Hybridizing black liquor-derived kraft lignin with Ag3PO4@ZnO to boost tetracycline and dye removal through synergistic adsorption-photocatalytic pathways.

Discharging pulping black liquor waste and industrial wastewater, such as that from the pharmaceuticals and textiles, into surface water can poses risks to both human health and aquatic ecosystems. Accordingly, this research introduces an innovative method for valorizing pulping black liquor waste by extracting kraft lignin (KL) as a sustainable biopolymer for constructing a ternary KL-supported Ag3PO4@ZnO p-n heterojunction (designated as AZC-KL(x), with x denotes the wt. % ratios of AZC (Ag3PO4@ZnO): KL at 1: 0.2, 1: 0.5, and 1: 1). The AZC-KL(x) composites are evaluated for the enhanced removal of mixed textile (e.g., methylene blue (MB) and methyl orange (MO) dyes) and pharmaceutical (e.g., tetracycline (TC)) pollutants through synergistic adsorption/photocatalytic mechanisms under dark/visible light conditions, in comparison to the pristine Ag3PO4@ZnO, as well as KL-supported Ag3PO4 or ZnO binary composites. In comparison to the AZC heterojunction, the AZC-KL(x) significantly improves the adsorption rate of three pollutants by a factor of 1.66 - 3.81. This enhancement is attributed to π-π stacking and hydrogen bonding with the oxygen-containing groups and aromatic structure of the KL substrate. In particular, AZC-KL(1) demonstrates superior adsorption-photocatalytic activity, sustaining its effectiveness over five cycles. It exhibits the highest removal rate for MB dye with a kapp of 3.26 × 10-2 min-1 and an apparent quantum yield (AQY) of 16.4 × 10-7 mol/E across 180 min of visible light irradiation, which is 6.4 and 9.5 times higher than that of TC and MO, respectively. Lowering the KL ratio to 1: 0.2 in the AZC-KL(x) structure leads to a performance decline by 1.18 - 2.1 times (relative to AZC-KL(1)), demonstrating the importance of KL in tailoring the surface chemistry and optoelectronic characteristics of AZC-KL(x), as verified by various analytical methods. This collaborative method lowers the energy required for effective wastewater treatment and decreases treatment costs, as demonstrated in the techno-economical analysis.