Tissue-Level Heterogeneity in Raw Giant Juncao Straw and Its Influence on Ammonium and Phosphate Adsorption

Raw lignocellulosic biomass offers a low-input route for nutrient capture, yet its internal tissue heterogeneity is rarely considered when adsorption behavior is interpreted. Here, Giant Juncao (Pennisetum giganteum; also reported as Cenchrus fungigraminus) straw was separated into rind and pith to determine how native tissue architecture and cell-wall chemistry relate to ammonium (NH4+) and phosphate adsorption. The pith occupied 72.33% of stem volume but only 26.85% of dry mass and had a much lower basic density than the rind (0.072 vs. 0.570 g·cm−3). It also showed a larger N2-accessible specific surface area (2.67 vs. 1.13 m2·g−1), greater pore volume (0.006 vs. 0.001 cm3·g−1), higher pectin content (20.58 vs. 14.81 mg·g−1), and lower cellulose crystallinity (48.28% vs. 57.30%). These contrasts were accompanied by tissue-dependent NH4+ uptake: the rind and pith adsorbed 4.19 ± 1.02 and 10.36 ± 1.54 mg·g−1, respectively, whereas whole straw reached 13.80 ± 0.23 mg·g−1 under the same comparison conditions. Phosphate uptake was less tissue-sensitive (19.57–22.40 mg·g−1). More than 90% of the 24 h uptake was reached within 60 min. Nonlinear least-squares fitting, evaluated by multiple criteria (R2, SSE, RMSE, χ2, or AIC), identified the pseudo-second-order kinetic model and the Langmuir–Freundlich isotherm as the best empirical descriptions of the measured data. Spectroscopic changes were consistent with participation of oxygen-containing cell-wall groups in nutrient retention, but did not uniquely resolve molecular binding pathways. The results show that tissue-scale architecture and heterogeneous lignocellulosic chemistry jointly shape nutrient uptake in minimally processed Giant Juncao and support further evaluation of the unseparated feedstock in real wastewater and nutrient-reuse scenarios.