Publication: Synthesis and characterization of schwertmannite-goethite as an adsorbent in arsenic removal
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Date
2022-07-01
Authors
Mahmad, Mohd Khairul Nizam
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Abstract
Mining activities contributed to the major arsenic pollution and acid mine drainage (AMD) since most of the valuable minerals like tin, gold and iron are associated with pyrite and arsenopyrite. Schwertmannite mineral has the potential to be a solution for treating arsenic in drinking water and mining wastewater. This research work involved the synthesis of schwertmannite using a modified neutralization method. The effect of operating pH (3.0 and 3.5), temperature (25°C and 65°C), agitation speed (150 rpm, 250 rpm and 350 rpm) and concentration of Fe3+ (0.05 M and 0.1 M) in the Fe3+ system, in the Fe2+ system and in the Fe3+-co-existing metal ion system (Fe3+- Sn2+, Fe3+- Zn2+, Fe3+- Mn2+ and Fe3+- Cu2+) towards the formation and transformation of the synthesized schwertmannite were investigated. The varying synthesis condition and system exhibited different degree of transformation of schwertmannite to goethite based on the respective X-ray Diffraction (XRD) characteristic peaks with matching d-spacing’s values to the natural schwertmannite and goethite mineral. Effect of different synthesis conditions have showed different characteristic of the mineral phases present in the schwertmannite. Complete schwertmannite formation was accomplished with partial or complete goethite phases. The field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) photomicrographs showed the schwertmannite produced has a relatively rough surface and void or cavities with compact aggregates of > 200nm diameter and very fine needles like shapemorphology. These enhanced the adsorption capabilities of the schwertmannite-goethite in the removal of As (V). From the study on the effect of operating pH, adsorbent dosage and operating time, the schwertmannite-goethite were able to remove As (V) to nearly 100% in both acidic and alkaline conditions. The optimum dosage determined at 0.10g (1.0 g/L) gave a high % of As (V) removal at 92-99%. The presence of different co-existing metal ions showed significant characteristic that influenced the % removal of As (V) where almost rapid removal occurred within the first 30 min and almost 100% removal were achieved within the 24 h run. The adsorption kinetic analysis, showed a goodagreement to the pseudo-second-order kinetic model, with high correlation coefficient values (R2 > 0.90), where the rapid removal of As (V) not only dependent on the arsenic concentration but also due to the presence of goethite mineral and the co-existing metal ions in the schwertmannite-goethite. Hence, the schwertmannite-goethite synthesized in this research work served as a good and cost-effective adsorbent in the removal of As (V) to help solve and minimise the environmental problem due to arsenic contamination and pollution in mining wastewater.