Study On Adsorption And Stabilization Of Cadmium And Zinc By Dairy Manure And Walnut Shell Biochars

The pollution of heavy metals in water and soil in China is serious,and the treatment and disposal of agricultural biomass waste is unreasonable.Therefore,it is of great significance and prospect to study the adsorption of heavy metals in water and the stabilization of heavy metals in soil by using biochar produced by pyrolysis of agricultural biomass wastes.This study first investigated and evaluated the heavy metal pollution condition in the downstream area of an industry zone in western Yunnan Province,China,which found that target heavy metals in this area were Cd and Zn.Using the Slow-Speed Oxygen-Limiting Pyrolysis Method（300℃,700℃respectively）,dairy manure and plant residue walnut shells were pyrolyzed into biochars.And then their adsorption of Cd,Zn in water and stabilization of Cd,Zn in soil were measured.Finally,adsorption and remediation mechanisms were revealed by means of modern instrumental analysis.The main findings were as followed:The results of investigation on heavy metal pollution in the lower reaches of a lead and zinc smelter in western Yunnan showed that heavy metal pollution in surface soil was mainly caused by Cd,Zn,Cu was light and surface water and groundwater were mainly polluted by Cd.From the spatial distribution,the Cd and Zn sites of high concentration soil were mainly distributed in marginal areas which close to the lead-zinc smelter.From the vertical distribution,Cd and Zn accumulated obviously in the topsoil of the profile,and the concentrations of Cd and Zn decreased gradually with the increase of sampling depth.The results of Nemero comprehensive pollution index evaluation,geo accumulation index evaluation and potential ecological risk evaluation all indicated that the region had been seriously polluted by Cd.Health risk assessment showed that Cd in soil of the region had significant potential health risk to exposed population.The results show that the area is polluted by Cd,Zn.Cd²⁺and Zn²⁺are two common heavy metal pollutants that have characteristics of a low toxicity threshold and are easily bioconcentrated.In order to remove Cd²⁺and Zn²⁺from aqueous solutions efficiently,using dairy manure and walnut shells as raw materials,biochar was prepared at different pyrolysis temperatures.The adsorption equilibrium method and dynamic adsorption method were used to study the kinetic characteristics of biochar and the adsorption effect of biochar on Cd²⁺and Zn²⁺in aqueous solutions.The adsorption mechanism was investigated by detecting biochar before and after adsorption using X-ray diffraction and Fourier transform infrared spectroscopy.The results showed that biomass materials and pyrolysis temperature were two major factors affecting the adsorption effect of biochar.The adsorption effect of dairy manure biochar was greater than that of walnut shell biochar,and the adsorption effect of biochar prepared at700°C was greater than that of biochar prepared at 300°C.Langmuir isotherms were more suitable for describing the Cd²⁺and Zn²⁺adsorption mechanisms of biochar.Dairy manure biochar prepared at 700°C（DM 700）was the most effective adsorption material for Cd²⁺and Zn²⁺,with maximum adsorption capacities of 117.5mg·g^-1 and 59.4mg·g^-1,respectively.The adsorption kinetics of DM700 fit better with pseudo-second order kinetics and included the rapid stage and slow stage.Ion exchange and the complex reaction between hydroxyl and carboxyl groups in biochar with Cd²⁺and Zn²⁺were the main adsorption mechanisms.CdCO₃ and Zn₃（PO₄）₂precipitates were formed after Cd²⁺and Zn²⁺were adsorbed.Therefore,DM700 has great potential as an adsorbent for Cd²⁺and Zn²⁺.This study provides a theoretical basis and application reference for biochar removal of heavy metals from water.The results of Cd and Zn tests showed that after biochar remediation,the pH value of high Cd,Zn contaminated soil around a lead and zinc smelter in western Yunnan increased,and the CaCl₂ extractable state content of Cd and Zn decreased.The acid extraction states of Cd and Zn were gradually transformed into reducible,oxidizable and residual states.The effect of DM700 on Cd and Zn remediation was the best,and the pH value of soil was increased from 5.84 to 8.15 after the addition of 5%（w/w）DM700biochar,and the effective concentration of CaCl₂ extracted from soil Cd,Zn decreased 99.6%and 99.7%,respectively.The acid extraction state of Cd decreased by 13.8%,and the reducible state,oxidizable state and residue state increased by 5.5%and 7.7%,respectively.The acid extraction state of Zn decreased by 30.6%,and the reducible state,oxidable state and residue state increased by 21.8%and 7.1%respectively.This indicates that DM700is an ideal stabilized remediation material for heavy metal Cd,Zn in soil.In conclusion,the environmental effects of biochar are different due to biomass raw materials and pyrolysis temperature.Among them,dairy manure biochar prepared at 700℃could effectively adsorb Cd and Zn,in water and stabilize Cd and Zn in polluted soil.This study provides theoretical basis and application reference for removing heavy metals in water and remediation of soil heavy metal pollution by biochar.