Enhanced electron transfer for superior Cr(VI) removal in water using ball-milled aluminum/biochar composite with embedded structure: Efficiency and mechanism

Biochar, being a cost-effective material, is widely used for water pollution control. However, the further degradation of most contaminants is limited due to the weak electron donor properties of raw biochar. Herein, we combined banana peel biochar with microscale zero-valent aluminum (mZVAl) using ball milling technology and demonstrated the effective chromium removal by the composite. Nearly complete removal of Cr(VI) could be achieved within 30 mins, alongside an impressive efficiency of 98.08% in total Cr elimination. Multiple parameters, including water matrices, composite ratio (m mZVAl /m biochar ), and initial solution pH, were explored. Additionally, detection of Cr species revealed that aqueous Cr(VI) was reduced to Cr(III) via chemisorption, followed by the formation of Cr(OH) 3 , which adhered to the surface of the composite. Structural analysis showed that the pristine mZVAl deformed into the plate-like structure under mechanical forces, with biochar distinctly embedded in the mZVAl matrix, increasing lattice spacings. Mechanistic studies suggested that mZVAl acted as the ‘electron donor’, while biochar functioned as the ‘electron shuttle’, accelerating electron transfer through galvanic cell structures. This study clarifies the electron-dominated mechanism in Cr(VI) removal and provides technical support for heavy metal elimination.