Effects and mechanisms on Cr(VI) and methylene blue adsorption by acid (NH4)2S2O8 modified sludge biochar

[Display omitted] •HKBC was prepared via a two-step pyrolysis and APS modification.•Modification enhanced the specific surface area and functional groups of biochar.•HKBC could effectively remove Cr(VI) (97.42 %) and methylene blue (99.98 %) in water.•The adsorption mechanisms for Cr(VI) and methylene blue removal were explored. To explore a modification pathway for enhancing the uptake performance of sludge biochar for Cr(VI) and methylene blue (MB) effective removal, hydrophilic sludge biochar (HKBC) was prepared by sewage sludge via two-step pyrolysis, KHCO3 activation, and acidic (NH4)2S2O8 modification. Results showed that HKBC presents higher hydrophilicity and polarity, and contains more oxygenic and amino functional groups such as COH, COC, CO, NH and NCH3. Benefiting from modification, the specific surface area of biochar enhanced from 143.16 to 295.77 m2·g−1. The maximum uptake amount and removal efficiency of HKBC for Cr(VI) reached 41.23 mg·g−1 and 97.42 %, for MB reached 209.58 mg·g−1 and 99.98 %, respectively. The uptake processes well fitted the Pseudo-second-order kinetics and the Langmuir isotherm model. Thermodynamic analysis indicated that both uptake processes were spontaneous and endothermic. The predominant mechanisms for Cr(VI) removal were surface complexation, reduction, ion exchange, and electrostatic force, while, surface complexation, π-π interactions, electrostatic, and hydrogen bond force were involved in MB removal. This work presents a viable approach for the resourceful use of sludge as it offers improved adsorption capacity of sludge biochar for Cr(VI) and MB removal.