Study On Adsorption Of Hexavalent Chromium In Electroplating Wastewater By Modified Magnetic Litchi Shell Adsorbent

Cr（Ⅵ）is a common water pollutant that poses a great hazard to the environment and the human body.Moreover,Cr（Ⅵ）is a pollutant with high pollution load in Shenzhen,so Cr（Ⅵ）is the treatment target in this study.The traditional method treating Cr（Ⅵ）is costly and may produce a large amount of chromium-containing sludge which is difficult to handle,and the adsorption method has the advantages of high efficiency,low cost,and recyclability of the adsorbent.In this paper,Litchi shell and litchi shell biochar were used as carriers to prepare modified magnetic litchi shell adsorbent and modified magnetic litchi shell biochar adsorbent by co-precipitation method with ferric oxide（Fe₃O₄）and layered double hydroxide（LDH,layered double hydroxide）.The single-factor and orthogonal experiments were carried out to investigate the calcined temperature of litchi shell biochar,the loading of Fe₃O₄,the loading of LDH and the diameter of adsorbent particles.By comparing the adsorbents prepared by two different carriers,it was determined that the removal of Cr（Ⅵ）by the adsorbent with litchi shell as the carrier was the best.The specific preparation conditions are as follows:10 g of litchi shell,the amount of ferric chloride and ferrous chloride are 0.06 mol and 0.03 mol respectively,and the amounts of magnesium chloride and aluminum chloride are 0.15 mol and 0.05mol respectively,and the particle diameter is 0.3-0.9 mm.The results of XRD,FT-IR and SEM showed that Fe₃O₄ and layered double hydroxide were successfully loaded in the litchi shell,and the morphology of the adsorbent changed significantly after loading.In this study,the effects of solution pH,adsorbent dosage and initial concentration of Cr（Ⅵ）on the adsorption efficiency of the adsorbent were investigated by static adsorption experiments.Based on the adsorption capacity and Cr（Ⅵ）removal rate,the pH of the solution is optimally 3,the adsorbent dosage is 6 g/L,and the initial concentration of Cr（Ⅵ）is 100 mg/L.In addition,the effect of coexisting ions on the adsorption efficiency of the adsorbent was investigated.It was found that the anion in the solution inhibited the adsorption efficiency of the adsorbent to a large extent.At the same time,pseudo-first-order kinetics,pseudo-second-order kinetics,intraparticle diffusion model and Elovich model were used to fit the experimental data,and pseudo-second-order kinetics fits best.The Langmuir,Freundlich and Sips adsorption isotherms were used to fit the experimental data.It was found that the experimental data was more consistent with the Sips model,and the maximum adsorption capacity of the adsorbent was up to 58.8 mg/g.The adsorption kinetics and adsorption isotherm fitting results show that the adsorption process of the adsorbent is mainly chemical adsorption.The adsorption performance of the adsorbent in electroplating effluent including the actual chrome-plated rinse water and mixed chromium-containing wastewater was investigated in this article.It was found that the adsorbent could almost completely remove Cr（Ⅵ）from the chrome-plated rinse water.But owing to complex composition for the mixed chromium-containing wastewater,Removing all Cr（Ⅵ）requires more adsorbent dosage.The adsorbent in this study is suitable for chromium-containing wastewater with the hexavalent chromium concentration below100 mg/L.The results of recycling experiment showed that the adsorbent still had a high adsorption capacity after six cycles for the chrome rinse water.