Application of Compost as a Cheap Sorbent for Dyes Removal from Aqueous Solutions

This study was aimed at determining the feasibility of using compost as an unconventional sorbent for dyes removal from aqueous solutions. Compost applied in the study was a product of sewage sludge composting with plant residues (birch wood chips and rapeseed straw). Experiments were conducted for two anionic reactive dyes: Reactive Yellow 84 [RY84] (1701 g/mol - lambda max = 357.5 nm) and Reactive Black 5 [RB5] (991 g/mol - lambda max = 600.0 nm), and for two cationic dyes: Basic Green 4 [BG4] (365 g/mol - lambda max = 618.0 nm) and Basic Violet 10 [BV10] (479 g/mol - lambda max = 547.5 nm). The scope of the study included: determination of the optimal pH value of dyes adsorption onto compost, determination of equilibrium time of dyes sorption onto compost, determination of the maximum sorption capacity of the analyzed dyes on compost. During each experiment, sorbent concentration in the solution was at 5 g d.m./dm(3). The concentration of dye remaining in the solution was determined with the spectrophotometric method on a UV-VIS SP 2000 spectrophotometer. Analyses were conducted at a room temperature - T = 22 degrees C. The pH value of solutions was adjusted with aqueous solutions of HCl and NaOH. Sorption of the anionic reactive dyes RY84 and RB5 was the most effective at pH = 3. The optimal pH value of cationic dyes sorption was established at pH = 5. At a high pH value (pH = 9), the sorption of each type of dye was impaired due to partial solubilization of the compost sorbent. The equilibrium time of dyes sorption on compost accounted for 180 min in the case of RY84, RB5 and BG4, and for 240 mm in the case of BV10 - 240 min. Analyses of the maximum sorption capacity of the selected dyes on compost were carried out already after establishing the optimal pH value of the sorption process for each dye. Results obtained were described with the use of two sorption models: a heterogenous Langmuir 2 model (double Langmuir equation), and a heterogenousFreundlich model. The Langmuir 2 model showed the best fit to experimental data (R-2>0.99). Due to the presence of functional groups -COOH and -OH, compost is negatively charged in aqueous solutions owing to which it prefers compounds with a positive charge (cationic dyes) during sorption. Electrostatic repulsion significantly impairs the adsorption of anionic dyes. The maximum sorption capacity of compost in the case of RY84 and RB5 reached 2.15 mg/g d.m. and 4.79 mg/g d.m., whereas in the case of BG4 and BV10 - 26.41 mg/g d.m. and 27.20 mg/g d.m., respectively. Results of the maximum sorption capacity of dyes on compost were referred to results obtained with other unconventional and cheap sorbents.