Synthesis of polyacrylamide-modified diatomite via visible-light photoactivation for the efficient removal of hexavalent chromium from aqueous solutions

Industrial discharges containing hexavalent chromium [Cr (VI)] present severe environmental and public health risks due to the toxicity, mobility, and mutagenic properties of the metal. This study explores the synthesis, characterization, and batch adsorption application of a novel polyacrylamide (PAM) modified diatomite composite engineered for enhanced Cr (VI) removal from aqueous matrices. The PAM modifier was successfully synthesized via a visible-light photoactivation pathway utilizing a green Mohr’s salt/H 2 O 2 redox initiator system, before being integrated with raw natural diatomite. Fourier-transform infrared (FT-IR) spectroscopy confirmed the successful grafting of acrylamide units and the incorporation of active amine (-NH 2 ) groups onto the porous silica surface. Batch adsorption experiments systematically investigated the effects of contact time (15–360 min), solution pH (2–6), adsorbent dosage (0.07–0.5 g), and initial Cr (VI) concentrations (1–100 mg/L). The optimal operating parameters were identified at pH 2, an adsorbent dose of 0.25 g, and an equilibrium contact time of 120 min, achieving a maximum Cr (VI) removal efficiency of 98.1% ± 0.116% for a 1 mg/L solution. Isotherm modelling revealed that the equilibrium data fit exceptionally well to the Langmuir model (R 2 = 0.998) relative to the Freundlich framework (R 2 = 0.970), establishing a monolayer adsorption capacity (q m ) of 1.91 mg/g at 25 °C. These results demonstrate that the PAM-modified diatomite composite is a highly effective, low-cost, and environmentally benign adsorbent for industrial wastewater treatment.