Mechanism For Remediation Of Cadmium Contaminated Soils With Red Gypsum And Bioavailability Study Of Immobilized Cadmium

Red gypsum(abbrev.RG) is a solid waste product from industrial production of titanium dioxide,which is extracted by sulphuric acid digestion.The waster water of dilute sulphuric acid is neutralized with slaked lime slurry to produce RG,whose red colour is imparted by oxidative conversion of Fe2+ to Fe3+.The yield of RG is high and still in increase with the market demand of titanium dioxide.The application of RG as construction materials such as cement retarder is restricted due to the high water content and high impurity content.Land application has been proposed as a disposal option for RG,which is an environmental friendly and economic method for disposal of solid waste.In this study,a novel method for disposal and utilization of RG as a kind of resource has been developed based on the feactures of RGThe feasibility of RG as Cd-contaminated soils amendment was determined with detailed characterization of physio-chemical properties,combined with survey on adsorption of cadmium by RG and influence factors of Cd species in soils.The main mechanism and remediation effects have been studied by plot experiments and in-situ field experiments.The main research work are summarized as follows.1.Detailed analysis of physico-chemical properties of RG were carried out and the feasibility for use of RG as amendment was thus confirmed.It is experimentally shown that RG is slightly alkaline,which consists of small size particles and contains 55% water.The impurity element of highest content is iron in the form of colloidal Fe(OH)3,which carries negative charges.ANC is 6625 mmol/g.RG is rich in essential elements for plants,such as Fe,Mg,Ca,S and so on.RG is a feasible amendment for remediation of Cd-contaminated soils, which can lead to slight increase in pH value of the treated soil.2.The adsorption of Cadmium with RG and the effect on adsorption capacity by related factor have been studied, and henceforth, the potential for remediation of cadmium contaminated soils with the aid of RG adsorption has been evalated.The Langmuir model can be use to describe adsorption capacity with satisfactory precision,while maximum adsorption capacity is 18.29mg/g at pH 6.The adsorption capacity is pH dependent undergoing a decrease with H+ concentration increase in the range of 3-6.The optimum dosage of RG in Cd adsorption is 10g/L.3.Investigation on presence of Cd species in soils and mechanism for immobilization of Cd with RG has been conducted.The application of RG is found to increase the pH,CEC,and decrease Eh value of soil,the highest increase beeing obtained at the 6.4% RG dosage,which is 24%,15% and 11% respectively. With the addition of RG, EXC-Cd is reduced from 20.7% to 13.8% and CA-Cd is reduced from 23.6% to 13.6% in the soil,while FeMnOx-Cd exhibits a slight increase from 11.3% to 12.4% and RES-Cd is increased from 33.3% to 44.6%. Precipitation and adsorption are the main mechanisms for immobilization of Cd with red gypsum.4.RG was then applied to remediate soils in field experiments,and Cd bio-availability was investigated.Results indicated that the addition of red gypsum significantly reduce the MgCl2,DTPA and HCl-extractable Cd concentrations by 39%,33% and 17% respectively, resulting in a notable decrease in the Cd concentration in the grain.It can be concluded that RG is beneficial to prevent the uptake of Cd in rice and red gypsum is a feasible and effective amendment for Cd-contaminated soil.The application of RG had no adverse effect on plants growth.Based on the corresponding research,an effective method for utilization of RG as a resource has been developed. In consideration of the large area of Cd polluted lands, use of RG in soil remediation provides a feasible outlet for disposal of the huge amount of RG generated in titanium dioxide production. Additionally, the results of this research work can be applied in remediation of soils polluted by other heavy metal elements.