Research On The Effect Of Hollow Amaranth Biochar, Bentonite And Their Composite Adsorbent On Cd Retention In Tea Garden Soil

With the rapid development of industry,agriculture,tourism,transportation and other industries,heavy metal pollution is increasing in the soil.The control of heavy metal pollution in the soil has become a hot spot of current research.Cadmium is a typical heavy metal pollutant in soil.It has the characteristics of a long cycle,high mobility,high biotoxicity and refractory degradation in the environment.It is easily absorbed by plants and remained in the body,and endangers the health of animals and human beings through food chain enrichment.Solidification/stabilization technology has become a relative mature method for treating heavy metal contaminated soil due to its simple operation and good effect.At present,there are many researches on the application of single curing materials for the repair of heavy metal pollution,but there are few studies on materials that combine two or more materials to obtain more excellent effects of heavy metal pollution.Therefore,it is a great theoretical significance for the in-depth study of curing repair materials.In this study,the inorganic material-calcium bentonite and organic material-alternanthera philoxeroides were used as raw materials,the bentonite adsorbent material?BE?with higher purity was obtained by simple physical purification.Biochar?PB?and composite adsorbent?APB?were prepared at 300,500 and 700?respectively.The effects of pyrolysis temperature and material type on the physicochemical properties of the adsorbents were investigated by elemental analysis,scanning electron microscopy and Fourier transform infrared spectroscopy.The adsorption characteristics and mechanism of Cd²⁺in the aqueous phase were studied by batch adsorption experiments.The immobilization of Cd²⁺in soil of tea garden by organic and inorganic single adsorbents and their composite adsorbents was examined and evaluated.The feasibility of single and composite adsorbents for remediation of Cd²⁺contaminated soil was discussed.The main conclusions were as follows:?1?The results showed that the adsorbents were alkaline,with the increase of pyrolysis temperature,the pH value of alternanthera philoxeroides biochar and composite adsorbent increased,the yield of alternanthera philoxeroides biochar and composite adsorbent decreased,ash content increased,the contents of C,H,N and O elements decreased and the atomic ratios decreased;While the acidic functional groups on the surface of alternanthera philoxeroides biochar and composite adsorbent decreased,and aromaticity increased.The surface pore of alternanthera philoxeroides biochar was more abundant with the increase of pyrolysis temperature,while the increase of pyrolysis temperature was not conducive to the development of pore structure on the surface of composite adsorbents.The peak values of Si,Ca,Mg,Al and the functional groups of Si-O-Si and Si-O stretching vibration absorption peaks on the surface of composite adsorbents indicate that bentonite had been loaded on alternanthera philoxeroides biochar.?2?Biochar at 700?and the composite adsorbents at 300?exhibited good capacities.Quasi-second-order adsorption kinetic model could describe accurately the adsorption kinetics of adsorbents for Cd²⁺.Adsorption process of adsorbent for Cd²⁺could be divided into two stages:surface adsorption and slow adsorption.It showed that diffusion in particles was the main but not the only speed control step in the adsorption process.The adsorptive behavior of adsorbents for Cd²⁺conformed to the heterogeneous multilayer adsorption described by Freundlich model,and adsorption of Cd²⁺by adsorbent was a spontaneous endothermic process.The pH value of the solution could affect significantly the adsorbent's adsorptive capacity for Cd²⁺.When the pH value reached 6.0,the adsorptive capacity reached equilibrium.The adsorption mechanisms of adsorbents for Cd²⁺were affected by material morphology,complexation,cation-pi interaction,coprecipitation and ion exchange.?3?Soil pH value could be increased effectively after the application of adsorbent,and it increased with the increase of the amount of adsorbent applied.After the application of adsorbent,the contents of available Cd in soil could be reduced significantly,the Cd in soil could be effectively fixed,and its migration could be reduced,which was proportional to the amount of adsorbent applied and pyrolysis temperature.Adsorbent application could reduce effectively the content of exchangeable Cd in soil,and the content of one or more forms of carbonate-bound,residue-bound,iron-manganese oxide-bound and organic-bound forms increased with the increase of the amount of adsorbent applied.Biochar at 700?had the most significant effect on reducing the exchangeable Cd²⁺content in soil under 3%application.The composite adsorbent was slightly less effective than biochar at 700?,and also achieved good results.