Removal of Methylene Blue Dye Using Crushed Walnut Shell Adsorbent –Determining Comminution Energy and Optimal Operational Conditions

Abstract

Serious environmental issues might be associated with Methylene Blue(MB) contaminated wastewaters as this synthetic dye is environmentally persistent, toxic, carcinogenic and mutagenic. Crushed-raw Walnut shell(WS) biosorption process provides an effective treatment method. Although with an increase in the specific surface area of the fine adsorbent particles higher efficiency in dye removal could be expected, the comminution energy will simultaneously increase. Consequently, we used a developed Bond-grindability-test, which measures the amount of energy required for comminution, to estimate energy consumption. In addition to adsorbent particle size(course/moderate/fine), two affective parameters of adsorbent to dye ratio and adsorbent dosage, were considered. According to the central composite design-circumscribed, 33 experiments were planned in three groups associated with each size ranges. The results of FTIR spectroscopy confirmed that MB molecules interact with WS functional groups as shifts in the spectrum of treated vs raw WS particles were observed. The Bond-work-index estimated as 564 kWh/t, revealing the standard WS comminution energy. The formation of extended cracks on the fine particles’ surface, due to significant required crushing energy, provided a porous medium for multilayer coatings, validated the compatibility of the Freundlich model for this category. The maximum adsorption efficiency for course particles was over 96% corresponding to 4.61 g/L of WS and 108.58 mg/L of MB. Accordingly, the energy consumption for each mg of adsorbed dye was 7.65 Joules. The removal efficiency of moderate to fine particles depended only on the adsorbent to dye ratio (i.e., 30.5 ~ 34.8 and 31.6 ~ 37.4 g/g, respectively, to achieve maximum MB removal efficiency).