Evaluation of snail shell as a coagulant aid in the alum precipitation of aniline blue from aqueous solution.

The present studies aimed to evaluate the potential of snail shell (SS) as a coagulant aid in the alum precipitation of aniline blue (AB), an acid dye, from an aqueous system. The proximate physico-chemical characteristics of the SS were determined, and the results obtained showed that the pH(solution) was 8.01, the SS was made up of a high fraction of inorganic constituents (ash content of 93.76%), Ca2+ was present as the major metal ion (99.74%), and the point of zero charge (PZC) was found to be at pH 7.9. The mineralogical assemblage was studied using an X-ray diffractometer, and the results obtained revealed the presence of aragonite. The stability and leaching of the SS, tested in different aqueous media (acidic, basic and neutral solutions) showed that the SS was less stable in the acidic medium. Both alum and SS were used, differently, for dye precipitation. The use of alum alone showed no precipitating effect on the AB dye molecules, whereas SS alone was able to reduce the dye concentration considerably. When the SS was used as a coagulant aid in alum precipitation, the percentage of the AB dye molecule removed increased. The effects of some process variables (coagulant/coagulant aid dosage, pH and flocculation time) were optimized by the method of continuous variation. The results obtained showed that an appreciable amount of the AB dye molecules was abstracted across the pH range, but the highest value was achieved at the lowest pH values. Studies on the effect of time on the flocculation of the precipitated AB molecule showed that the problem of redispersion and restabilization encountered in alum precipitation could be overcome using an alum-SS combination. The settling characteristics of the sludge obtained from the use of SS alone and an alum-SS combination were studied by measuring the sludge volume index (SVI, mg/g) over time. The value of the SVI showed that the sludge produced from the alum-SS combination had better settling characteristics than the sludge from the use of SS alone. Subsequent to the results obtained from the present studies, the use of the SS as a coagulant aid in the alum precipitation of the anionic dye as a unit process tertiary method of treatment in a suspended-growth wastewater treatment system was proposed.