Zero-waste approach to fluoride removal by contact precipitation using Hyriopsis cumingii shells with value-added products

Abstract

We propose a zero-waste approach using Hyriopsis cumingii shells for fluoride (F) removal via contact precipitation with value-added products. H3PO4 was used as the precipitant for F removal. The F concentration could be reduced from 9.5 mg/L to 0.09–0.91 mg/L for over 900 BV with running-backwashing cycle. The precipitates formed during contact precipitation were easily stripped from the shell matrix by periodic backwashing, thus supporting the continuous dissolution of calcium (Ca) from the shell particles. X-ray diffraction (XRD)-Rietveld analysis showed that the backwashing residue was fluorapatite (FAP) with a small amount of hydroxy substitution. Therefore, FAP can be used as a raw material in phosphate fertilizer manufacturing. Economic analysis and sustainability evaluation showed that the contact precipitation process with H. cumingii shells as the substrate not only reduces chemical costs but also generates value-added products; thus, this process is environmentally friendly compared to traditional methods of F removal. The proposed approach for F elimination, waste shell utilization, and F and Ca resource recovery in the contact precipitation process represents a novel sustainable water treatment technology.