Optimization of Lignocellulose Esterification in Biosorption Research: A Case Study

Biosorbents based on chemically modified lignocellulose (LC) are promising for sustainable water treatment. However, a limited understanding of how modification factors influence adsorptive performance and a lack of system optimization hinder comparability across studies. In this work, brewers’ spent grain (BSG) was esterified solvent-free with citric acid (CA), varying temperature, time, and CA concentration. The impact of these factors on methylene blue (MB) adsorption was screened using response surface methodology (RSM). Langmuir isotherm parameters and equilibrium adsorption capacities (qe) were evaluated for their suitability as model responses using analysis of variance (ANOVA). BSG was successfully esterified, with temperature and time showing strong individual and interactive effects on the maximum adsorption capacity (qmax), while the CA concentration had a smaller, yet positive, influence. At optimal esterification conditions (170 °C, 30 min, 2 M CA), qmax reached 128.4 mg/g against unmodified BSG (94.8 mg/g). Here, qmax was validated as the most accurate and reliable response metric (R2adj 88.5%), while the Langmuir constant (KL) failed to capture variance meaningfully. Although qe values at initial concentration (C0) 500 mg/L explained system trends with reduced accuracy (R2adj 81%), they produced a response surface similar to that of qmax, indicating that simplified experimental strategies may be viable in future optimization studies if appropriately validated.