Preparation And Performance Of Heavy Metal Adsorbent Mercaptoacetyl Corn Straw

With the large-scale development of industry and manufacturing,a great deal of wastewater containing heavy metal ions is produced in the production process.If proper treatment measures are not taken,it will not only destroy the ecological environment,but also bring harm to human body.There are several of methods for the treatment of heavy metal wastewater,such as chemical precipitation,ferrite,membrane separation and so on.Most of these methods have complex operation,long treatment cycle and high cost,and the by-products are easy to cause secondary pollution and other problems,which result in restrictions to be used in practical application.Because of the advantages of convenient operation,low cost and environment-friendly,the adsorption method is more and more popular with some researchers.At present,some mercaptoacetyl groups and amino groups are introduced into the molecules of low-cost biomass through chemical modification,such as agricultural and forestry wastes.They have good adsorption capacity for heavy metal ions owing to their porous physical properties and chelating ability for heavy metal ions,resulting in good economic benefits.And in this paper,a novel heavy metal adsorbent,called mercaptoacetyl corn straw?MACS?,was prepared by introducing mercaptoacetyl groups?-SH?onto agricultural waste corn straw through chemical modification.In this paper,the effects of six factors on the preparation of MACS were researched by single factor experiments,using corn straw?CS?and thioglycolic acid?TGA?as raw materials,and N-HydroxySuccinimide?NHS?and EDC·HCl as catalysts.The preparation conditions of MACS were optimized by orthogonal design and response surface methodology.In response surface methodology,Plackett-Burman experiment was used to screen the key factors affecting the preparation of MACS,but the results showed that the model was not significant for whole research range;finally,the factors selected by orthogonal experiment were taken as main influencing factors,and the central composite design?CCD?was selected to optimize the preparation conditions of MACS.The structure and surface morphology of MACS were characterized with FTIR,BET,SEM and elemental analysis.Water samples containing Cd?II?and Cu?II?were investigated,and the effects of adsorption time,amount of adsorbent,adsorption temperature,oscillation rate,initial pH value,and initial concentration of heavy metal ions on the adsorption of Cd?II?and Cu?II?with MACS were also studied.The mechanism of adsorption for MACS with Cd?II?and Cu?II?was discussed preliminarily in combination with adsorption kinetics and adsorption isotherm experiments.The major research results are as follows:?1?Based on the single factor of determination affecting preparation conditions of MACS,seven influencing factors were selected,including corn straw diameter,the ratio of reactants CS and TGA,the ratio of catalyst EDC and TGA,the ratio of activator NHS and EDC·HCl,the pH value of reaction medium,reaction temperature and reaction time,the removal rates of Cd?II?and Cu?II?from water samples were researched.The results showed that the orthogonal optimization scheme of MACS preparation was inconsistent.In order to optimize the preparation process of MACS and remove multiple heavy metal ions as much as possible,the effect of removal Cd?II?and Cu?II?simultaneously was comprehensively evaluated,and the preparation conditions were unified.The optimum preparation conditions of MACS confirmed by orthogonal experiments were as follows:the diameter of corn straw was 80 mesh?0.2 mm?,the mass-volume ratio of CS and TGA was 1:6,the molar ratio of catalyst EDC·HCl and TGA was 0.075:1,the mass ratio of activator NHS and EDC·HCl was0.5:1,the reaction medium pH value was 6.0,the reaction temperature is 30?,and the reaction time is 1 h,after CS was continuous oscillation activated with 0.505 mol/L of HNO₃solution at 25?for 6 h.The highest removal rates of Cd?II?and Cu?II?of simulated water samples by MACS prepared under these conditions were 88.42%and 100%,respectively.?2?MACS was prepared by CCD response surface optimization with three factors:pH value of reaction medium,CS particle size and reaction temperature.The optimized preparation conditions of recommendation of Expert8.06 software was analyzed and compared The preparation method of MACS was obtained as follows:The size of CS particle was 80 mesh?0.2 mm?,and the mass-volume ratio of CS and TGA is 1:6,the molar ratio of EDC·HCl and TGA is 0.075:1,the mass ratio of NHS and EDC·HCl was 0.5:1,the reaction medium pH value was 5.72,the reaction temperature was 47?,and the reaction time was 60min.The removal rates of Cd?II?and Cu?II?by MACS were 88.14%and 98.62%,respectively.?3?FTIR analysis and BET specific surface area showed that the modified CS had a weak S-H stretching vibration peak at 2519.0 cm-1,the specific surface area decreased by19.9%,the pore diameter increased by 3.2%and the pore volume decreased by 17.2%.The content of S element was found to be 0.448%by element analysis.The surface of MACS was observed to become smoother and its structure was looser and more pores by SEM analysis.The results showed that mercaptoacetyl groups were successfully introduced onto the CS molecular structure.?4?The influences of MACS dosage,adsorption time,adsorption temperature,oscillating rate,initial pH value and initial concentration of heavy metal ions on the adsorption of Cd?II?and Cu?II?were researched.The results showed that MACS dosage was 30 g/L,the adsorption time was 60 min,the adsorption temperature was 40?,the oscillating rate was100 r/min,the pH value of water samples was 6.0,and the initial concentration of Cd?II?was50 mg/L.In these conditions,the maximum adsorption rate of Cu?II?by MACS was 89.69%,and the adsorption capacity was 4.20 mg/g.Furthermore,when MACS dosage was 15 g/L,the adsorption time was 60min,the adsorption temperature was 25?,the oscillating rate was200 r/min,the pH value of water samples was 6.0,and the initial concentration of Cu?II?was50 mg/L,the maximum adsorption rate of Cu?II?by MACS was 100%,and the adsorption capacity was 8.94 mg/g.?5?The adsorption of Cd?II?and Cu?II?on MACS accorded with the quasi-second-order kinetic model and the Langmuir adsorption isothermal model.The highest quasi-second-order Kinetic fitting correlation coefficients R² is 0.9859 and 0.9927 for Cd?II?and Cu?II?,respectively.The Langmuir isotherm fitting correlation coefficients R² is 0.9669 and 0.9914for Cd?II?and Cu?II?,respectively.These results indicated that the adsorption process was mainly single molecular layer adsorption by chemical control.In conclusion,MACS,as a novel heavy metal adsorbent,has a better adsorption efficiency on Cd?II?and Cu?II?in wastewater.Moreover,the preparation method of MACS is simple and low-cost.Therefore,MACS has certain application prospect,and it is expected to become an effective treating agent for heavy metal wastewater.