Face centered central composite design-Response surface methodology (FCC-RSM) a multivariate optimization of crystal violet dye adsorption by Cajanus cajan residues with thermodynamic, kinetic and mechanistic insights

The present study explores the potential of ecofriendly biosorbents derived from pigeon pea ( Cajanus cajan) residues in the remediation of crystal violet (CV) dye. To access detailed chemical structure and morphological information, the as-prepared adsorbents pigeon pea peel (PPP) and pigeon pea seed (PPS), obtained from the peel and seeds of the Cajanus cajan , were characterised by advanced instrumental techniques, viz., powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FT-IR), Brauner-Emmet-Teller (BET), Field emission scanning emission microscopy (FESEM), energy dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS). Response surface methodology (RSM) with a face-centred central composite design (FCC) was successfully developed to model and optimize the adsorption process. The interactive influence of the adsorbent dosages (0.04–0.14 g), initial concentration (5–50 mg/L), and contact time (10–100 min) on the efficiency of CV removal % was comprehensively evaluated and optimized through the statistical modelling. The FCC-RSM methodology showed outstanding effectiveness in process optimization. The experimental results envisage that the adsorption process followed a polynomial quadratic model, as evident from the higher regression coefficient obtained for both adsorbents, PPP (R 2 = 0.992) and PPS (R 2 = 0.998), respectively. The optimal conditions under the studied range of adsorption parameters corresponding to maximum CV removal % for the PPP/CV (C o = 35 mg/g, dosages = 0.092 g, time = 89.4 min, efficiency = 77.45 %) and PPS/CV system (C o = 42.64 mg/g, dosages = 0.133 g, time = 58.92 min, efficiency = 73.83 %) were obtained. The thermodynamic and kinetic studies suggested the spontaneous, endothermic and chemisorption nature of the process. In addition, the reusability experiments manifested stable performance at low initial dye concentration and can be effectively reused up to three consecutive cycles. Thus, the proposed adsorbents (PPP, PPS) emerged as sustainable and low-cost adsorbents, offering efficient and ecofriendly alternatives for dye treatment from aqueous media.