Evaluation of straw ash as a cost‒effective adsorbent for the removal of phosphate and fluoride from aqueous solution

Abstract

In the present study, the straw ash (SA) was proposed to remove phosphate and fluoride from aqueous solution. The effects of SA dosage, initial contaminants concentration and initial pH on removal efficiency, adsorption kinetics and isotherms were investigated in batch experiments. The removal mechanisms were also discussed. The results show that solution pH was a critical parameter for the adsorption and the removal efficiency decreased with the increase of initial pH. At initial solution pH of 2, the removal efficiency of SA (25 g/L) for phosphate (200 mg/L) achieved 88.20%, and the fluoride (100 mg/L) removal rate by SA (50 g/L) was 87.52%. The adsorption for phosphate/fluoride followed the pseudo‒second‒order kinetic model. The adsorption isotherm data were well fitted by the Langmuir model, and the maximum adsorption capacity of SA reached 92.16 mg/g for phosphate and 66.23 mg/g for fluoride. The SEM-EDS and XRD tests show that phosphate/fluoride were mainly removed by chemically precipitating as CaHPO4, Ca4H(PO4)3·2.5H2O, Ca5(PO4)3(OH) and AlF3, CaF2 on the SA surface. Electrostatic attraction and ligand exchange with hydroxyl groups also exited in the removal process. Besides, SA has been proved to be a promising adsorbent for the treatment of industrial wastewater contaminated with phosphate and fluoride.