Kinetics of Copper Desorption from Highly Calcareous Soils

Abstract

Desorption of copper (Cu) is an important factor in determining Cu availability in calcareous soils. Kinetics of native and added Cu desorption by DTPA (diethylene‐triaminepentaacetic‐acid) from 15 highly calcareous soils of southern Iran were studied in a laboratory experiment. Our results showed that two constant‐rate, Elovich, simple Elovich, and parabolic‐diffusion equations were the best‐fitted equations among eight kinetic models used. The copper desorption pattern based on the parabolic‐diffusion equation revealed that the rate of native Cu desorption was higher in the first 2 h followed by a slower release rate, which suggests that two different mechanisms are involved. The trend may describe why the DTPA soil test has been considerably successful in predicting Cu availability in calcareous soils. Stepwise multiple regression equations indicated that CCE (calcium carbonate equivalent), CEC (cation exchange capacity), and clay content are the most important soil characteristics that predict the rate constants of the kinetic models. Mean extractant recovery percentage (ERP) of the soils was only 20%, which indicated that after 20 days, DTPA extracted only one‐fifth of added Cu. Regression equations indicated that as soil OM (organic matter) content increased, the value of ERP decreased. From results reported herein it seems that CCE, CEC, and clay are the most important factors controlling Cu release from highly calcareous soils of southern Iran. However, the initial soil Cu desorption rate is probably controlled by CEC.     

Cadmium Sorption and Desorption Characteristics of Tropical Alfisols from Different Land Uses

Cadmium (Cd) sorption and desorption characteristics by Alfisols from different land uses were examined, and the relationships between soil and sorption/desorption characteristics were investigated. Adsorption studies were done using Cd concentrations (0–100 mg Cd kg^?1) in 0.01 M CaCl₂. The Cd sorbed by the soils was then subjected to two desorption runs. The soils' adsorption conformed to Freundlich and Langmuir equations. The amount of Cd sorbed by the soils varied. Two desorption runs detached more than 95% of sorbed Cd, but the first accounted for more than 80% of the total. Desorption of Cd in degraded soils was more than in soils from other land uses. The amount of Cd desorbed correlated with amount applied (r = 0.90??), solution concentration (r = 0.83??), and amount sorbed (r = 0.70??). A positive relationship exists between the adsorption maxima of the soils and soil organic matter (r = 0.13, p = 0.87). The relationship between amount of Cd desorbed and sorbed is quadratic for all the soil.     

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%.     

Evaluation of copper desorption characteristics using DTPA and citric acid for wheat (Triticum aestivum L.) rhizosphere of soils amended with sewage sludge

A greenhouse experiment was carried out to determine copper (Cu) desorption characteristics in ten bulk and rhizosphere soils (Typic Calcixerepts) amended with sewage sludge (1% w/w) using rhizoboxes. The kinetics of Cu desorption in the bulk and the rhizosphere soils were determined after successive extraction with DTPA‐TEA and 10 mM citric acid in a period of 1 to 504 h at (25 ± 1)°C. The results show that Cu extracted after 504 h using DTPA‐TEA were significantly (P < 1%) lower in the rhizosphere than the bulk soils. However, Cu extracted after 504 h using citric was significantly (P < 1%) higher in the rhizosphere than in the bulk soils. The results illustrated that, on average, citric acid extracted 56% more Cu from the bulk soils than DTPA‐TEA, and citric acid extracted 85% more Cu from the rhizosphere soils than DTPA‐TEA. Desorption kinetics of Cu in the two extractants was well described by power‐function, parabolic diffusion, and first‐order equations. The results show that a 10 mM citric acid extractant may be recommended to determine the kinetics of Cu desorption in calcareous soils amended with sewage sludge.     

Effect of Organic Matter on the Release Behavior and Extractability of Copper and Cadmium in Soil

Land disposal of organic waste materials may alter the heavy-metal status of the soil by affecting metal solubility or dissociation kinetics. The aim of this investigation was to study the influences of poultry manure and pistachio compost (at the rate of 300 g kg^?1) on release behavior and extractability of copper (Cu) and cadmium (Cd) in two soils of Iran. For release studies, samples after incubation were extracted with ethylenediaminetetraacetic acid (EDTA) at pH 7.0 with shaker for periods of 5 to 2880 min. Results showed that during the reaction periods, Cu and Cd release rates were rapid at first and then became slower until equilibrium was achieved. In the two soils, extractability of Cu increased with poultry manure treatment and decreased with pistachio compost treatment as compared to the control soil, and extractability of Cd decreased with both pistachio compost and poultry manure treatments as compared to the control soil.     

Adsorption-desorption of atrazine on four soils of Hyderabad

The adsorption-desorption equilibrium of atrazine (2-chloro, 4-ethylamino, 6-isopropyl amino-1, 3, 5 triazine) was studied by the batch equilibration method at 27 ± 1 °C on four soils of Hyderabad. Adsorption isotherms conformed to the Freundlich equation (A = KC^1/n ). K increased in the same order as the organic C content of the soils. Desorption studies were conducted by repeated replacement of 5 mL of the supernatant equilibrium solutions after adsorption, with 0.01 M CaCl₂. Desorption isotherms showed considerable hysteresis which was more prominent when the desorption was carried out with higher adsorbed concentration of atrazine. Desorption from the lowest level of adsorbed atrazine (3 to 5 μg g^?1 soil) was close to the adsorption isotherm. The cumulative desorption after four desorption steps covering five days was significantly different at the 1% level, for different initial adsorbed concentrations of atrazine. Desorption was significantly higher at the lowest adsorbed level of atrazine. The soils differed significantly at 6% level for desorption and the amount desorbed decreased in the inverse order of organic C. Desorption isotherms also conformed to Freundlich equation but K andn values were both higher than that for adsorption and increased with increase in initially adsorbed concentration of atrazine. Desorption thus confirmed the irreversible nature of the adsorption of atrazine on these soils. The quantitative factors and reasons for desorption are discussed.     

塿土表面铜吸附解吸的动力学特征及滞后效应

采用间歇法发现土娄土及不同处理土样对铜离子的吸附过程遵循二级反应动力学方程 ,吸附速率常数ka 呈 :土娄土 >去OM土娄土 >去Ca土娄土 >去Ca、OM土娄土 ;解吸过程符合两点位一级动力学方程 ,第一点位解吸速率常数kd1大于第二点位解吸速率常数kd2 ;各土样解吸速率常数的大小顺序与ka相反 ;吸附 解吸之间的滞后效应用ka/kd来衡量 ,滞后程度大小与ka大小顺序一致     

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.     

有机酸对土壤中镉的解吸及影响因素

研究了酒石酸和柠檬酸对两种不同处理土壤 (镉质土和污染土 )中Cd的解吸行为 ,探讨了介质pH值、支持电解质等对其解吸镉质土和污染土中Cd的影响。结果表明 ,随有机酸浓度升高 ,柠檬酸和酒石酸对镉质土和污染土中Cd的解吸率呈先降低后升高变化 ,出现一波谷 ,波谷位置与有机酸和土壤类型有关。当解吸液起始pH值由 2升至 8时 ,柠檬酸对镉质土和污染土中Cd的解吸率呈降低—升高—降低的变化 ,先后出现一波谷和一波峰 ,且柠檬酸浓度高时变化趋势尤为明显 ;而酒石酸对镉质土和污染土中Cd的解吸率则一直降低。解吸液中支持电解质浓度提高 ,酒石酸对镉质土和污染土中Cd的解吸率增大 ,且以KCl为支持电解质时的解吸率要高于KNO3。     

Adsorption and Desorption of Copper in Pasture Soils

Abstract

Copper (Cu) is bound strongly to organic matter, oxides of iron (Fe) and manganese (Mn), and clay minerals in soils. To investigate the relative contribution of different soil components in the sorption of Cu, sorption was measured after the removal of various other soil components; organic matter and aluminum (Al) and Fe oxides are important in Cu adsorption. Both adsorption and desorption of Cu at various pH values were also measured by using diverse pasture soils. The differences in the sorption of Cu between the soils are attributed to the differences in the chemical characteristics of the soils. Copper sorption, as measured by the Freundlich equation sorption constants [potassium (K) and nitrogen (N)], was strongly correlated with soil properties, such as silt content, organic carbon, and soil pH. The relative importance of organic matter and oxides on Cu adsorption decreased and increased, respectively, with increasing solution Cu concentrations. In all soils, Cu sorption increased with increasing pH, but the solution Cu concentration decreased with increasing soil pH. The cumulative amounts of native and added soil Cu desorbed from two contrasting soils (Manawatu and Ngamoka) during desorption periods showed that the differences in the desorbability of Cu were a result of differences in the physico‐chemical properties of the soil matrix. This finding suggests that soil organic matter complexes of Cu added through fertilizer, resulted in decreased desorption. The proportions of added Cu desorbed during 10 desorption periods were low, ranging from 2.5% in the 24‐h to 6% in the 2‐h desorption periods. The desorption of Cu decreased with increasing soil pH. The irreversible retention of Cu might be the result of complex formation with Cu at high pH.     

有机肥非水溶性分解产物对 铜 、镉吸附及解吸的影响

研究了稻草、紫云英和猪粪淹水培养的非水溶性分解产物、两种土壤( 红壤、潮土) 与这 3 种有机肥共同淹水培养后的非水溶性产物( 水溶性物质被除去) 对铜、镉的沉淀、吸附及解吸作 用的影响。 结果表明, 当铜初始浓度为 10-4mol/ L, pH <6 时有机残渣促进铜的沉淀;当 pH >6 时则抑制了铜的沉淀。 当铜初始浓度降为 10-5mol/ L 时, 有机残渣对铜沉淀的促进作用加强。 3 种有机残渣均促进镉的沉淀, 但促进程度比铜低。 与有机肥共同培养的红壤, 在相同的 pH 条 件下, 提高对铜、镉的吸附;在不调节 pH 条件下, 由于有机肥料有提高 pH 的作用, 进一步提高 对铜 、镉的吸附。 与有机肥共同培养的潮土, 在相同的 pH 条件下, 对铜、镉吸附的影响很小;在 不调节 pH 时, 提高了潮土对铜的吸附, 但对镉吸附的影响则较复杂。 上述结果表明, 有机肥的 非水溶性分解产物主要通过提高体系的 pH 值、与铜、镉形成不溶性的络合物而影响铜、镉的吸 附。 与有机物料共同培养的红壤所吸附的铜、镉的解吸率均不同程度降低。     

Influence of potassium chloride on desorption of cadmium sorbed on hydroxyaluminosilicate-montmorillonite complex

Cadmium adsorption and desorption on a synthetic hydroxyaluminosilicate-montmorillonite (HAS-Mt) complex were investigated in comparison with a montmorillonite (Mt) and hydroxyaluminum-montmorillonite (HyA-Mt) complex.

Kinetics studies of Cd adsorption on Mt, HyA-Mt and HAS-Mt complexes were conducted at pH 5.0, using a 10^-7 M Cd(NO₃)₂ solution in a solid to solution ratio of 100 mg to 100 mL. After 240 min reaction, 94, 88, 32% of the added Cd were removed from the solutions of the Mt, HyA-Mt, and HAS-Mt systems, respectively. Considering the magnitude of the cation exchange capacity of the clay and complexes, affinity of Cd ions was highest for the HAS-Mt complex.

Desorption experiment was conducted using 1 M KC1, 1 M KNO₃, and water. The desorption rate of Cd sorbed on the Mt was the highest, 90 and 59%, using KC1 and KNO₃, respectively, while, that on the HyA-Mt and HAS-Mt was around 30% and much lower than in the case of Mt, using both chemical reagents. Water could not desorb the adsorbed Cd at all. The difference in the desorption rate between KC1 and KNO₃ was due to the complex ion formation of Cd with the anionic species.

The adsorption of Cd by Mt varied with the nature of intercalated materials, namely, HyA and HAS. The extent of the desorption of Cd decreased due to intercalation of HAS and HyA. In the soil environment, Mt is commonly intercalated with HyA and/or HAS. Our data indicated that the formation of HyA- and HyA-Mt complexes should lead to Cd accumulation in soil environments. In addition, the application of potash (KC1) fertilizer would enhance the desorption of the Cd accumulated in soils.     

Immobilization of Cu and Cd in a contaminated soil: one- and four-year field effects

Purpose

Many amendments have been applied to immobilize heavy metals in soil. However, little information is available on the changes of immobilization efficiencies of heavy metals in contaminated soils over time. This work investigated the immobilization efficiencies of copper (Cu) and cadmium (Cd) in contaminated soils in situ remediated with one-time application of three amendments for 1 year and 4 years.

Materials and methods

Apatite, lime, and charcoal were mixed with the topsoil of each plot with the amounts of 22.3, 4.45, and 66.8 t/ha, respectively. Soil chemical properties and fractions of Cu and Cd were examined after in situ remediation for 1 year and 4 years. Soil sorption and retention capacities and desorption proportions for Cu and Cd were investigated by batch experiments.

Results and discussion

The addition of amendments significantly increased soil pH, but decreased exchange acid and aluminum (Al). The amendments significantly decreased the CaCl₂ extractable Cu and Cd and transformed them from active to inactive fractions. After the application of amendments for 1 year, the maximum sorption capacities ranged from 35.6 to 38.8 mmol/kg for Cu and from 14.4 to 17.0 mmol/kg for Cd, which were markedly higher than those of the application of amendments for 4 years (Cu, 29.6–34.7 mmol/kg; Cd, 10.9–16.4 mmol/kg). Desorption proportions (D) of Cu and Cd using three extractants followed the order of \( {D}_{{\mathrm{NaNO}}_3}<{D}_{{\mathrm{CaCI}}_2}<{D}_{{\mathrm{MgCI}}_2} \) . Moreover, the retention capacities (R) of Cu and Cd both increased and followed the order of R _apatite?>?R _lime?>?R _charcoal, resulting in higher Cu and Cd in the amended soils than the untreated soil.

Conclusions

Apatite, lime, and charcoal increased the soil sorption and retention capacities of Cu and Cd and resulted in higher immobilization efficiencies in the amended soils than the untreated soil. However, the immobilization efficiencies of Cu and Cd decreased with the decrease of sorption capacities after 4 years. It was concluded that apatite had the best effect on the long-term stability of immobilized Cu and Cd and can be applied to immobilize heavy metals in contaminated soils.     

不同温度下镉在典型农田土壤中的吸附动力学特征

按土重的3%和5%向采自海南和广西的3种可变电荷土壤中添加由稻草制备的生物质炭,混合培养30 d后用一次平衡法研究了生物质炭对土壤吸附Cd（Ⅱ）的影响及其与土壤表面电化学性质的关系,旨在阐明生物质炭促进可变电荷土壤吸附和固定Cd（Ⅱ）的机制。结果表明,添加稻草炭显著提高了3种土壤的阳离子交换量（CEC）和土壤pH,并使土壤胶体Zeta电位向负值方向位移。因此,添加稻草炭增加了土壤表面的负电荷量,土壤表面对Cd（Ⅱ）的吸附容量增强,使3种可变电荷土壤对Cd（Ⅱ）的吸附量增加,且Cd（Ⅱ）吸附量的增幅随稻草炭添加水平的提高而增加。Freundlich方程和Langmuir方程可以拟合3种土壤对Cd（Ⅱ）的吸附等温线,但Freundlich方程拟合效果更好,该方程表征吸附容量的常数k也随着稻草炭添加水平提高而增大。研究表明在pH3.0~5.0范围内,稻草炭均增加土壤对Cd（Ⅱ）的吸附量。添加稻草炭提高土壤pH,促进Cd（Ⅱ）的吸附,因为Cd（Ⅱ）的吸附量随pH升高而增加。解吸实验表明,添加稻草炭处理Cd（Ⅱ）的解吸量高于对照处理,说明生物质炭提高了土壤对Cd（Ⅱ）的静电吸附量。     

Adsorption and desorption of arsenate in Louisiana rice soils

Adsorption and desorption of arsenic (As) in the soil are dominant parameters that affect the mobility and bioavailability of arsenic. Batch arsenate adsorption and desorption experiments were conducted using soils collected from three Louisiana, USA, aquaculture ponds representing different crayfish farming and rice cultural practices. Arsenate adsorption behavior in the soils was investigated using Freundlich and Langmuir sorption equations. Results demonstrated that the Langmuir isotherm model was the best fit based on statistical correlation with soil properties governing adsorption, for the entire range of arsenate concentrations for all soils. Adsorption of As(V) was governed by soil physicochemical properties especially Fe and Al oxides, clay and organic matter. Desorption of As(V) was initially fast, but with increasing incubation times desorption occurred progressively slower. Chemical fractionation of arsenic in the soils showed that the most mobile fraction represented 4.74–5.18% of the total arsenic. A part of this mobile fraction could potentially be taken up by rice and enter the food chain, but would require additional research to quantify.     

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

Purpose

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

Materials and methods

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

Results and discussion

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

Conclusions

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

     

Bioavailability of Heavy Metals in Sewage Sludge-Amended Thai Soils

Plough and subsoil layers of two majoragricultural soil series, Rangsit and Thonburi, foundin Bangkok area of Thailand were studied fordetermining the bioavailability and solubilitybehavior of heavy metals (Cd, Cu, Zn, Mn, and Fe)following application of sewage sludge from awastewater treatment plant and a series of laboratoryexperiments. The soils contained low indigenous heavymetals while the sludge contained higher amounts ofheavy metals but in an acceptable range forapplication as plant nutrient source in agriculturalsoil. Applications of sewage sludge increased pH ofthe acid soil and available plant nutrients to thesoils. The heavy metal concentration levels in thesoils also increased. Most partitioned into easilymobile pools and later into sparingly mobile poolsfollowing 12 weeks of incubation time. Bioavailabilityforms of Cd in soil solution were low but that of Cu,Zn, and Mn remained elevated even at 12 weeks of thereaction time. Applied inorganic Zn depressed Cdsorption capacity of two soils studied but it had noeffect on Cd desorption.     

Solubility,sorption and desorption of native and added cadmium in relation to properties of soils in New Zealand

Solution cadmium (Cd) concentrations and sorption and desorption of native and added Cd were studied in a range of New Zealand soils. The concentration of Cd in solution and the concentrations and patterns of native soil Cd desorbed and added Cd sorbed and desorbed varied greatly between the 29 soils studied. Correlation analysis revealed that pH was the most dominant soil variable affecting solution Cd concentration and sorption and desorption of native and added Cd in these soils. However, organic matter, cation exchange capacity (CEC) and total soil Cd were also found to be important. Multiple regression analysis showed that the log concentration of Cd in solution was strongly related to soil pH, organic matter and total Cd, which in combination explained 76% of the variation between soils. When data from the present study were combined into a single multiple regression with soil data from a previously published study, the equation generated could explain 81% of the variation in log Cd solution concentration. This reinforces the importance of pH, organic matter and total Cd in controlling solution Cd concentrations. Simple linear regression analysis could at best explain 53% of the total variation in Cd sorption or desorption for the soils studied. Multiple regression analysis showed that native Cd desorption was related to pH, organic matter and total Cd, which in combination explained 85% of the variation between soils. For sorption of Cd (from 2 μg Cd g^–1 soil added), pH and organic matter in combination explained 75% of the variation between soils. However, for added Cd desorption (%), pH and CEC explained 77%. It is clear that the combined effects of a range of soil properties control the concentration of Cd in solution, and of sorption and desorption of Cd in soils. The fraction of potentially desorbable added Cd in soils could also be predicted from a soil’s K_d value. This could have value for assessing both the mobility of Cd in soil and its likely availability to plants.     

可溶性有机物对土壤中绿麦隆吸附与解吸的影响

A batch equilibrium techniques was used to examine the effect of dissolved organic matter （DOM） extracted from both non-treated sludge （NTS） and heat-expanded sludge （HES） on the sorption and desorption of chlorotoluron （3-（3-chloro-p-tolyl）-1,1-dimethylurea） in two types of soils, a yellow fluvo-aquic and a red soil from China. Without DOM,sorption of chlorotoluron was significantly greater （P 〈 0.05） in the red soil than in the yellow fluvo-aquic soil. However,with DOM the effect was dependent on the soil type and nature of DOM. Chlorotoluron sorption was lower in the yellow fluvo-aquic soil than in the red soil, suggesting that with the same DOM levels the yellow fluvo-aquic soil had a lower sorption capacity for this herbicide. Application of DOM from both NTS and HES led to a general decrease in sorption to the soils and an increase in desorption from the soils. Desorption of chlorotoluron also significantly increased （P 〈 0.05） with an increase in the DOM concentration. Additionally, for sorption and desorption, at each DOM treatment level the NTS treatments were significantly lower （P 〈 0.05） than the HES treatments. This implied that non-treated sludge had a greater effect on the sorption and desorption of chlorotoluron than heat-expanded sludge.     

Kinetics of Desorption of Heavy Metals from Polluted Soils: Influence of Soil Type and Metal Source

Kinetics of desorption of heavy metal ions (Cd, Cu, Ni and Zn) from the surface (0–15 cm) samples of an acidic soil (Inceptisol) and a neutral soil (Mollisol) spiked with inorganic salts of these metals or through an acidic sludge were studied by the column method. The rate of desorption of soil applied heavy metals was initially rapid and gradually declined with time. In general, the release of heavy metals from soils polluted by inorganic or sludge sources conformed to a multi-site model of first order kinetics; however, the release of Cd and Ni applied through inorganic sources to the neutral soil could be adequately accounted for by single-site model. The double-site model could adequately explain the release of Cd from sludge amended acidic soil and the release of Zn applied though inorganic salt or sludge to the neutral soil. In acidic soil, the apparent desorption rate coefficients of heavy metals applied through inorganic sources were higher than those for the sludge source. In neutral soil, however, the apparent desorption rate coefficients of heavy metals added through sludge were higher than for inorganic sources. Among the heavy metals, the higher apparent desorption coefficient value and percent desorption of Cd indicated a higher potential of this metal for leaching and ground water contamination. The results also suggested that the acidic soil pH might reduce the ability of the soil to naturally sequester heavy metal cations and lead to their leaching.