Sustainable purification of phosphoric acid contaminated with Cr(VI) by Ag/Ag3PO4 coated activated carbon/montmorillonite under UV and solar light: Materials design and photocatalytic mechanism

Abstract

Wet production of phosphoric acid, the most used low-cost industrial approach, first leads to an impure phosphoric acid contaminated by organic and heavy metal pollutants which requires costly purification before its usage. The present work has investigated for the first time the recovery of Cr(VI), one of the most co-existed heavy metals, from the phosphoric acid medium. For this purpose, three composites based on Ag3PO4 coated on Montmorillonite (MAg3PO4), commercial activated carbon (ACAg3PO4), and synthetic activated carbon (SACAg3PO4) were fabricated. The photocatalytic reduction of Cr(VI) was studied comparatively in the absence and presence of citric acid as a hole scavenger under UV and solar light irradiations. Under UV light, MAg3PO4 was the most efficient, while activated carbon-based photocatalysts showed higher reduction rates under solar light due to the photosensitizing effect. Compared with an aqueous medium, the photocatalytic reduction was very effective without citric acid in a phosphoric acid medium because of the low photogeneration of reactive oxygen species (ROSs). The chemical bonds formed between Ag3PO4 and support materials, as proved by XPS, may boost the photocatalytic activity due to enhanced charges of photogeneration and separation. SACAg3PO4 showed higher reduction ability even at higher Cr(VI) concentrations (100–300 ppm). A comparison study showed that SACAg3PO4 is more effective than as-prepared ACTiO2. This investigation shows promising findings to widen the application of photocatalytic technology in different industrial fields.