Hydrochar effectively removes aqueous Cr(VI) through synergistic adsorption and photoreduction

Photoreduction is recognized as a desirable treatment method for hexavalent chromium Cr(VI) because of the advantages of high efficiency, energy-saving, and environmentally friendly. Herein, three kinds of hydrochars were synthesized by one-step hydrothermal carbonization of agricultural and forestry residuals for synergistic adsorption and photoreduction of Cr(VI). All of them showed relatively strong adsorption capability to Cr(VI). Without illumination, the maximum adsorption capacities of bamboo hydrochar (BHC), cornstalk hydrochar (CHC), and pine wood hydrochar (PHC) were 177.0, 164.2, and 156.5 mg/g, respectively. Moreover, the hydrochars also had excellent photocatalytic properties as indicated by various characterization techniques. Under light irradiation, they created synergistic effects between adsorption and photoreduction to effectively remove Cr(VI) under various conditions. The synergistic removal of Cr(VI) by adsorption and photoreduction in ultrapure water, tap water, and lake water on BHC all reached almost 100% in 40 min. Besides, the photoreduction of Cr(VI) was almost unaffected with various coexisting ions (Cl-, SO42-, PO43-), and the removal rate of Cr(VI) remained above 90% within 60 min after 5 runs of synergistic adsorption-photoreduction. The photocatalytic mechanism suggests that the BHC had suitable band energies which can reduce Cr(VI) to Cr(III) by photo-induced electron. Findings of this study demonstrate that hydrochar is a promising dual-functional material for the treatment of Cr(VI) contaminated water.