Study On The Adsorption Of Cd（Ⅱ） And Pb（Ⅱ） By Mg-Fe/LDO Loaded Sludge Biochar

The production of urban sludge in China has increased dramatically in recent years,and its subsequent treatment and disposal is under great pressure.The preparation of sludge biochar adsorbent and its use for removing heavy metals from water has become a new direction of sludge treatment and disposal.In recent years,how to improve the performance of sludge biochar has become the focus of sludge biochar research.Layered Double Hydroxide（LDHs）have high specific surface area and good adsorption effect,but are easy to agglomerate and have low mechanical strength.The adsorption performance of the composites prepared by using sludge biochar as a carrier and loaded with LDO is greatly improved,which provides a direction for the preparation of adsorbent materials with low price and excellent adsorption performance.In this study,sludge biochar carriers were prepared by KOH activation using sludge as the raw material,and the preparation parameters were optimized.Meanwhile,the sludge biochar was prepared as a carrier loaded with LDHs,and the sludge biochar-LDOs composites were prepared after pyrolyzing to investigate the adsorption performance and adsorption mechanism of the prepared materials on Pb（Ⅱ）and Cd（Ⅱ）in water,and to investigate the competitive adsorption of Pb（Ⅱ）and Cd（Ⅱ）in the multivariate system.First,the sludge biochar carriers were optimally prepared and characterized by response surface methodology using KOH as the activator.The results showed that the KOH soaking concentration was the most significant factor,and higher soaking concentration was beneficial to improve the adsorption performance of sludge biochar（ASB）;increasing the KOH soaking concentration and increasing the pyrolysis temperature could synergistically improve the adsorption performance of ASB;the optimal preparation conditions were 2.5 mol·L^-1 KOH impregnation concentration,7 h impregnation time,631℃pyrolysis temperature and 44 min pyrolysis time.The surface roughness of the KOH-modified sludge biochar,the increase in specific surface area,and the increase in the number of micropores resulted in a specific surface area of 141.22 m~2·g^-1 for ASB,which was 24 times higher than that of the original sludge biochar(SB:5.93 m~2·g^-1).Next,ASB-LDO composites with the best adsorption performance were prepared and characterized by single-factor experiments with ASB as the carrier and pyrolyzed after loading LDHs.The results showed that among the four composites,ASB-Mg-Al/LDO,ASB-Mg-Fe/LDO,ASB-Mn-Al/LDO and ASB-Mn-Fe/LDO,ASB-Mg-Fe/LDO had the best adsorption effect on Cd（Ⅱ）and Pb（Ⅱ）.The optimal preparation parameters of ASB-Mg-Fe/LDO were:Mg,Fe molar ratio of 3:1,loading concentration of Mg²⁺of 0.4 mol·L^-1,and roasting temperature of 500°C.The surface of ASB-Mg-Fe/LDO was rough and contained a large number of fine lines tunnels,which provided larger adsorption sites for biochar adsorption,while the content of Mg and Fe elements was greatly increased;ASB-Mg-Fe/LDO detected the same crystal structure as Mg O,and its-OH oxygen-containing functional groups were also greatly increased compared with ASB.Then,the effects of ASB-Mg-Fe/LDO on the adsorption of Cd（Ⅱ）and Pb（Ⅱ）at different adsorption times,adsorption temperatures,initial concentrations and initial p H were investigated,and their adsorption behaviors were explored;the adsorption mechanisms were investigated by combining characterization means and adsorption contribution experiments.The results showed that at the optimum adsorption temperature of 35℃and adsorption p H of5,the maximum adsorption capacity of ASB-Mg-Fe/LDO for Cd（Ⅱ）was 302.09 mg·g^-1,and for Pb（Ⅱ）was 440.48 mg·g^-1.The adsorption of Cd（Ⅱ）and Pb（Ⅱ）by ASB-Mg-Fe/LDO were both mixed adsorption of multi-molecular layers with chemisorption as the main rate control step,and the adsorption was consistent with the Freundlich isothermal adsorption model,and increasing the solution p H and temperature could promote the adsorption.After adsorption of Cd（Ⅱ）and Pb（Ⅱ）,white solids were attached to the surface of both ASB-Mg-Fe/LDO,while rod-like compounds were also present in ASB-Mg-Fe/LDO after adsorption of Pb（Ⅱ）.The content of K⁺,Ca²⁺and Mg²⁺elements on the surface of ASB-Mg-Fe/LDO decreased,and the intensity of the peaks of-OH and-COOH oxygen-containing functional groups decreased.The adsorption mechanism involved mineral precipitation(Q_mp),ion exchange(Q_ie),complexation of oxygen-containing functional groups(Q_oc)and metalπ-bond binding(Q_mπ),and the order of contribution of different adsorption mechanisms was:Q_mp>Q_ie>Q_oc>Q_mπ,with mineral precipitation and ion exchange accounting for about 95%and dominating in adsorption.Finally,the effects of different adsorption times,initial concentrations,and adsorption p H on the adsorption of Cd（Ⅱ）and Pb（Ⅱ）by ASB-Mg-Fe/LDO,respectively,under the binary system of Cd（Ⅱ）-Pb（Ⅱ）were investigated.The effects of competitive adsorption of Cd（Ⅱ）and Pb（Ⅱ）in the same system and the presence of other metal cations Na⁺,Ca²⁺and Mg²⁺on the adsorption of Cd（Ⅱ）and Pb（Ⅱ）by ASB-Mg-Fe/LDO were investigated.The results showed that under the binary system of Cd（Ⅱ）-Pb（Ⅱ）,the increase of adsorption time,p H and adsorption concentration were favorable to adsorption,while the adsorption amount of Pb（Ⅱ）was always higher than that of Cd（Ⅱ）.In the process of adsorption,Cd（Ⅱ）only inhibited the adsorption rate of Pb（Ⅱ）in the initial stage of adsorption,but did not affect the equilibrium adsorption amount of Pb（Ⅱ）,while Pb（Ⅱ）inhibited the equilibrium adsorption amount of Cd（Ⅱ）.The effects of coexisting cations Na⁺,Mg²⁺and Ca²⁺on the adsorption of Cd（Ⅱ）and Pb（Ⅱ）by ASB-Mg-Fe/LDO were in the order of Mg²⁺>Ca²⁺>Na⁺,Na⁺hardly affected the adsorption of Pb（Ⅱ）,and Mg²⁺and Ca²⁺inhibited the adsorption of both Cd（Ⅱ）and Pb（Ⅱ）,with a greater inhibition on Cd（Ⅱ）.