Effects Of Peat Application On Chemical Fractions And Bioavailability Of Cadmium In Contaminated Soils

Studies on the eco-toxicology and techniques for remediation of Cd polluted farmlands has been paying much attention in agriculture and environmental sciences, since 1961 when "Itai-itai" disease in Japan was reported due to cadmium(Cd) pollution..The effect of organic materials on environmental chemical behavior of heavy metals in soil- plant system has been important in the field of research,organic materials are usually considered as natural materials to inhibit the activity and the bioavailability of heavy metals in polluted soils,but there are also many research results show that,the application of organic materials was promoting absorption and translocation of Cd from the soils by plant,increasing Cd contents in the plant. Therefore,the interaction between organic materials and heavy metals in the soil is a very complex phenomenon,far away from clear understanding of people.The objective of this research was to investigate the effect of Cd application on peat of the soil adsorption,desorption,chemical speciation and bioavailability of the plant using soil culture experiments and pot experiments under well-controlled laboratory conditions.Main results are as follows:(1)Effects of the batch adsorption of cadmium on peat were studied and the isotherm kinetic studies were conducted in order to determine the effects of contact time,pH of reaction solution,initial concentration of the adsorbate,concentration of solvent,amount of applied peat and shake velocity.It was found that a contact time of 1 h was necessary for the adsorption to reach equilibrium and the absorption rate of cadmium on peat was achieved 99.1%.The dynamics equation for the absorption of cadmium on peat was described better fit by Elovich equation(R² = 0.807).The isotherms adsorption equation for the absorption of cadmium were described well by the Langmuir model and Freundlich model,however,the statistical analysis of the data showed that Langmuir equation was fitted better(R² = 0.9985),and the maximum capacity of Cd²⁺on peat from Lungmuir equation obtained was 13699 mg/kg in the single component systems.The absorption of cadmium on peat was significantly affected by pH of reaction solution,and the optimum pH range was 6.0 to 7.0.In addition,the increase in the concentration of medium ion decreased the removals by peat from solution.Add peat concentration of the choice of 2.5 g/L suitable,the best oscillation rate of 100 rpm.The concentration of added peat was suitable to 2.5 g/L,and the best of shake velocity was 100 rpm.(2)The isothermal adsorption of cadmium in two paddy soils(yellow mottled paddy soil and blue clayed paddy soil)was measured.The two soils were collected from Jiaxing,Zhejiang Province,China.The results showed that the isothermal sorption of cadmium fitted well with three Lungmuir equation,Freundlich equation and Temkin equation,and the correlation coefficient was 0.95 and was highly significant.The statistical analysis showed that Langmuir equation was fitted better. The maximum capacity of Cd²⁺on soil obtained from Langmuir equation was 1883.2 mg/kg in the yellow mottled paddy soil and 5208.3 mg/kg in the blue clayed paddy soil,respectively,and the application of peat to two soils did not affect the adsorption of cadmium.At the yellow mottled paddy soil Cd²⁺was added as initial concentration at a rate of 150 ml/L,the peat added at a rate of 5.0 g/kg and 10.0 g/kg increased the amount of adsorption of Cd²⁺in soil from 1375.3 mg/kg and 1401.3 mg/kg, respectively,the rates of increase was 1.85%and 3.77%in compared with the control.At the blue clayed paddy soil,the peat added at a rate of 5.0 g/kg and 10.0 g/kg increased the amount of adsorption of Cd²⁺in soil from 4517.5 mg/kg and 4532.8 mg/kg,respectively,the rates of increase were 0.34%and 0.68%compared with the control.The research results on the isothermal desorption of Cd²⁺showed that the isothermal desorption of cadmium was significantly correlated with isothermally absorbed cadmium.In addition,the increase of the amount of Cd²⁺adsorption in soil increased the amount of Cd²⁺desorption in soil,and that was significantly increasing trend.The amount of desorbed Cd²⁺in the yellow mottled paddy soil was from52 to 83%of the amount of adsorption,in the blue clayed paddy soil,was from 6 to 20%.After peat was added,the amount of Cd²⁺desorption in the yellow mottled paddy soil decreased,but the desorption rate in the blue clayed paddy soil was not obvious.(3)The influence of peat on soil DTPA extractable Cd was investigated under laboratory incubation conditions using two paddy soils contaminated with different levels of Cd(Cd 0 and 5 mg/kg)under different levels of peat.The results showed that the contents of DTPA extractable Cd increased with the increase of peat levels of two contaminated soils,whereas the influence of the different levels of peat on soil DTPA extractable Cd was not obvious.The exchangeable Cd fraction in control soils increased with the increase of peat levels of two contaminated soils,whereas the iron-manganese oxide bound Cd fraction decreased.These two Cd fractions in the soils treated with 5.0g peat kg^-1soil are statistically different with the control.The rising peat levels tend to increase the content of organic matter bound Cd fraction in soils.The rising peat levels tend to decrease the content of water-soluble Cd fraction in yellow mottled paddy soil.The water-soluble and exchangeable Cd fraction increased with the increase of peat levels of yellow mottled paddy soil contaminated with 1.0 and 5.0 mg/kg Cd, whereas the iron-manganese oxide bound Cd fraction decreased.The exchangeable and organic matter bound Cd fraction increased with the increase of peat levels of blue clayed paddy soil,whereas the iron-manganese oxide bound Cd fraction decreased.The transformation of soil Cd is related to the change of soil pH and water soluble organic matter after the application of peat in soils.(4)A glasshouse experiment was conducted using a specially designed rhizobox where Hangzhou Youdonger seedlings were grown for 100 day in two soils spiked with Cd at a concentration of 5 mg/kg to investigate the effect of peat application on Cd fractions in the rhizosphere and bulk soils.The soil in the rhizobox was divided into six separate compartments at various distances from the root surface.The research results shown that,the dry biomass of the shoots increased with the increase of peat levels of soils,whereas the application of peat to two Cd contaminated soils did not adversely affect dry matter production of plant.The dry biomass of the shoots decreased with the rising Cd concentration in the soils,and the concentration and amount of Cd in the shoots increased with the increase of peat levels of soils.In condition of added or not added peat,it revealed that the form of distribution of Cd in the rhizosphere and bulk of two soils were same,the degradation gradient followed the order:exchangeable Cd＞iron-manganese oxide bound Cd＞organic matter bond Cd.The content of the exchangeable Cd in the rhizosphere was lower than in bulk soil,whereas the content of the iron-manganese oxide bound Cd and organic matter bound Cd decreased.The content of the extractable Cd in the non-rhizosphere varied with the distance from the root surface,that was especially small within 0-3 mm,the content of the extractable Cd in bulk soil was slightly higher than in rhizosphere soil.The rising distance from root surface increase content of the extractable Cd in the soil of the rhizosphere more than 3 mm.Compared with the bulk soil,the rising distance from root surface tend to increase the content of the iron-manganese oxide buond Cd and organic matter bound Cd in the control soil and added peat soil,whereas tend to decrease the content of the iron-manganese oxide bound Cd.