Inorganic-Chemistry Approaches to the Treatment of Dye-Polluted Water: A Comparative Review of Five Methods

Abstract

Synthetic dyes are among the most visible and the most refractory of industrial water pollutants. With global production now exceeding 700,000 tonnes per year and 10–15% routinely lost to effluent streams, the textile, leather, paper and food industries together discharge coloured wastewaters whose toxicity, complexity and resistance to biodegradation make conventional treatment processes either ineffective or expensive. This review surveys five established treatment strategies through the lens of inorganic chemistry — the family of methods in which metal cations, metal-oxide semiconductors, transition-metal redox couples, coordination polymers and electroactive metal interfaces are the central actors. We examine, in turn: (i) adsorption on inorganic adsorbents (metal oxides, layered double hydroxides, zeolites, clays and mesoporous silicas); (ii) photocatalytic degradation on inorganic semiconductors (TiO₂, ZnO, BiVO₄, g-C₃N₄ and their heterojunctions); (iii) Fenton and Fenton-like advanced oxidation processes, including the more recent sulfate-radical AOPs centred on Co²⁺, Cu²⁺ and Fe-based activation of peroxymonosulfate and peroxydisulfate; (iv) metal–organic frameworks and coordination polymers, in which the metal node is simultaneously the binding pocket and the catalytic centre; and (v) electrochemical methods, including electrocoagulation on Fe/Al sacrificial anodes, electro-Fenton and electrocatalytic oxidation on metal-oxide and boron-doped diamond anodes. For each method we summarise the underlying coordination and redox chemistry, the operational variables that govern performance, and representative literature data on dye removal efficiencies and mineralisation. The review closes with a comparative overview, the practical challenges of cost, sludge and stability, and the directions in which the inorganicchemistry toolkit is likely to expand in the next decade.

Highlights
Five inorganic-chemistry families cover the physical, chemical and electrochemical removal of synthetic dyes.
LDH and metal-oxide adsorbents reach Langmuir capacities of several hundred to >1500 mg g⁻¹ for azo and basic dyes.
TiO₂, ZnO, Bi-based oxides and g-C₃N₄ heterojunctions degrade 90–99% of model dyes under solar irradiation.
Co²⁺/Fe²⁺ activation of peroxymonosulfate gives complete decolorisation in minutes through SO₄•⁻.
Fe/Al electrocoagulation and Fe-based MOF photo-Fenton are the most deployable inorganic technologies at present.