Synergistic adsorption of anionic and cationic dyes using commingled food waste biochar from microwave-assisted pyrolysis

Biochar is a versatile carbon-rich material with multifaceted applications in environmental remediation, sustainable agriculture, waste valorization, carbon sequestration, and energy systems, and is particularly valued as a highly effective and tunable adsorbent for the removal of diverse contaminants—including dyes, heavy metals, nutrients, pharmaceuticals, and emerging pollutants—from water and wastewater. The current study employed commingled food waste biochar (CFWB), along with its acid-modified (A-CFWB) and alkali-modified (Al-CFWB) forms, produced from food waste and LDPE through microwave-assisted pyrolysis to remove Congo red (CR) and Methylene blue (MB) from single and binary dye systems. CR and MB removal from single dye system were in the order A-CFWB (79.5%) > CFWB (75.4 %) > Al-CFWB (63 %) and Al-CFWB (83.7%) > CFWB (70.1%) > A-CFWB (59.5%) respectively. Al-CFWB displayed superior CR (71.2%) and MB (96.7%) removal from binary dye system compared to A-CFWB (63.3% and 51.1%) and pristine CFWB (59.7% and 61.9%), with an overall maximum adsorption capacity of 33.6 mg/g. Better CR removal from binary dye system (71.2%) was observed due to complex formation between oppositely charged CR and MB, compared to single dye system (63%). Monolayer CR and MB adsorption on Al-CFWB was represented by the Langmuir isotherm. Chemisorption was the rate-limiting mechanism for dye removal from single and binary dye systems, with a strong co-relation with pseudo-second order kinetic model. Al-CFWB was reusable, removing ~86% of MB from binary dye system after five reutilization cycles. The oxygen-containing functional groups and distinct porous structure of Al-CFWB (5.95 µm average pore size) facilitated surface accessibility for dye adsorption. These results indicated that Al-CFWB is an effective adsorbent for the removal of CR and MB from binary dye system. Moreover, this research emphasized on the conversion of abundantly available commingled food waste into CFWB and its application as an effective adsorbent for multi-dye removal from wastewaters which serves the dual purpose of efficacious waste valorization and pollution control for sustainable development and environmental safety contemplating to UN Sustainable Development Goals.