Surface engineering of chitosan nanoparticles via polyamidoxime grafting and Response Surface Methodology optimization for hexavalent chromium remediation.

Herein, we report the synthesis of polyamidoxime-functionalized chitosan-nanoadsorbent capable of efficiently removing hexavalent chromium (Cr(VI)) from aqueous media. Chitosan-nanoparticles were prepared by reverse-micelle method followed by vinyl-modification to incorporate grafting sites. Polyacrylonitrile was then grafted onto modified-nanoparticles by emulsion-graft polymerization followed by chemical-transformation to polyamidoxime (PAO-g-MCN). Gravimetric analyses revealed the maximum grafting-percentage of 183% under optimized reaction parameters comprising 5% monomer, 1.0% surfactant and 0.1% initiator. Chitosan-nanoparticles were characterized via DLS and UV-Visible spectroscopy to assess particle size distribution (167 nm) and characteristic absorption peak (254 nm). The structural and morphological characteristics of the bio-nanoadsorbent were examined using FTIR, XRD, FESEM, EDX and BET. Adsorption experiments were conducted to optimize the effects of contact time, pH and initial chromium concentration using Box-Behnken design within response-surface methodology framework. The experimental findings were investigated by the ANOVA showing that the model regression is acceptable. The R2 determination coefficients were found to be 0.9781 for adsorption capacity and 0.9672 for %Removal signifying an excellent connection between predicted and experimental responses with optimal adsorption at pH = 4.5, time = 30 min, initial-concentration = 50 mg/L, adsorbate volume = 10 mL and adsorbent mass = 10 mg. The developed bio-nanoadsorbent exhibited a maximum adsorption-capacity of 320.47 mg g-1, with experimental data best described by pseudo-second order and Langmuir isothermal-model, indicating monolayer chemisorption on homogeneous surface. The cyclic-reuse performance of PAO-g-MCN bio-nanoadsorbent was evaluated through nine cycles, which demonstrated that it maintained >90% removal-efficiency over five cycles indicating its promising durability and reusability. In conclusion, PAO-g-MCN demonstrates significant potential as an effective and environmentally friendly bio-nanoadsorbent for the remediation of Cr(VI) contaminated aqueous-systems.