Adsorption Of Methylene Blue And Heavy Metals Lead And Cadmium On Modified Coffee Grounds

Coffee is one of the three best-selling beverages in the world,but it has a residue production rate of 90%.Coffee grounds contain mainly lignin,cellulose and hemicellulose making it seen as a very promising and sustainable resource.At present,as a large industrial country,China has serious water pollution problems,among which dyes and heavy metals are particularly prominent.Coffee grounds are still an emerging waste biomass,and there is a lack of research on the preparation of coffee grounds as adsorbent for the removal of dyes and heavy metals from wastewater.Therefore,in this paper,methylene blue and heavy metals Pb and Cd were selected as typical pollutants in two types of wastewater as target pollutants,and coffee grounds were used as raw materials to prepare adsorbents by hydrothermal modification and L-cysteine modification of coffee grounds to carry out studies on adsorption of typical dyes methylene blue wastewater and typical heavy metals Pb and Cd by L-cysteine modification of coffee grounds.Through experimental methods such as coffee grounds characterization,adsorption equilibrium experiments and data fitting using kinetic isotherms and other models,the effects of modified coffee grounds on the adsorption of target pollutants and the inferred adsorption mechanism of modified coffee grounds on pollutants were elucidated.The results are summarized as follows:(1)The optimal hydrothermal conditions for hydrothermally modified coffee grounds were hydrothermal time of 1.5h,temperature of 240℃,and Na OH concentration of 5%.The parameters of the hydrothermally modified coffee grounds adsorption process based on the response surface methodology were optimized as follows: p H 8,hydrothermally modified coffee grounds coffee grounds dose of 2g/L,and equilibrium adsorption was achieved within244 min.Under the above experimental conditions,the fitted data yielded quasi-secondary kinetics as well as Langmuir adsorption isotherms that are consistent with unimolecular layer adsorption,which is a spontaneous,entropy-increasing,heat-absorbing reaction.The adsorption may include electrostatic gravitational forces,van der Waals forces,hydrogen bonding interactions,and π-π interactions.(2)The parameters of the L-cysteine modified coffee grounds adsorption process based on the response surface methodology were optimized as follows: p H of 4,L-cysteine modified coffee grounds dose of 2 g/L,and equilibrium adsorption was achieved within 62 min.Under the above experimental conditions,the fitted data yielded quasi-secondary kinetics as well as Freundlich adsorption isotherms,which are multi-molecular layer adsorption and are spontaneous,entropy-increasing,heat-absorbing reactions.Adsorption may include surface adsorption,precipitation,ion exchange,electrostatic adsorption,functional group complexation,and cation-π interaction.(3)The parameters of the L-cysteine modified coffee grounds adsorption process of heavy metal cadmium were optimized based on the response surface methodology: p H 8,L-cysteine modified coffee grounds dose of 2 g/L,and equilibrium adsorption was achieved within 67 min.Under the above experimental conditions,the fitted data yielded quasi-secondary kinetics as well as Freundlich adsorption isotherms,which are multi-molecular layer adsorption and are spontaneous,entropy-increasing,heat-absorbing reactions.Adsorption may include surface adsorption,ion exchange,electrostatic adsorption,functional group complexation,and cation-π interaction.(4)The coffee grounds were characterized and the modified coffee grounds were more fluffy and porous,and the specific surface area and total pore volume were effectively increased compared with those before modification.The adsorption of methylene blue and L-cysteine modified coffee grounds was compared with that of unmodified coffee grounds,and the adsorption of heavy metals Pb and Cd was better after modification than before.