Preparation And Adsorption Mechanism Of Cd(Ⅱ) From Water In Mining Area By Modified Phytolacca Acinosa Roxb.biochars

The acidic wastewater,tailings,and waste rock produced by industrial activities such as smelting and mining not only damage the vegetation on the surface but also produce heavy metal pollutants that enter the soil,atmosphere,and water bodies with heavy metals,causing heavy metal pollution around the mining area.Among them,cadmium（Cd）is the main pollutant.Heavy metal cadmium（Cd）,which is an important pollutant produced during smelting and mining with the characteristics of strong biological toxicity,persistent hazard,and easy accumulation,poses a serious threat to the public health and environmental quality after entering the water body.At present,methods such as chemical precipitation,membrane separation,ion exchange,bioremediation,and adsorption are widely used in the adsorption and removal of Cd in water.The adsorption method is considered to be an effective method for simple,efficient,and low-cost removal of Cd in water.Biochar has attracted extensive attention as a heavy metal adsorption material because of its advantages of wide source,low cost,and good adsorption performance.In this paper,carbonate-modified biochar and magnetic hydrochar were prepared by Phytolacca acinosa Roxb.derived from Xiangtan manganese mine,which is a heavy metal hyperaccumulator plant.The physicochemical properties of carbonate-modified biochar at different pyrolysis temperatures and magnetic hydrochar were analyzed by Fourier transform infrared spectroscopy,scanning electron microscope,X-ray diffraction pattern,and X-ray electron spectroscopy.The adsorption performance of carbonate-modified biochar at different pyrolysis temperatures and magnetic hydrochar on Cd（Ⅱ）was studied by designing batch experiments.The adsorption mechanism and the performance in the treatment Cd（Ⅱ）of simulated mine wastewater of carbonate-modified biochar at different pyrolysis temperatures and magnetic hydrochar were discussed evaluated.The main results are summarized as follows:（1）Phytolacca acinosa Roxb.was impregnated into Ca Cl₂and Na₂CO₃solution to obtain modified biomass with a high carbonate.Carbonate-modified biochars were prepared by pyrolysis with modified biomass at four temperatures of 300°C,400°C,500°C,and 600°C,and were recorded as Bio C300,Bio C400,Bio C500,and Bio C600.The pyrolysis temperature has a great influence on the physicochemical properties of carbonate-modified biochar.The results showed that the ash contents of Bio C300,Bio C400,Bio C500,and Bio C600 were17.87%,21.52%,23.96%,and 27.96%,and the yields were 42.22%,34.09%,31.05%,and30.19%,respectively,indicating that the higher the pyrolysis temperature,the higher the ash content of carbonate-modified biochar,the higher the degree of carbonization,and the lower the yield.The effects of pyrolysis temperature,solution p H,and adsorbent dosage on Cd（Ⅱ）adsorption of carbonate-modified biochar were significant.The higher the amount of adsorbent,the higher the pyrolysis temperature and the p H of the solution,the better the effect of carbonate-modified biochar on Cd（Ⅱ）adsorption.Bio C400,Bio C500,and Bio C600 were able to rapidly flocculate after adsorbing Cd（Ⅱ）within 1 minute and settle within 20 minutes.The adsorption of Cd（Ⅱ）on carbonate-modified biochar is regulated by multiple mechanisms,including surface complexation,Cd-πinteraction,and surface precipitation,confirming that surface precipitation plays a major role in the adsorption of Cd（Ⅱ）by carbonate-modified biochar.（2）During the growth of Phytolacca acinosa Roxb.,iron salt（Fe SO₄solution）was irrigated,and iron-rich Phytolacca acinosa Roxb.was obtained through bioconcentration.Magnetic hydrochar was prepared by hydrothermal carbonization（180℃,8 h）of iron-rich Phytolacca acinosa Roxb..The iron content in the magnetic hydrochar was 8.49±0.41 mg/g,Fe₃O₄nanoparticles were formed and distributed on the surface of magnetic hydrochar.The initial concentration of the solution and the p H of the solution have a great influence on the adsorption of Cd（Ⅱ）by magnetic hydrochar.The lower the initial concentration of Cd（Ⅱ）solution and the higher the p H of the solution,the higher the removal rate of heavy metal Cd（Ⅱ）by magnetic hydrochar.The adsorption of Cd（Ⅱ）by magnetic hydrochar conformed to pseudo-second-order kinetics and Langmuir model.At 25℃,the maximum adsorption capacity(q_max)of Cd（Ⅱ）by magnetic hydrochar was 246.6 mg/g.The adsorption of Cd（Ⅱ）by magnetic hydrochar is affected by various mechanisms such as surface precipitation,surface complexation,electrostatic attraction,and ion exchange.The reusability experiment of magnetic hydrochar confirmed that the removal rate of Cd（Ⅱ）by magnetic hydrochar decreased after the second cycle,but the magnetic separation could still be achieved.（3）Although the physicochemical properties（including carbonization degree,ash content,point of zero charge,element content,functional groups,etc.）of carbonate-modified biochar and magnetic hydrochar are quite different,both of them have good effects on the adsorption of heavy metal Cd（Ⅱ）in water.And the adsorption mechanism of Cd（Ⅱ）by carbonate-modified biochar and magnetic hydrochar involves surface precipitation,surface complexation,electrostatic attraction,ion exchange,and Cd-πinteraction.The surface precipitation plays a major role in the removal of Cd（Ⅱ）from water by the two biochar,indicating that the use of biochar to adsorb heavy metal Cd（Ⅱ）in water should give priority to the role of surface precipitation.In summary,both carbonate-modified biochar and magnetic hydrochar,which have the prospect of application in wastewater treatment in mining areas,can be used to remove heavy metal Cd（Ⅱ）in water,and also have a good adsorption effect on simulated mining wastewater.