Co-doped black carbon nitride (BCN)@TiO₂ heterojunction: A bifunctional material for simultaneous solar evaporation and photothermal catalytic wastewater purification

In response to the dual challenges of water scarcity and organic pollution, this study developed a dual-functional solar evaporator based on cobalt-doped BCN@TiO₂ heterojunctions (Co-BCT). The Co-BCT composite was prepared using a combination of sol-gel method and the hydrothermal method, and is uniformly embedded in a superhydrophilic melamine foam scaffold. Distinct from most reported integrated SDIE (solar desalination integrated with evaporation) -AOP (advanced oxidation process) systems that solely rely on photothermal-photocatalytic coupling, this system further integrates a Z -scheme heterojunction for efficient charge separation, a Co 2+ /Co 3+ redox cycle for boosted peroxymonosulfate (PMS) activation, and a superhydrophilic SA (sodium alginate) -MF (melamine foam) scaffold for self-desalination and rapid water transport—enabling multi-field synergy that addresses the trade-off between evaporation efficiency and pollutant degradation in complex systems. When exposed to simulated sunlight (1 Sun), the device achieves an evaporation rate of 2.51 kg·m −2 ·h −1 in pure water and 2.42 kg·m −2 ·h −1 in simulated seawater (3.5 wt% NaCl solution) with electrical stimulation. The system can efficiently degrade organic pollutants under solar power by activating PMS. Thanks to Co 2+ /Co 3+ -mediated PMS activation and a multi-pathway reactive oxygen species (ROS) generation mechanism (including ·OH, SO₄· − , ·O₂ − and h + ), over 98 % of Rhodamine B (RhB) is degraded within 30 min. Compared with recent Co-based PMS activators that suffer from limited ROS types or Co leaching, the atomically dispersed Co anchored at BCN nitrogen vacancies in Co-BCT not only enhances the generation of multi-type ROS but also improves catalytic stability, with no significant performance decay after 5 cycles. When applied to effluent from the biological treatment of coking wastewater, the system removes nearly all COD (51.36 mg/L to ~0 mg/L), with conductivity decreasing sharply from 2423.9 μS/cm to 21.2 μS/cm. The device also demonstrates excellent salt tolerance (evaporation rate ≥ 1.8 kg·m −2 ·h −1 in 15 wt% NaCl solution) and fouling resistance, as well as being applicable to wastewater treatment. This proves its feasibility and sustainability for use in complex wastewater purification and solar-driven seawater desalination.