Defluoridation by Al-based coagulation and adsorption: Species transformation of aluminum and fluoride

Abstract

a b s t r a c tThe species transformation of Al and fluoride in fluoride removal by Al-based coagulation and adsorption is far from being well-characterized. This study used batch-and pilot-scale experiments to compare the fluoride removal efficiency and species transformation of Al and fluoride in Al coagulation with that in Al precipitate (AlOxHy) adsorption. Al coagulation showed superior removal of fluoride compared to AlOxHy adsorption. In the coagulation process, the formation of complexed fluoride (Com-F) occurred, and Com-F concentrations increased with elevated Al dose, i.e., lowered pH. At initial pH of 8.6, free fluoride (Free-F) dominated in the adsorption system even at the high AlOxHy dose of 200 mg Al/L whereas at pH < 7.0, Com-F was the dominant species in both systems. Fluoride had an influence on Al 3+ hydrolysis and favored the transformation of Al 3+ and Al-F complexes to Al-F precipitates, and the removal of fluoride by coagulation was improved thereafter. Fluoride also promoted the dissolution of AlOxHy, the extent of which was more significant at lowered pH from 7.0 to 4.0 and at elevated ratios of fluoride to Al from 0.15 to 2.45. The formation of Al-F complexes impaired fluoride removal by adsorption. A pilot-scale field study also indicated that coagulation showed higher efficiency than adsorption in the removal of fluoride. The average ratios of Com-F to total fluoride were 0.14:1 and 0.04:1, respectively, in coagulation and adsorption processes.

Fluoride recovery using porous calcium silicate hydrates via spontaneous Ca2+ and OH− release

“…In the recent years, various methods have been investigated to remove or recover fluoride from wastewater, such as chemical precipitation [5], adsorption [6,7], ion exchange [8], electrocoagulation [9], and membrane filtration [10]. Among these methods, the sludge produced by chemical precipitation cannot be reused due to low fluoride content, which is difficult to be treated with high water content.…”

Characteristics of poly-silicate aluminum sulfate prepared by sol method and its application in Congo red dye wastewater treatment

“…Well knows, coagulation is a physic-chemical process, which is highly dependent on solution pH, especially in the case of aluminum-based coagulants. 34,35 So, in subsequent performance tests, we focus on PSAS 1.11 (Al molar ratio of 12, Al/Si of 20) coagulant. The effects of the starting pH of the solution and the coagulant dose on the color removal rate are provided in Fig.…”

A Mini Review of Recent Findings in Cellulose-, Polymer- and Graphene-Based Membranes for Fluoride Removal from Drinking Water

“…Another method, adsorption, is a widely used technique [19] that has the advantage of being a manual method with a low-cost operation, with an increased selectivity and adsorbent readiness. Several materials such as alumina [20], activated carbon [21], ion exchange resins, silica gel, natural materials like clay, mud and low-cost alternative adsorbents like fly ash, bone charcoal, etc., are used [13,22,23].…”