Study Of Adsorption Behavior Of Cd²⁺ And Pb²⁺ By Vermicomposts And Vermicompost Biochars

The application of biochar in the remediation of heavy metal pollution in water environment has been widely concerned by the scientific community.Characteristics of raw materials for preparation of biochar and the types of heavy metals affect the adsorption efficiency and mechanism of biochar on heavy metals.How to utilize municipal sludge to solve the problem of heavy metals pollution in water based on the idea of"treating waste with waste"is a very promising and challenging research topic.Vermicomposts produced by vermicomposting with organic wastes are rich in carboxyl,and hydroxyl groups,along with various minerals that have a strong adsorption ability for heavy metals.At present,there are still few studies on the removal of heavy metals from water by vermicompost and its by-products.In this study,vermicompost produced by vermicomposting with municipal sludge was used as the basic raw material to prepare biochar by anaerobic pyrolysis,and then activated by oxidation and magnetic modification technology.Vermicompost and vermicompost biochars were used as adsorbents to study the adsorption efficiency and mechanism of Cd²⁺and Pb²⁺in aqueous solution by static adsorption test,model fitting,optics,spectroscopy and element analysis.The results were as follows:(1)The vermicompost was mainly mesoporous structures,which was a slit pore formed by stacking flake particles.The p H was weak acidity,and the ash content w as high.The main functional groups were OH,CH₂,aromatic ring(C=O,C=C or C-C),amide,nitro compounds,carbonate(C=O and C-O),silicate(Si-O),C-O of polysaccharides,alkenes,alcohols and halogen compounds.The change in p H value of the solution significantly affected vermicompost adsorption of Cd²⁺/Pb²⁺.However,the effect of Na NO₃ concentration on Cd²⁺/Pb²⁺adsorption was not obvious.At room temperature(25?),the maximum adsorption capacity of Cd²⁺/Pb²⁺by vermicompost was41.06 mg/g and 116.65 mg/g,respectively.The adsorption capacity of Cd²⁺/Pb²⁺by vermicompost increased with solution temperature increasing.The adsorption process was spontaneous and endothermic monolayer chemisorption.CO₃^2-,C-F and-OH played a major role in the adsorption of Cd²⁺by VC,and CO₃^2-and-OH played a major role in the adsorption of Pb²⁺by VC.The main products were Cd CO₃,Cd F₂,Pb CO₃ and Pb(OH)₂.The contribution of vermicompost of Cd²⁺adsorption were in the order of Q_p>Q_{p H}>Q_e>Q_o,while that of Pb²⁺was Q_p>Q_o>Q_{p H}>Q_e.The percentage of mineral adsorption of Cd²⁺/Pb²⁺by vermicompost(the sum of surface precipitation and ion exchange)was 64.05%and 70.19%,respectively,which was the dominant mechanism of the adsorption process.The surface precipitation accounted for 44.85%and 63.76%.When Cd²⁺and Pb²⁺coexist,vermicompost first adsorbed Pb²⁺,which means that the affinity of Pb²⁺to vermicompost is greater than that of Cd²⁺.(2)The biochars(VBC300?VBC500 and VBC700)prepared at 300?,500?and 700?were in the form of lumpy and granular structures,and the pores were slit pores formed by stacking flake particles.With the pyrolysis temperature increasing from 300?to 700?,the specific surface area increased,while the average pore diameter decreased.As the pyrolysis temperature increasing,the yield of biochar decreased,p H changed from neutral to alkaline,the ash content increased,the content of C,N,H and O decreased,and the stability increased.The types and contents of functional groups in vermicompost biochars were significantly lower than those in vermicompost.The main functional groups were OH,aromatic ring(C=O,C=C or C-C),carbonate(C=O and C-O),silicate(Si-O),C-O of polysaccharides and halogen compounds.The adsorption of Cd²⁺/Pb²⁺by vermicompost biochars was significantly affected by the p H value of the solution.However,the effect of Na NO₃ concentration on Cd²⁺/Pb²⁺adsorption was not obvious.At room temperature(25?),the maximum adsorption capacities of Cd²⁺/Pb²⁺by VBC300,VBC500 and VBC700 were 31.86/43.36,26.60/46.30 and 28.90/37.63 mg/g,respectively.The adsorption capacity of Cd²⁺/Pb²⁺by vermicompost biochars increased as solution temperature increasing.The adsorption process was spontaneous and endothermic monolaye r chemisorption.CO₃^2-,C-F and OH played a major role in the adsorption of Cd²⁺by vermicompost biochars,and CO₃^2-and OH played a major role in the adsorption of Pb²⁺by vermicompost biochars.The main products were Cd CO₃,Cd(OH)₂,Cd F₂,Pb CO₃ and Pb(OH)₂.The contributions of four mechanisms of Cd²⁺adsorption by vermicompost biochars were Q_p>Q_e>Q_{p H}>Q_o.With the pyrolysis temperature increasing from 300?to 700?,the contribution of minerals in biochar to Cd²⁺increased from 61.48%to 76.02%(the contribution rate of ion exchange decreased from 28.20%to 16.27%,and that of surface precipitation increased from33.29%to 59.75%).The contribution rate of oxygen-containing functional group complexation and other mechanisms decreased from 21.64%and 16.88%to 13.79%and 10.19%,respectively.The order of contribution of four mechanisms of Pb²⁺adsorption by vermicompost biochars was the same as that of Cd²⁺.As the pyrolysis temperature increasing from 300?to 500?,the interaction mechanism of minerals between Pb²⁺and vermicompost biochars increased from 70.21%to 80.35%.But as the temperature continued to increase from 500?to 700?,the interaction mechanism of minerals between Pb²⁺and biochar changed from 80.35%to 79.55%,the change range was very small.An increase in the pyrolysis temperature from 300°C to 700°C led to a decrease in the contribution rate of ion exchange mechanism from 21.31%to 13.97%,and a increase in the contribution rate of surface precipitation from 48.90%to 65.59%.With the pyrolysis temperature increasing from 300?to 500?,the contribution rate of oxygen-containing functional groups and other mechanisms decreased from 19.97%and 8.49%to 12.55%and 7.09%,respectively.But as the temperature continued to increase from 500?to 700?,the contribution rate of oxygen-containing functional groups increased slightly from 12.55%to 13.48%,while the contribution rate of other mechanisms did not change.When Cd²⁺and Pb²⁺coexist in the solution,vermicompost biochars first adsorbed Pb²⁺,which means that the affinity of Pb²⁺to vermicompost biochars are greater than that of Cd²⁺.(3)Modification with KMn O₄ resulted in a successful loading of Mn O₂ in the vermicompost biochars,the modified biochars were mainly mesoporous structures and the pore distribution were uniform.The average pore diameters were lower than that of the original biochars,but the specific surface areas are higher than that of the original biochars.The p H values are strongly alkaline,and the p H_IEP are slightly lower than that of the original biochars.After KMn O₄ treatment,amide or nitro compounds were added in KBC300 and aromatic ring(C=C and C=O)was added in KBC700.The other functional groups contained were basically consistent with those of unmodified biochars.The adsorption of Cd²⁺/Pb²⁺by KMn O₄-modified vermicompost biochars were significantly affected by the p H value of the solution.However,the effect of Na NO ₃ concentration on Cd²⁺/Pb²⁺adsorption were not obvious.The adsorption capacity of Cd²⁺/Pb²⁺on the modified biochars increased significantly.The maximum adsorption capacity of KBC300,KBC500 and KBC700 for Cd²⁺/Pb²⁺at room temperature(298K)increased by 58.35%/116.40%,132.26%/186.33%and88.65%/223.49%,respectively.The adsorption capacity of Cd²⁺/Pb²⁺by KMn O₄-modified biochars increased with solution temperature increasing.The adsorption process was spontaneous and endothermic monolayer chemisorption.CO₃^2-,OH and Mn O₂ played a major role in the adsorption of Cd²⁺and Pb²⁺by KMn O₄-modified biochars.The main products were Cd CO₃,Cd(OH)₂,Pb CO₃ and Pb(OH)₂.Compared with the unmodified biochar,the contributions of other mechanisms of Cd²⁺adsorption by KMn O₄-modified biochars increased significantly,while the oxygen-containing functional group complexation mechanism decreased significantly,the surface precipitation mechanism increased slightly,and the ion exchange mechanism almost unchanged.Among these mechanisms of Pb²⁺by KMn O₄-modified biochars,the contributions of surface precipitation mechanism and other mechanisms increased,while the ion exchange mechanism and oxygen-containing functional group complexation mechanism decreased.The affinity of Cd²⁺and Pb²⁺to biochars was significantly increased by KMn O₄ modification.When Cd²⁺and Pb²⁺coexist in the solution,KMn O₄-modified biochars first adsorb Pb²⁺,which means that the affinity of Pb²⁺to KMn O₄-modified biochars are greater than that of Cd²⁺.(4)Fe₃O₄ was successfully loaded onto biochar by magnetization modification.MBC was mainly mesoporous structure,the specific surface area was slightly lower than that of KBC500,and the pores were mostly slit pores formed by stacking flake particles.The saturation magnetization of MBC was 29.12 emu/g,which was representative of superparamagnetism.p H was alkaline and ash content was high.The composition of functional groups was basically consistent with those of KBC500,but Fe₃O₄was added to the original functional group.The adsorption of Cd²⁺/Pb²⁺by MBC was significantly affected by the p H value of the solution.However,the effect of Na NO₃ concentration on Cd²⁺/Pb²⁺adsorption was not obvious.At room temperature(25?),the maximum adsorption capacity of Cd²⁺/Pb²⁺by MBC was 59.49 mg/g and 127.07 mg/g,respectively.The maximum adsorption capacity of Cd²⁺/Pb²⁺was slightly lower than that of KBC500.The adsorption capacity of Cd²⁺/Pb²⁺by MBC increased as solution temperature increasing.The adsorption process was spontaneous and endothermic monol ayer chemisorption.CO₃^2-,OH and Mn O₂ and Fe₃O₄played a major role in the adsorption of Cd²⁺and Pb²⁺by MBC.The main products were Cd CO₃,Pb CO₃ and Pb O.The contributions of four mechanisms of Cd²⁺adsorption by MBC were Q_p>Q_o>Q_{p H}>Q_e,in which precipitation mechanism accounted for 52.21%,other mechanisms accounted for 25.37%,ion exchange and oxygen-containing functional group complexation mechanisms accounted for17.25%and 5.17%respectively.The contributions of four mechanisms of Pb²⁺adsorption by MBC were Q_p>Q_o>Q_{p H}>Q_e,in which precipitation mechanism accounted for 62.53%,other mechanisms accounted for 19.38%,ion exchange and organic functional group complexation mechanisms accounted for 10.71%and 7.38%,respectively.The interaction me chanism of mineral precipitation between Cd²⁺/Pb²⁺and the MBC was the largest among these mechanisms,with value of 69.46%and 73.24%,respectively.When Cd²⁺and Pb²⁺coexist in the solution,MBC first adsorbed Pb²⁺,which means that the affinity of Pb²⁺to MBC is greater than that of Cd²⁺.After the MBC was eluted with 0.2 mol/L HCl,the adsorption effect of MBC on Cd²⁺/Pb²⁺was the best.After 5 times of elution,the adsorption capacity of Cd²⁺/Pb²⁺by MBC was 64.75%and 76.17%of that of the untreated MBC,respectively.The results showed that MBC has high regeneration ability after adsorption of Cd²⁺/Pb²⁺.