Adsorption Characteristics Of Cadmium And Chromium In Electroplating Wastewater By Auricularia Auricula Spent Substrate Biochar Modified By Carbon Disulfide And Mercaptoacetic Acid

In recent years,with the rapid development of electroplating industry,a large number of electroplating wastewater is produced.Electroplating wastewater contains a variety of heavy metals,such as chromium,zinc,copper,cadmium and nickel.The characteristics of high toxicity,bioaccumulation and difficult biodegradation posing a serious threat to the ecological environment and human health.Traditional chemical precipitation,electrolysis and other treatment processes are costly and easy to produce secondary pollution.Adsorption method has the advantages of no secondary pollution and simple process.Therefore,the development of low-cost and efficient heavy metal adsorbents has attracted much attention.Biochar has an excellent adsorption performance and is often used for heavy metal removal.However,there are many kinds of heavy metal ions with different charge properties in the comprehensive electroplating wastewater,such as the cadmium in the form of divalent cation Cd（II）,while the highly toxic hexavalent chromium Cr（VI）exists in the form of anion(HCr O₄^2-、Cr O₄^2-、Cr₂O₇^2-).The adsorption efficiency of biochar for different ions is different,therefore,it is difficult to achieve the standard discharge of all ions in the comprehensive electroplating wastewater.In order to improve the ability of biochar to capture the target ions and reduce their toxicity,carbon disulfide（CS₂）modified biochar（SBC）and mercaptoacetic acid（C₂H₄O₂S）modified biochar（HSBC）were prepared with Auricularia auricula spent substrate biochar（BC）for Cd（II）and Cr（VI）respectively.The adsorption efficiency and characteristics of SBC for Cd（II）and HSBC for Cr（VI）were studied,the corresponding adsorption mechanisms were analyzed and revealed,and the synergistic removal efficiency of multi-ions in comprehensive electroplating wastewater by the combined application of two kinds of modified biochar was further investigated.The results showed that the sulfur containing groups were introduced into the SBC by CS₂modification,which increased the content of S by 9%.In the batch adsorption test,the adsorption rate（R）and adsorption capacity（q）of Cd（II）by SBC were higher than those of unmodified BC under the same conditions.The optimum adsorption conditions for Cd（II）by SBC were as follows:adsorbent dosage 2 g/L,adsorption time 120 min,adsorption temperature 25℃,initial Cd（II）concentration 20 mg/L and p H 5.0.In order to explore the adsorption mechanism,scanning electron microscope X-ray energy dispersion spectroscopy（SEM-EDX）,Fourier transform infrared spectroscopy（FTIR）and X-ray diffraction（XRD）were used to analyze the physicochemical characteristics of BC and SBC before and after the Cd（II）adsorption.Compared with BC,the surface S content of SBC increased significantly,and combined with Cd（II）in the solution in the form of S^2-、SO₃^2-and SO₄^2-,so as to improve the removal rate of Cd（II）.The adsorption kinetics and adsorption isotherm shows that the adsorption of Cd（II）by SBC conforms to Langmuir isothermal model and Pseudo-second-order kinetic equation.The adsorption is monolayer homogeneous,and the maximum theoretical adsorption capacity(q_max)reaches 458.35mg/g.Thermodynamic analysis shows that biosorption is a spontaneous（△G⁰<0）endothermic reaction（△H⁰>0）.In the fixed-bed system,higher bed depth,lower flow rate and influent Cd（II）concentration lead to longer penetration and depletion time.When the bed depth is 30 cm,the flow rate is 5 ml/min and the influent Cd（II）concentration is 20 mg/L,the best performance is obtained（the equilibrium adsorption capacity q_eis 6.91 mg/g,the penetration time is 90 min and the depletion time is 1140 min）.Thomas model fitted the breakthrough curve of continuous flow adsorption better（R²>0.90）.In the modification of C₂H₄O₂S,the-OH on the surface of biochar reacted with-COOH on C₂H₄O₂S,and finally-SH was successfully immobilized on the surface of biochar.In the batch adsorption test,the adsorption rate（R）and adsorption capacity（q）of HSBC for Cr（VI）were higher than those of BC under the same conditions.The best adsorption conditions for Cr（VI）by HSBC are the dosage of adsorbent 1 g/L,adsorption time 180 min,adsorption temperature 30℃,initial concentration 20 mg/L and p H 2.0.SEM-EDX,FTIR,XRD and XPS analysis of BC and HSBC were carried out respectively.It was found that functional groups such as-SH,S^2-,SO₃^2-and SO₄^2-appeared on the surface of modified biochar.After adsorbing Cr（VI）,the modified biochar can effectively reduce high toxic Cr（VI）to low toxic Cr（III）.The analysis of adsorption kinetics and adsorption isotherm showed that the adsorption of Cr（VI）by HSBC accorded with Langmuir isotherm model and Pseudo-second-order kinetic equation,and q_maxreached 61.17 mg/g.Thermodynamic analysis shows that the adsorption is a spontaneous endothermic reaction.In the flow bed adsorption,the best performance was obtained when the bed depth was 30 cm,the flow rate was 3 ml/min and the influent Cr（VI）concentration was 5 mg/L（the equilibrium adsorption capacity q_ewas 36.76 mg/g,the penetration time was 330 min and the depletion time was 2340min）.Thomas model better fitted with the penetration curve（R²>0.95）.In the experiment of treating actual electroplating wastewater,two kinds of adsorbents were applied through the two-stage upflow fixed-bed system.During the operation,Cd（II）,Cr（VI）and Cr（III）were not detected in the effluent within 90 minutes.During continuous operation,Cd（II）,Cr（VI）and Cr（III）reached penetration time at 360,450 and 150 min respectively,and reached exhaustion at 2580,2940 and 1320 min.Within 300 min,the concentration of Cd（II）in the effluent was less than 0.1 mg/L,and before 780 min,the concentration of Cr（VI）in the effluent was less than 0.5 mg/L.