Efficient removal of hexavalent chromium from wastewater using a novel sodium alginate-biochar composite adsorbent

Discharging hexavalent chromium (Cr(VI))-containing wastewater has become a critical environmental and ecological concern. Coffee grounds biochar (CGB), rich in oxygen-containing functional groups, can reduce Cr(VI) to the less toxic trivalent chromium (Cr(III)) without secondary contamination. This study developed a composite adsorbent by incorporating sodium alginate (SA) into CGB, exploiting their synergistic adsorption and ion exchange reduction effects for efficient Cr(VI) removal. The maximum adsorption capacity of SACGB, fitted by the Langmuir isotherm model, was 30.66 ± 1.89 mg/g. Under optimal conditions (pH = 2, SA: CGB mass ratio = 3:2, Cr(VI) concentration = 50 mg/L), SACGB achieved 99.79 % Cr(VI) removal from synthetic wastewater. Significant Cr(VI) reduction to Cr(III) (58.97 % removal) occurred at an SA:CGB ratio of 1:1. The adsorption mechanism involved hydrogen bonding, electrostatic interactions, and ion exchange reduction. Removal of Cr(VI) retained 93 % removal despite interference from coexisting Cu(II), Zn(II), and Ni(II) ions, and the ions promoted Cr(VI) adsorption onto SACGB. In actual wastewater, SACGB exhibited excellent Cr(VI) removal (99.26 %), meeting the Chinese industrial effluent discharge standard (0.2 mg/L). After 6 recycling cycles, SACGB demonstrated stable removal performance and contained no biotoxic components. The cost of preparation and adsorption process was analyzed and also compared with earlier studies to evaluate and highlight the economic feasibility of the adsorbent for a truly green removal method. This study highlights SACGB as an economical and efficient adsorbent with practical application potential for Cr(VI) removal from industrial wastewater.