Adsorption-desorption Characteristics And Environmental Behavior Of Cadmium In Particulate Organic Matter Of Purple Paddy Soil

Rice is a major staple food crop in China.It has a strong ability to accumulate cadmium?Cd?.Purple paddy soil is an important soil resource in China.Studying the transformation and mobility of Cd in purple paddy soil is of great significance to the prevention of soil pollution.The adsorption and desorption is an important process to control the transformation of Cd in soil.The adsorption and desorption behavior of Cd depends on the quantity and composition of soil organic matter.Particulate organic matter?POM?has the characteristics of high activity and low humus,and participates in the process of soil carbon,nitrogen,sulfur cycle and nutrient supply.It is found that POM is porous,and has obvious adsorption effect on heavy metals.However,there are few studies on the adsorption characteristics of POM on heavy metals in purple paddy soil.The effect of POM on the adsorption capacity of Cd in soil and the adsorption effect of POM on bioavailability of Cd in soil also need further discussion.The characteristics of cadmium ion(Cd²⁺)adsorption on POM isolated from typical acid purple paddy soil?APPS?and calcareous purple paddy soil?CPPS?were investigated by means of scanning electron microscopy-energy dispersive spectrometer?SEM-EDS?,fourier transform infrared spectroscopy?FTIR?,and the fractionation of Cd²⁺in POM before and after adsorption.Moreover,the influencing factors of POM on Cd adsorption were analyzed.Culture experiments were conducted to study the impact of POM and montmorillonite on the adsorption capacity of Cd in purple paddy soil and the adsorption effect of POM on the bioavailability of Cd in purple paddy soil.The conclusions are as follows:?1?The affinity of Cd in two types of POM were much higher than those to the original soil,and the affinity in CPPS-POM was higher than APPS-POM.The adsorption kinetic data could be well described by the pseudo-secondary order equation.The Freundlich equation was better than Langmuir model in describing Cd adsorption isotherm of POM,indicating that the adsorption of Cd on POM was a heterogeneous multi-molecular layer adsorption.The thermodynamic parameters illustrated that the adsorption was a spontaneous endothermic process.The value of?H^?and desorption experiments indicated that Cd was bond to POM primarily through chemical reactions.The fractionation showed that the adsorption process increased the percentage of exchangeable and complex forms in POM.Combining SEM-EDS and FTIR results,the mechanisms of Cd adsorption on POM included complexation of oxygen-containing functional groups,ion exchange,cation-?bonding and electrostatic adsorption.The optimal solid-liquid ratio for POM adsorption of Cd was 15g·L^-1;the optimal ionic strengths was 0.01mol·L^-1;and the smaller the particle size of POM was,the larger the adsorption capacity of Cd was.?2?In APPS and CPPS,the addition of POM and montmorillonite alone or together could increase the Cd adsorption capacity of soil,and the increase was the largest when they are added together.In APPS,the amount of Cd desorption capacity in montmorillonite and POM combined montmorillonite was significantly greater than that of POM and control treatment.In CPPS,there was no significant difference between all treatments.The pH,CEC and organic carbon content had very significant positive correlations with Cd adsorption capacity and desorption capacity.Therefore,POM and montmorillonite could affect the Cd adsorption ability by changing p H,CEC and organic carbon content.?3?In APPS and CPPS,the effectiveness of Cd adsorbed in POM was lower than that of directly cultivating equivalent Cd contaminated soil.The effective Cd content in APPS was greater than CPPS.Compared with the control,Cd adsorbed in POM and cultivating Cd contaminated soil treatment promoted the conversion of Cd to tightly bound organic matter form in APPS,while to the mineral form in CPPS.Moreover,the proportion of exchangeable Cd in POM treatment was the smallest,indicating that POM can reduce its environmental risks.Both pH and organic carbon content were significantly negatively correlated with effective Cd content.Compared with the control,the other two treatments increased the pH of APPS and CPPS,but there was no significant difference between them.The POM treatment improved the organic carbon content of APPS and CPPS,while cultivating Cd contaminated soil has been reduced,which again proved that the bioavailability of Cd in POM was lower.