Polyaniline-based hierarchical heterostructures for photocatalytic degradation of organic pollutants for water treatment – A critical review

• PANI composites achieved >90 % pollutant degradation under visible light. • Review emphasizes synthetic strategies & interfacial charge transfer routes. • PANI-based heterojunctions show superior stability & recyclability vs. others. • Challenges include fouling, catalyst deactivation & limited scalability. • Future directions: solar-driven hybrids & reusable catalyst design. Conjugated polymers (CPs) have opened new avenues for green technologies. It is widely used as an alternative candidate for most active materials of E-textiles, offering promising ways in industrial sheet fabrication, which has been employed simply because of its outstanding environmental stability and low cost. Moreover, like others, molecular doping and depositing it in conjunction with an inorganic semiconductor to form a heterojunction will enable much better physical and chemical properties. Together, the tuneable electrochemistry of PANI-based systems (absorbing pH-dependence) and their visible-light photo response, and high perceptibility of coaster bead structures make them ideal for wastewater remediation applications as well as solar energy conversion. In recent years, the global industrial chain and human life have continuously been threatened by organic pollutants (OPs) from various industrial and municipal effluents due to their extreme ecotoxicity and long-term health risks. The ability of photocatalysis to act as a green solution and an economically appropriate treatment for the mineralization of these pollutants is becoming increasingly important. Inorganic Semiconductor Photocatalysts: Over the past few decades, intensive studies have been conducted on inorganic semiconductor photocatalysts. However, PANI-based nanocomposites have exhibited improved charge transportation, wide-spectrum photocatalytic response, and enhanced environmental compatibility. For instance, PANI/graphene oxide (GO) and PANI/MIL-88A(Fe) systems have achieved >90 % degradation of organic pollutants under visible light, while maintaining reusability over multiple cycles. In contrast to other reported photocatalysts, which have narrow absorption windows, photo corrosion, and challenges in post-catalyst recovery, these often hinder large-scale usage. This review critically examines the recent advances in PANI-based heterostructures with 2-D materials, metal organic frameworks (MOFs), surface-mounted MOFs (SURMOFs), and other advanced heterojunctions for photocatalytic water treatment. Emphasis is placed on synthetic strategies, interfacial charge transfer mechanisms, performance metrics, and degradation kinetics. Highlight existing research gaps and outline potential directions for the design of next-generation PANI-based photocatalysts. Despite these advances, challenges remain in translating laboratory-scale performance to real-world applications, including catalyst stability under variable water chemistries, fouling and deactivation in complex effluents, limited recyclability, and scalability barriers. Potential solutions include protective coating, surface engineering with corrosion-resistant materials, and integration of continuous-flow photocatalytic reactors. Future research should focus on optimizing hierarchical structures for simultaneous pollutant degradation and disinfection, exploiting solar-driven hybrid systems, and employing machine-learning-assisted materials discovery to accelerate the design of robust, high-performance PANI-based photocatalysts for sustainable water purification.