Quantitative N-S chelation speciation and pH-dependent selectivity of Pb(II) and Cu(II) on thiourea-functionalized...

Selective and quantitative speciation of Pb(II) and Cu(II) in strongly acidic matrices (pH 2) remains analytically challenging because conventional oxygen-donor chelation platforms lose coordination capacity under extreme proton competition. Here, we report thiourea-functionalized rice husk cellulose (RH6) as a quantitative N-S coordination platform for pH-resolved metal speciation. High-resolution XPS deconvolution of the N 1s and S 2p core levels, the primary analytical outputs, reveals that, despite 65-67% nitrogen-site protonation at pH 2, 18-22 at.% N-M and 30-35 at.% S-M coordination bonds persist (Tables 1 and 2), quantifying the competing protonation and coordination equilibria directly at the binding interface. Selectivity coefficients determined in multi-ion matrices (α(Pb/Fe) = 25 ± 6; α(Pb/Al) = 22 ± 5; α(Pb/Cu) = 3.1 ± 0.6) are quantitatively predictable from Pearson's HSAB softness parameters and the Irving-Williams stability series, establishing a mechanistic analytical framework for matrix-tolerant speciation at pH 2. AIC-guided kinetic model selection (PSO: R² = 0.992, AIC = -150) identifies N-S coordination-bond formation as the rate-limiting step in the analytical process. Langmuir isotherm fitting yields q_m(Pb) = 625 mg g^-1 and q_m(Cu) = 542 mg g^-1 (pH 2), with favorable separation factors (R_L < 0.12). Platform reproducibility across 10 analytical cycles (>90% capacity retention; thiourea leaching <0.8 mg L^-1 per cycle) confirms chemical stability and analytical repeatability. This work establishes a quantitative, mechanism-grounded analytical framework for speciation-based determination of soft Lewis acid metals in complex, strongly acidic matrices.