250 sample pairs. The liquid–solid ratios were 2 L kg^–1 and 5 L kg^–1. At 5 L kg^–1, the ionic strength was adjusted with Ca(NO₃)₂ to a sample‐specific value similar to in situ, while at 2 L kg^–1, the ionic strength was not adjusted. The extracted concentrations of most trace elements exhibited a statistically significant but weak correlation (p value < 0.01) to the corresponding in situ concentrations. In the liquid–solid ratio of 2 L kg^–1 extracts, Pb and Cr showed very poor comparability with the in situ equivalent. A likely cause was the enhanced dissolved‐organic‐C release in the extract due to the lower ionic strength compared to in situ conditions in combination with effects from drying and moistening soil samples. For the other elements, correlation increased in the order As < Cu, Zn, Sb, Mo, V < Cd, Ni, Co where adjustment of the ionic strength led to slightly better results. In addition to the element‐specific shortcomings, it appeared that low concentration levels of in situ concentrations were generally underestimated by batch extraction methods. The liquid–solid ratio of 2 L kg^–1 extracts could only be used as a method to predict exceedance of thresholds if a safety margin of approximately one order of magnitude higher than the thresholds was adopted. The ability of the batch‐extraction methods to estimate in situ concentrations was equally limited.     

Soil properties affecting solid–liquid distribution of As(V) in soils

The prediction of the mobility of arsenic (As) is crucial for predicting risks in soils contaminated with As. The objective of this study is to predict the distribution of As between solid and solution in soils based on soil properties and the fraction of As in soil that is reversibly adsorbed. We studied adsorption of As(V) in suspensions at radiotrace concentrations for 30 uncontaminated soils (pH 4.4–6.6). The solid–liquid distribution coefficient of As (K_d) varied from 14 to 4430 l kg^?1. The logarithm of the concentration of oxalate‐extractable Fe explained 63% of the variation in log K_d; by introducing the logarithm of the concentration of oxalate‐extractable P in the regression model, 85% of the variation in log K_d is explained. Double labelling experiments with ⁷³As(V) and ³²P(V) showed that the As to P adsorption selectivity coefficient decreased from 3.1 to 0.2 with increasing degree of P saturation of the amorphous oxides. The addition of As(V) (0–6 mmol kg^?1) reduced the K_d of ⁷³As up to 17‐fold, whereas corresponding additions of P(V) had smaller effects. These studies suggest that As(V) is adsorbed to amorphous oxides in soils and that sites of adsorption vary in their selectivity in respect of As and P. The concentration of isotopically exchangeable As in 27 contaminated soils (total As 13–1080 mg kg^?1) was between 1.2 and 19% (mean 8.2%) of its total concentration, illustrating that a major fraction of As is fixed. We propose a two‐site model of competitive As(V)–P(V) sorption in which amorphous Fe and Al oxides represent the site capacity and the isotopically exchangeable As represents the adsorbed phase. This model is fitted to ⁷³As adsorption data of uncontaminated soils and explains 69% of the variation of log K_d in these soils. The log K_d in contaminated soils predicted using this two‐site model correlated well with the observed log K_d (r = 0.75). We conclude that solubility of As is related to the available binding sites on amorphous oxides and to the fraction of As that is fixed.     

Chlorophenol binding to dissolved and particulate soil organic matter determined in controlled equilibrium systems

We determined the sorption of 2,4‐dichlorophenol (DCP), 2,4,5‐trichlorophenol (TCP) and pentachlorophenol (PCP) to dissolved (DOM) and particulate soil organic matter (POM) from the same soil in controlled equilibrium systems, using ¹⁴C‐labelled chlorophenols in combination with reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and liquid scintillation. Associations of DCP, TCP and PCP to DOM and POM were satisfactorily described by linear adsorption isotherms. Together with the absence of substantial competition between DCP and TCP for binding sites, this indicates a hydrophobic partitioning mechanism. The organic carbon normalized partitioning coefficient (K_OC) for the binding of DCP was similar in magnitude for POM (K_POC) and for DOM (K_DOC), whereas K_POC for the more hydrophobic compounds TCP and PCP were approximately one order of magnitude greater than K_DOC. On the basis of the relationships between log K_OC and the organic carbon normalized partitioning coefficient (log K_OW), the extent of association to POM increases more with the hydrophobicity of the chlorophenol than the extent of association to DOM. This holds for our data obtained for DOM and POM of similar origin, as well as for various sources of POM and DOM reported in the literature. Differences in the magnitude of K_POC and K_DOC in our study could not be accounted for by differences in gross carbon chemistry of POM and DOM, as determined by nuclear magnetic resonance (¹³C‐NMR) and X‐ray photoelectron spectroscopy (XPS). Thus, other factors such as the average size and capacity of hydrophobic moieties could explain differences in chlorophenol association between POM and DOM. We conclude that K_POC and K_DOC need to be determined explicitly, when the transport and retention of chlorophenols is modelled, and not calculated from relationships between log K_OC and log K_OW.     

Measuring bioavailable trace metals by diffusive gradients in thin films (DGT): soil moisture effects on its performance in soils

Conventional methods of measuring labile chemical species of trace metals in soil solutions, such as chemical competition following centrifuging, are inadequate if the speciation changes during sampling and extraction. A new technique, diffusive gradients in thin films (DGT), measures labile species of trace metals in natural waters and sediments in situ. A well-defined diffusive gel layer distinguishes it from other resin-based techniques. It perturbs the soil in a controlled way by introducing an in situ local sink for metal ions. Resulting fluxes to the device are quantitatively measured, allowing assessment of re-supply kinetics and in some cases measurement of in situ soil solution concentrations. We used DGT to measure fluxes of Cd, Co, Cu, Ni, Pb and Zn in a sludge-treated soil at various moisture contents (27–106%). Replicate measurements showed that the precision of DGT-measured fluxes was within 10%. For moisture contents exceeding the field capacity (42%), the DGT response reflected soil water concentrations. At smaller moisture contents, changes related to tortuosity and dilution were reflected in the measurements. This technique has the potential for in situ measurements in the field where it should provide quantitative flux data on individual soils and provide a good surrogate for bioavailable metal.     

土壤对镉的吸附与解吸——Ⅱ.吸附势与解吸势

本文提出了吸附势(logK_a)和解吸势(logK_d或相对解吸势logK_dr)两个有关吸附和解吸的强度概念,并进行了理论推导和实验验证。logK_a和logK_d或logK_dr是影响土壤或胶体吸附和解吸因素的综合反应。实验证实logK_a可用于表征土壤胶体对Cd的相对选择性,并且对胶体吸附Cd有着良好的预测性;logK_dr可用于表征土壤胶体对Cd的相对固定能力,并可用于估测Cd的污染程度。盆栽试验表明,随着土壤胶体logK_a的增加,或logK_dr的降低,稻草或糙米中Cd的含量下降。可以预期,吸附势和解吸势不但在土壤物理化学,土壤环境化学研究中,而且在植物营养化学、水化学、以及界面化学等方面有可能获得实际应用。     

Methodologically controlled variations in laboratory and field pH measurements in waterlogged soils

We have tested the reliability and consistency of conventional pH measurements made on water‐soil mixtures with respect to sieving, drying, ratio of water to soil, and time of shaking prior to measurement. The focus is on a waterlogged soil where the preservation potential of archaeological artefacts is critical. But the study includes agricultural and forest soils for comparison. At a waterlogged site, laboratory results were compared with three different field methods: calomel pH probes inserted in the soil from pits, pH measurements of soil solution extracted from the soil, and pH profiles using a solid‐state pH electrode pushed into the soil from the surface. Comparisons between in situ and laboratory methods revealed differences of more than 1 pH unit. The content of dissolved ions in soil solution and field observations of O₂ and CO₂ concentrations were used in the speciation model PHREEQE in order to predict gas exchange processes. Changes in pH in soil solution following equilibrium in the laboratory could be explained mainly by CO₂ degassing. Only soil pH measured in situ using either calomel or solid‐state probes inserted directly into the soil was not affected by gas exchange processes. Variations on the order of 0.2–0.5 pH unit in different laboratory methods could not be explained by degassing and seem to be soil‐type specific and strongly influenced by drying and shaking. Further attention should be given to standardization of pH measurements, particularly before pH measurements from different soil types are compared.     

Radio-labile cadmium and zinc in soils as affected by pH and source of contamination

The determination of radio‐labile metals in soil has gained renewed interest for predicting metal availability. There is little information on to what extent the fraction of labile metal is affected by the soil properties and the source of metal contamination. The radio‐labile content (E value) of Cd and Zn was measured in field‐collected soils with Cd and Zn originating from different sources. The E values were erratic and sometimes even exceeded total metal content when the concentration in the soil extract was less than 8 μg Zn l^?1 or less than 3 μg Cd l^?1. Addition of EDTA (0.1 mm ) to the radio‐labelled soil suspension resulted in larger concentrations of Cd and Zn in solution and smaller E values for these soils. The E values were, however, unaffected by the presence of EDTA (0.1 mm ) in soils with larger concentrations of Cd and Zn in solution. The %E values (E value relative to metal soluble in aqua regia) ranged from 9% to 92% (mean 61%) for Cd and from 3% to 72% (mean 33%) for Zn. No correlation between soil properties and %E was observed for Cd, and the %E of Zn was negatively correlated with soil pH (r = ?0.65). There was a strong negative correlation between pH and %E in soils enriched with metals in soluble form (e.g. metal salts, corrosion of galvanized structures). In soils where Cd or Zn were added in a less soluble form, no such correlation was found, and %E values were generally less than in soils spiked with metal salts, suggesting that the source of the contamination controls mainly the labile fractions of Cd and Zn.     

Measurement of Si adsorption and “Active Si” in a soil amended with slag fertilizers by using stable isotope 30SP

The silicic acid adsorption by a soil (Eutric Gleysols) where slag fertilizers were applied was measured by the addition of a silicic acid solution labelled with ³⁰Si after soil incubation, in order to study the effect of slag application on the specific Si adsorption by the soil and to estimate the amount of Si in the soil solid phase which can easily enter the soil solution. It was evident that the application of slags increased the ability of soil to adsorb Si. It was also shown that the ³⁰Si added was diluted with not only the Si present in the soil solution but also the Si dissolved from the soil solid phase. We proposed the use of the term “active” for Si in soil which can take part in the isotopic dilution within 1 h. The amount of active Si in the soil solid and liquid phases (D ₆₀ - value) was calculated from the ³⁰Si content in the soil solution and compared with the amount of Si taken up by rice plant (Oryza sativa L.), which was determined in our previous study. The buffering capacity of the soil for Si, which can reflect the ability of soil to maintain the Si concentration in the soil solution constant when Si is added to or removed from the soil, was also determined. The D ₆₀ - value and the Si buffering capacity of the soil increased by slag application. These increases were large when the alkalinity of the applied slags was high. The correlation study revealed that the D ₆₀ - value was a better index of Si availability of the soil than the amount of Si dissolved from the soil solid phase during the incubation when the slags were previously applied.     

Adsorption isotherms of some heavy metals under conditions of their competitive adsorption onto highly calcareous soils of southern Iran

Recently, application of sewage sludge or effluents resulted in raising the concentrations of some heavy metals in some agricultural soils of Iran. Experiments were conducted to evaluate the competitive adsorption of lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) on six calcareous soils. Adsorption characteristics were evaluated by equilibration of 1 g of each soil sample with 20 ml of 0, 10, 20, 30, 40, 50, 100, or 200 mg L^?1 of their nitrate solutions and 0.01 M NaNO₃ as background electrolyte. Furthermore, solid/liquid distribution coefficients (K_d) of studied metals, as an index of soil capacity to resist a change of the soil solution concentration, were calculated. Results indicated that amounts of adsorbed Pb, Cu, Zn, and Cd increased with increase in their concentrations in the contact solutions, but this trend was more pronounced for Pb and Cu than the others. For all studied soils and metals, Langmuir equation described the adsorption behavior fairly well. Furthermore, Langmuir and Freundlich equation parameters were positively correlated to cation exchange capacity (CEC) and smectite contents; whereas, they were negatively correlated to sand content. Considering K_d values, the selectivity sequence of the metal adsorption was Pb > Cu > Zn > Cd. Therefore, the risk of leaching and also plant uptake of Zn and Cd will be higher as compared to those of the other elements.     

Analysis of the variable charge of two organic soils by means of the NICA-Donnan model

We have tested to see if the generic set of NICA‐Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back‐titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non‐ideal, competitive‐adsorption NICA‐Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H⁺ affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V_D, varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K_i^int) and the heterogeneity of the site (m_i), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by ¹³C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.     

Adsorption and solubility of ten metals in soil samples of different composition

We conducted batch experiments for ten metals [Mg, Cr(III), Fe(III), Co, Ni, Cu, Zn, Sr, Cd, Pb] and four soil samples of different composition to determine the relation of the soluble fraction (’intensity’︁) to an adsorbed or precipitated metal pool (’quantity’︁) and, thus, to investigate the buffer function of soils. The soil samples were spiked with 6 to 12 exponentially increasing metal doses added as metal nitrates. The native metal pool involved in sorption processes was characterized by an extraction with 0.025 M (NH₄)₂EDTA (pH 4.6). The quantity-intensity (Q/I) relations of eight metals [except Cr(III) and Fe(III)] were governed by sorption and complexation processes and can be fitted by Freundlich isotherms. Q/I relations for Cr(III) and two soils indicate a sorption maximum, which can be approximated with the Langmuir isotherm. In a calcareous soil high Cr doses induced the precipitation of a Cr oxide. The solution concentrations of Fe are primarily a function of the pH-dependent solubility of ferrihydrite. For all metals pH was the predominant factor controlling the partitioning between the solid and the liquid phase. Drastic losses in the buffer function of soils primarily occurred in the slightly acidic range. Furthermore, adsorption was also metal specific. On the basis of median Freundlich K values, adsorption increased in the order [median K_F values and K_F range (mg kg^—1) in brackets]: Mg (2.9: 0.9—19) < Sr (4.7: 0.6—21) << Co (17.7: 1.1—143) < Zn (26.7: 1.8—301) = Ni (27.6: 2.4—120) < Cd (71: 2.5—405) << Cr(III) (329: 45—746) < Cu (352: 30—1200) < Pb (1730: 76—4110).     

Feldmethode zur Bestimmung der substrat‐induzierten Bodenatmung

A field method for the measurement of substrate‐induced soil respiration A novel method for in situ measurements of microbial soil activity using the CO₂ efflux combined with kinetic analysis is proposed. The results are compared with two conventional, laboratory methods, (1) substrate‐induced respiration using a ’︁Sapromat’ and (2) dehydrogenase activity. Soil respiration was measured in situ after addition of aqueous solutions containing 0 to 6 g glucose kg^—1 soil. The respiration data were analysed using kinetic models to describe the nutritional status of the soil bacteria employing few representative parameters. The two‐phase soil respiration response gave best fit results with the Hanes' or non‐parametric kinetic model with Michaelis‐Menten constants (K_m) of 0.05—0.1 g glucose kg^—1 soil. The maximum respiration rates (V_max) were obtained above 1 g glucose. Substrate‐induced respiration rates of the novel in situ method were significantly correlated to results of the ’︁Sapromat’ measurements (r² = 0.81***). The in situ method combined with kinetic analysis was suitable for the characterisation of microbial activity in soil; it showed respiration rates lower by 59% than measured in the laboratory with disturbed samples.     

施用碱稳定固体的酸性土壤的Cu和Zn的形态分布

Fractionation of metals in a granite-derived acid sandy loam soil amended with alkaline-stabilised sewagesIudge biosolids was conducted in order to assess metal bioavailability and environmental mobility soil solution was extracted by a centrifugation and filtration technique. Metal speciation in the soil solution wasdetermined by a cation exchange resin method. Acetic acid and EDTA extracting solutions were used forextraction of metals in soil solid surfaces. Metal distribution in different fractions of soil solid phase was determined using a three-step sequential extraction scheme. The results show that the metals in the soilsolution existed in different fractions with variable lability and metals in the soil solid phase were also presentin various chemical forms with potentially different bioavail ability and environmental mobility Alkaline-stabilised biosolids could elevate solubility of Cu and proportion of Cu in organically complexed fractionsboth in soil liquid and solid phases, and may therefore increase Cu mobility. In contrast, the biosolids lowered the concentrations of water-soluble Zn (labile fraction) and exchangeable Zn and may hence decrease bioavailability and mobility of Zn. However, Fe and Mn oxides bound and organic matter bound fractions are likely to be Zn pools in the sludge-amended soil. These consequences possibly result from the liming effect and metal speciation of the sludge product and the difference in the chemistry between the metals in soil.     

General purpose Freundlich isotherms for cadmium,copper and zinc in soils

Assessing the accumulation and transport of trace metals in soils and the associated toxicological risks on a national scale requires generally applicable sorption equations. Therefore Freundlich equations were derived for Cd, Zn and Cu using multiple linear regression on batch sorption data from the literature with a wide variety of soil and experimental characteristics, and metal concentrations ranging over five orders of magnitude. Equations were derived based on both total dissolved metal concentrations and free metal activities in solution. Free metal activities were calculated from total metal concentrations taking into account ionic activity, and inorganic (all metals) and organic complexation (Cu only). Cadmium and Zn were present in solution predominantly as free ions, while Cu was present as organic complexes. Since actual dissolved organic carbon (DOC) concentrations were not available they were estimated using an empirical field relation between DOC and organic matter content. The logarithmic transformation of the Freundlich constant for Cd was regressed on the logarithmic transformations of cation exchange capacity (CEC) (H⁺) and dissolved Ca, and for Zn with CEC and (H⁺). For Cu the log–log regression model of the Freundlich constant included the solid:solution ratio of the batch to account for dilution of DOC in the batch as compared with the field. The explained variance for the fitted Freundlich equations was 79% for Cd, 65% for Cu and 83% for Zn, using log-transformed adsorbed concentrations and soil solution activities. The Freundlich adsorption models underestimated metal contents determined from 1 m HNO₃ digestion on field samples, up to a factor of 6 (Cd and Cu) or 10 (Zn).     

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.