Modification of the coffee industry sludge-based adsorbent using acid treatment for the removal of tartrazine dye from aqueous solution

The massive amount of industrial sludge produced annually poses an environmental concern, and improper sludge disposal may harm the environment. To address this issue, sludge has been converted into a cost-effective adsorbent for treating wastewater containing dyes. This study aims to modify the coffee sludge-based adsorbent using various treatment methods for tartrazine dye removal. The raw sludge was modified through physical treatment (PT) via oven drying and calcination, chemical treatment (CT) using phosphoric acid and nitric acid, and physicochemical treatment (PCT) method. The modified sludge adsorbents were characterised using Micromeritics ASAP 2020 for nitrogen adsorption analysis. Subsequent batch adsorption experiments were performed using the best-modified adsorbent of PCTS-HNO3 prepared by physicochemical treatment method with nitric acid to assess the effect of various factors, including initial pH (3-11), adsorbent dosage (0.05-0.25 g/L), and initial concentration of tartrazine dye (25-100 mg/L), with a contact time of 2 hours. The best process conditions were achieved at 0.15 g/L of adsorbent dosage, initial pH of 3 and initial concentration of 50 mg/L with almost 100% dye removal. The characterization results revealed that the BET surface area of the modified adsorbent was significantly increased from 363.32 m2/g of RS to 599.64 m2/g for PCTS-HNO3 adsorbent. The Langmuir isotherm model provided an excellent fit to the adsorption isotherm data, with correlation coefficients R2 ≥ 0.99 and a maximum adsorption capacity (qmax) of 99.01 mg/g. The kinetic study observed that the adsorption process adheres remarkably well to the pseudo-second-order kinetic model, signifying chemisorption adsorption. The results of this study indicate that the modified coffee sludge-based adsorbent proves to be a viable and cost-efficient solution for eliminating tartrazine dye from aqueous solutions.