Nitrite Removal Efficiency from Tofu Wastewater Using Robusta Coffee Grounds as a Biosorbent

Abstract

Tofu wastewater may contain toxic nitrite ions (NO₂⁻) due to inadequate treatment before discharge into aquatic environments. This study investigated the effectiveness of Robusta spent coffee grounds as a low-cost biosorbent for nitrite removal from tofu wastewater using a batch adsorption process. The biosorbent was prepared by carbonising dried coffee grounds at 350°C for 3.5 hours, followed by chemical activation with 0.1 M HCl for two days, washing to neutral pH, and drying. Adsorption experiments were conducted using 100 mL of tofu wastewater with an initial nitrite concentration of 40 mg/L. Biosorbent masses of 0.2, 0.3, and 0.4 g and contact times of 10, 30, 50, 70, and 90 minutes were evaluated. The residual nitrite concentration was determined using UV–Visible spectrophotometry. The results demonstrated that nitrite-removal efficiency increased with increasing biosorbent mass and contact time. The highest removal efficiency of 77.75% was achieved using 0.4 g of biosorbent at a contact time of 90 minutes, reducing the nitrite concentration to 8.9 mg/L. Meanwhile, the highest adsorption capacity of 14.50 mg/g was obtained using 0.2 g of biosorbent at 90 minutes. Kinetic analysis showed that both pseudo-first-order and pseudo-second-order models adequately described the adsorption process, with coefficients of determination ranging from 0.86 to 0.96. However, the pseudo-second-order model produced adsorption-capacity estimates closer to the experimental values, suggesting that chemisorption contributed to the overall removal mechanism. A preliminary equilibrium analysis indicated that the Freundlich isotherm provided a better fit than the Langmuir model, although additional experiments involving different initial nitrite concentrations are required. Two-way ANOVA confirmed that biosorbent mass and contact time significantly affected nitrite-removal efficiency, with contact time identified as the dominant factor. These findings demonstrate that Robusta spent coffee grounds have considerable potential as an environmentally friendly biosorbent for treating nitrite-contaminated tofu wastewater.