Combined physical and chemical activation of sludge-based adsorbent enhances Cr(Ⅵ) removal from wastewater

To enhance the adsorption properties of sludge-based adsorbents (SBAs), the physical activation and the combined physical and chemical activation were examined comparatively. The surface composition and structure of modified SBAs were characterized using BET surface area, XRD and FTIR, the adsorption behavior of modified SBAs was investigated for Cr(Ⅵ) removal from wastewater. For CSU-NaOH, the optimum adsorption time and pH were 0.5 h and 2.5, respectively. The maximum adsorption capacity by CSU-NaOH was 15.3 mg g−1 for Cr(VI) removal, which was over 2 times of those by carbonized sludge and carbonized sludge with urea addition at 25 °C. The adsorption kinetics could be fitted with the pseudo second-order model for three adsorbents, the adsorption isotherm could be fitted with Langmuir model. The thermodynamic analysis indicated that the adsorption process of three adsorbents for Cr(Ⅵ) removal was spontaneous and endothermic. The reusability tests showed that the removal rate of Cr(VI) was kept over 95% by desorbed CSU-NaOH sample until the fifth cycle. The enhancement of adsorption properties can be attributed to both the increase of BET surface area and the improvement of surface functional groups of SBAs. The advantages of higher adsorption capacity and adsorption rate of CSU-NaOH suggest that CSU-NaOH is an effective adsorbent for Cr(Ⅵ) removal. [Display omitted] •A combining activation method was provided for preparation of SBA.•The equilibrium time of 30 min suggested high adsorption rate of CSU-NaOH for Cr(VI).•The adsorption capacity by CSU-NaOH was over 2 times than other SBAs.•The adsorbent of CSU-NaOH had high reusability for Cr(VI) removal.