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Novel hexagonal disc‐shaped barium hydrogen phosphate was synthesized and analyzed by X‐ray diffraction, X‐ray photoelectron spectroscope, scanning electron microscope, transmission electron microscopy, thermogravimetric analysis, Brunauer–Emmett–Teller, and FTIR. The effects of adsorbent dose, adsorption time, initial concentration, initial pH, temperature, and sodium chloride on hexavalent chromium, Cr (VI), adsorption by BaHPO4 were studied. Effective Cr (VI) adsorption capacity was observed in the pH range of 4–5. The temperature‐dependent adsorption data showed sorption is endothermic and spontaneous in nature. The adsorption occurs via electrostatic interaction and is described by second order kinetic, and the adsorption isotherm was best fitted with the Langmuir model. The equilibrium adsorption capacity and maximum adsorption capacity were found to be 128.7 mg/g and 146.8 mg/g, respectively at room temperature and pH 6. Adsorbent is stable, recyclable, environmentally friendly, and can be produced on a large scale at a low cost. This can be utilized for the treatment of Cr (VI) contaminated water.

The g-C 3 N 4 /Zn 3 (PO 4 ) 2 composites were synthesized in the aqueous medium via chemical precipitation route. The photocatalytic activity of these photocatalysts was evaluated for the degradation of aqueous crystal violet (CV) dye under solar light. The optimum photocatalytic activity of g-C 3 N 4 /Zn 3 (PO 4 ) 2 composite was found for 30 % weight of g-C 3 N 4 . The comparison experiments showed the photocatalytic activity of the g-C 3 N 4 / Zn 3 (PO 4 ) 2 composite increased 2 times compare to bare Zn 3 (PO 4 ) 2 and 1.5 times of g-C 3 N 4 . The enhanced photocatalytic activity originates due to the transfer of photogenerated electrons from the conduction band of g-C 3 N 4 to the Zn 3 (PO 4 ) 2 which makes well-separated electron-hole pairs. The degradation capacity increases with an increase in the pH of the solution up to 8 and then decreases up to 10 and then again starts increasing. The addition of scavengers decreases degradation capacity of g-C 3 N 4 /Zn 3 (PO 4 ) 2 composite in the order of Ethylene diamine tetra-acetic acid-Na 2 (EDTA) > tert-butyl alcohol (TBA) > L-ascorbic acid (AA). The degradation of CV over g-C 3 N 4 /Zn 3 (PO 4 ) 2 composite mainly attributed by the holes. The interfering anions (except Br À ) decrease the degradation capacity of Zn 3 (PO 4 ) 2 whereas in the case of composite no change in the degradation capacity was observed. The recycling experiment for the degradation of CV dye was performed on bare Zn 3 (PO 4 ) 2 and g-C 3 N 4 /Zn 3 (PO 4 ) 2 composite and found better stability and recyclability of the composite material.

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