Maximizing wheat straw utilization: An integrated, near-zero-waste sequential biorefining approach

This study introduces an integrated, near-zero-waste biorefining approach for comprehensive wheat straw (WS) valorization to multiple value-added products. Wheat straw underwent various chemical pretreatments wherein each step generated distinct solid and liquid streams serving as a feedstock for subsequent processing, rather than focusing on a single product. Initial dilute acid treatment (1 % (v/v) H₂SO₄) of WS (20 g) releases xylose (9.59 g/L, 51.12 %) and glucose (25 g/L, 42.14 %), which are fermented by Lactobacillus reuteri to produce lactic acid (LA) (4.12 g/L; yield: 0.139 g/g). Alkaline treatment of acid-pretreated solid residue using varying NaOH concentrations (1.5, 3 %, 5 %) produced black liquor, with maximum lignin recovery (32.96 ± 0.42 %) at 3 % NaOH alongside a phenolic rich extract (0.07 g) primarily composed of acetosyringone (27.20 %), p-coumaric acid (22.36 %), ferulic acid (10.48 %) and 2,3-dihydrobenzofuran (21.36 %). The final sodium chlorite treatment of residue produces cellulose pulp (3.26 g), effectively removing the residual lignin from alkaline pretreatment step. Structural characterization of lignin by FTIR and ¹H/¹³C NMR confirmed its guaiacyl-syringyl-hydroxyphenyl (GSH) type structure with preserved aromatic motifs and minimal degradation. Extracted lignin exhibited strong ABTS radical scavenging activity (IC₅₀ = 0.7 mg/mL) surpassing commercial alkaline lignin and phenolic extract. The cellulose pulp showed 9.98 % increase in crystallinity index compared to raw WS, complete absence of lignin associated FTIR peaks (1640 cm⁻¹, 1514 cm⁻¹, 1467 cm⁻¹) and fibrillar morphology confirming the effective delignification of biomass. A detailed mass balance elucidated compositional changes throughout the process and product accumulation.