Transformation of textile dyeing industrial sludge into economical biochar for sorption of ofloxacin: equilibrium, kinetic, and cost analysis

Textile dyeing industry sludge was successfully converted into efficient biochar to remove a persistent pharmaceutical pollutant, namely ofloxacin, from an aqueous solution. Raw sludge contained substantial calcium (Ca) attributable to lime as a coagulant in the effluent treatment plant (ETP). It had a reasonable amount of carbon from the cotton yarns dyed in the industry. The raw sludge and prepared biochars were comprehensively examined through characterization techniques like TGA, BET, XRD, XRF, FESEM, EDX, and FTIR. Batch adsorption studies were performed to optimize the operational parameters such as initial ofloxacin concentration, pH of the solution, biochar dose, temperature, and time. The biochar showed the maximum adsorption capacity of 21.6 mg g at optimum conditions (C = 30 mg L, T = 25 ± 2 °C, pH = 6 ± 0.2, dose = 5 g L, time = 240 min). The pseudo 2nd-order kinetics well explained the experimental adsorption data with regression coefficient in the range 0.944 ≤ R ≤ 0.998 and sum of square of error (SSE) varying in the range 0.3 ≤ SSE ≤ 29.5. Langmuir isotherm closely describes the ofloxacin adsorption with 0.877 ≤ R ≤ 0.965 and 5.1 ≤ SSE ≤ 12.4. The thermal regeneration study of biochar was also carried out, and the removal efficiency of ≈73% was obtained in the first cycle while it trends to decrease to ≈59% after five consecutive cycles. The cost analysis showed that biochar could be prepared for ≈$0.305 (₹22.8) per kg. Hence, biochar is reasonably economical and could be used for large-scale applications. Graphical abstract: [Figure not available: see fulltext.]