High-temperature calcination modified red clay as an efficient adsorbent for phosphate removal from water

Abstract

To develop an efficient and cost-effective adsorbent for phosphate removal from water bodies, this study utilized natural red clay (RC) as a carrier. The modified red clay (MRC) was prepared through three methods: acid modification, high-temperature calcination, and metal loading. The preparation conditions were optimized, and the adsorption effects on phosphate were compared across these different modifications. The key findings are as follows: (1) The adsorption capacity of MRC for phosphate, based on the modification method, is ranked as 700°C calcined modified red clay (MRC-700) > H₂SO₄ modified red clay (MRC-H) > AlCl₃·6H₂O modified red clay (MRC-Al) > FeCl₃·6H₂O modified red clay (MRC-Fe) > RC. The optimal preparation conditions were identified as high-temperature calcination at 700°C for 2 h, achieving a phosphate adsorption capacity of 9.92 mg/g and a removal rate of 98.42%. (2) pH, temperature, and coexisting ions had influence the adsorption of phosphate by MRC-700. The influence of coexisting anions on phosphate adsorption follows this order: CO₃^2- > NO₃^- > SO₄^2- > Cl^-. (3) Scanning Electron Microscopy observations indicated that the surface of MRC-700 is rougher, with the specific surface area increasing from 5.02 m²/g (RC) to 14.84 m²/g. This enhancement suggests that calcination treatment creates more active sites for phosphate adsorption, thereby improving adsorption capacity. The maximum phosphate monolayer adsorption capacity, as derived from the Langmuir adsorption isotherm model, is 29.98 mg/g for MRC-700, significantly higher than the 5.02 mg/g for RC. This indicates that MRC-700 holds promise as a cost-effective phosphate adsorbent.

Keywords: Environmental factors; High-temperature calcination; Modified red clay; Phosphate.