A pH-Sensitive Surface of Chitosan/Sepiolite Clay/Algae Biocomposite for the Removal of Malachite Green and Remazol Brilliant Blue R Dyes: Optimization and Adsorption Mechanism Study

A pH-sensitive chitosan/sepiolite clay/algae biocomposite (Chi/Sep/Alg) was fabricated to remove cationic (malachite green, MG) and anionic (remazol brilliant blue R, RBBR) dyes from aqueous. The characteristics of Chi/Sep/Alg biocomposite were investigated by pH_pzc, pH-potentiometric titration, CHNS, XRD, SEM–EDX, FTIR, and BET analyses. A Box–Behnken design (BBD) was adopted to optimize the adsorptive performance of the Chi/Sep/Alg biocomposite towards removal of MG and RBBR dyes using three controllable operating factors, namely Chi/Sep/Alg dose (0.02–0.1 g), solution pH (4–8), and process time (5–30 min). The ideal BBD model circumstances for MG dye removal efficiency were as follows: Chi/Sep/Alg dose (0.1 g/L), solution pH 8, and time (17.5 min), while for RBBR dye, the ideal circumstances were Chi/Sep/Alg dose (0.1 g/L), pH ~ 4, and time (17.5 min). The adsorption kinetic and isotherm reflect that the adsorption of MG and RBBR dyes onto Chi/Sep/Alg biocomposite obeyed pseudo-second-order and Freundlich isotherm model, respectively. The maximum adsorption capacities of Chi/Sep/Alg biocomposite towards MG (515.7 mg/g) at basic pH environment (pH 8) and RBBR (292.4 mg/g) at acidic pH environment (pH 4). An endothermic and spontaneous adsorption process of MG and RBBR dyes was confirmed by the calculated thermodynamic functions. The adsorption mechanism of MG and RBBR dyes on the surface of Chi/Sep/Alg biocomposite can be attributed to various interactions such as electrostatic, H-bonding, and n-π interactions. Thus, this pH-sensitive Chi/Sep/Alg biocomposite exhibits a great affinity towards capturing cationic and anionic dyes by adjusting the solution pH.