Removal Of Heavy Metals From Aqueous Solution By Biochars:Adsorption And Mechanisms

The properties of discarded mushroom-stick biochar(DMB),sheep manure biochar(SMB),robinia pseudoacacia biochar(RPB)derived from discarded mushroom-stick,sheep manure,and robinia pseudoacacia under different pyrolysis temperatures were investigated.Results showed that with the increased of pyrolysis temperature,the pH value,ash content,specific surface area,pore diameter,and total pore volume of these three biochars generally increased,while biochar particles and productivity decreased.The yield and surface oxygen-containing group of DMB,SMB and RPB prepared at 300?(DMB3,SMB3 and RPB3)were higher than that of DMB from higher pyrolysis temperature,whereas the DMB,SMB and RPB prepared at 800?(DMB8,SMB8 and RPB8)has higher pH,mineral elements,ash content and polarity.SMB surface contained abundant functional groups,especially DMB3 having a lot of oxygen containing functional groups on the surface.The pH of DMB8 was obviously higher than other biochar.At the same time,DMB8 has higher polarity and aromaticity,large specific surface area,high ash content and alkaline pH,which were conducive to the mineral precipitation of heavy metals by DMB8;Different pyrolysis temperatures will lead to differences in chemical and physical properties of biochar,thus affecting its adsorption effect on heavy metals.When DMB,SMB and RPB used as heavy metal adsorbents,the pH of wastewater increased.The heavy metals adsorption capacity of biochars were also examined.DMB8 has the maxmium adsorption capacity for Pb2+,Cu2+,Cd2+and Ni2+ in wastewater.The heavy metal adsorption capacity of DMB3 was higher than DMB5.The adsorption capacity of SMB was second,only inferior to DMB in these three biochar species.SMB3 has the highest adsorption capacity for heavy metals in all SMB.When the pyrolysis temperature higher than 300?,with the increased of pyrolysis temperature,the adsorption of heavy metals by SMB performanced degradation.RPB had the worst adsorption effect for heavy metals.The adsorption kinetics data fitted well by pseudo second-order model and sorption isotherms simulated well by Langmuir model,indicating heavy metal adsorption by biochars were monolayer chemical adsorption.Adsorption mechanisms of biochar for heavy metals include ion exchange,mineral precipitation,functional group complexation,and-? coordination.This study calculated the contribution of different adsorption mechanisms to the total adsorption quantity through the Calculation model.The result suggested that the interaction between heavy metal and the minerals was the dominant mechanisms of heavy metal sorption on DMB.In general,pyrolysis temperature has a great influence on the adsorption capacity and mechanism of biochar for heavy metals.DMB8 was considered most effective adsorbent for heavy metals,and discarded mushroom-stick was potential raw materials for the production of effective heavy metal adsorption biochar.This study demonstrated a successful example of waste refinery by converting discarded mushroom-stick,sheep manure and robinia pseudoacacia to feasible heavy metal adsorbents to implement the concept of circular economy.