Treatment Of Chromium-Contained Electroplating Effuent By Magnetic Biochar Prepared With Eucalyptus Urophylla Leaf Residue: A Mechanical And Pratical Study

Treatment of wastetwater by sorption-based techniques has attracted much attention over the last decades. However, the practical utilization of the conventional adsorbents is restricted due to the difficulty of solid-liquid separation. Magnetic adsorbents, currently, have been found to be one of the promising adsorbents as they can be easily and quickly separated from treated water by an external magnetic field, requiring no further separation treatment. In this study, magnetic biochar prepared with Eucalyptus urophylla leaf residue (MELRBC) was applied to the treatment of actual chromium-contained electroplating effluent by coupling of biochar sorption and magnetic separation processes. The results are as follows:(1) Physico-chemical characteristics of the biochar changed after magnetic modification. The iron oxide particles (Fe3O4) were dispersed on the specific parts of the biochar instead of covering its whole surface. The average grain size of magnetite crystallites was 22 nm. The surface area and micropore volume decreased from 808.2 m2 g-1 and 0.332 m3 g-1 in the pure biochar to 623.0 m2 g-1 and 0.263 m3 g-1 in MELRBC, respectively. The newly formed absorption band of Fe-O was observed and the saturation magnetization of MELRBC was 16.12 emu g-1.(2) The factors that influence Cr (Ⅵ) sorption capacity are magnetic modification, solution pH, coexisting ions, conrtact time and temperature. Lower removal efficiencies of Cr (Ⅵ) and TCr were found for MELRBC compared to these of biochar. Cr (Ⅵ) removal increased with decreasing solution pH. The presence of Cu (Ⅱ) and Ni (Ⅱ) enhanced the removal. The Cr (Ⅵ) sorption by MELRBC revealed a two-step kinetic process:a rapid initial sorption in the first 30 min and thereafter a much slower rate of sorption. Higher temperature was in favor of the sorption.(3) Cr (Ⅵ) removal by MELRBC was a physicochemical process (a comprehensive sorption process). Freundlich and Sips isotherm models had a good fitting for equilibrium data of both Cr (Ⅵ) and TCr. The kinetic data were well described by the pseudo-second-order model. The main sorption mechanisms contain electrostatic attraction, complexation and redox.(4) Both MELRBC content and solution pH play important roles in magnetic separation. To achieve the same separation performance, the more MELRBC was added, the longer separation time would be required. Separation efficiency reduced with solution pH decrease, while it no longer significantly changed when the pH was more than 3.(5) The most effective desorbent is NaOH solutiont, by which more than 80% of both Cr (Ⅵ) and TCr were desorbed with no Fe leaching. Increasing NaOH concentration did not affect Cr desorption. The best solid-liquid ratio was 160 g L-1.(6) Chromium-contained electroplating effluent can be effectively treated by MELRBC. The removal efficiencies of Cr (Ⅵ) and TCr were more than 97%, and the coexisting ions, Ni (Ⅱ) and Cu (Ⅱ), were removed along. The turbidity of supernatant was 21.8 NTU after 10-min separation. With the same dose, the Cr removal decreased slightly when using regenerated MELRBCs, while the removal of Ni (Ⅱ) and Cu (Ⅱ) remained unchanged. Moreover, no significant difference of separation performance between original and regenerated MELRBCs was observed.