Assessment of metals and sulfur content in lignocellulosic biomass: developing a simple method for characterizing ash composition by optical emission spectrometry

Lignocellulosic biomass presents a high potential for biofuel production. Knowledge of its chemical composition is essential to understand the contamination in the final products, and to select appropriate conversion technology. Hence, this study aimed to optimize an alternative analytical protocol for elemental determination in sugarcane straw (SS), sugarcane bagasse (SB), pine wood residue (PWR) and eucalyptus wood residue (EWR). Metals and sulfur were determined by inductively coupled plasma optical emission spectrometry and microwave-induced plasma optical emission spectrometry and optimization of a powerful and greener sample preparation protocol was investigated. The combination of nitric acid (0.5 mL), hydrogen peroxide (0.20 mL), hydrofluoric acid (0.06 mL) and boric acid (0.6 mL) for sample digestion resulted in accurate results (79 to 116%). The developed protocol reduced the reagent volume by up to sixteen times compared to the EN ISO 16967 standard method while maintaining accuracy within the confidence limit (ANOVA, p < 0.05). A certified reference material of sugarcane leaves (CRM Agro C1005a) was analyzed under optimized conditions to evaluate the accuracy of the proposed method. Limits of quantification ranged from 1.00 μg g⁻¹ (Cu) to 1096 μg g⁻¹ (Si) by MIP-OES. Regarding the elemental content, Ba, Cu, and Sr were typically found at lower levels in all samples, at <1 μg g⁻¹ (Cu) whereas Al, Ca, K, and Si reached 0.1 to 0.4% in SS and SB; Ca and K were the main contaminants in PWR and EWR (0.09 to 0.1%).