Core-shells of magnetite nanoparticles decorated by SBA-3-SO3H mesoporous silica for magnetic solid phase adsorption of paraquat herbicide from aqueous solutions

Abstract

Paraquat, the most well-known herbicides of toxicological class I, is widely used as an effective herbicide despite its high toxicity for humans and animals. In the current research, a simple synthesis approach was utilized to prepare Fe3O4 @SiO2 @SBA-3-SO3H core-shell mesoporous magnetic nanoparticles (MMNPs) as effective adsorbent for paraquat removal from the aqueous solutions. The characterization of nanoparticles was examined by XRD, BET, FT-IR, FESEM, VSM, mapping and TEM. To optimize the various experimental variables affecting the removal efficiency of paraquat, the effects of four experimental parameters at four levels including solution pH, adsorbent amount, contact time and ionic strength were evaluated by Taguchi fractional factorial design method (OA16). Based on the ANOVA results, ionic strength (80.69%) was recognized as the most significant factor contributing to the paraquat removal efficiency followed by the adsorbent mass (12.67%), pH of the solution (3.52%), and stirring time (0.141%). At the optimum conditions (sample volume = 50 mL, adsorbent dose = 2.4 g L−1, solution pH = 7 and ionic strength = without salt addition), the four kinetic models for the paraquat adsorption on the Fe3O4 @SiO2 @SBA-3-SO3H MMNPs were studied at two concentrations of 25 and 75 mg L−1 in the time range of 0.5–120 min. According to the results, the pseudo-second order kinetic model is the best model (R2 > 0.99, qe, cal =80.0 mg g−1), confirming diffusion plays a key role in determining the rate of adsorption process. Adsorption equilibrium data were investigated by four known isotherm models and the results demonstrated the best fitting with the Langmuir model. From the thermodynamic studies, ΔH°, ΔS°, ΔG°, and Ea of adsorption process were obtained as − 4697.08 J mol−1, − 3.938 J mol−1 K−1, − 3523.45 J mol−1 and − 4246.46 J mol−1, respectively. The negative ΔH° and ΔG° values indicated the exothermic and spontaneous nature of paraquat sorption on the surface of nanoparticles, respectively. Moreover, the obtained values of ΔH°, ΔG° and Ea confirmed the physisorption nature of interaction. The negative ΔS° and Ea shows a decrease in the randomness at the interface of solution-adsorbent with the progress of the sorption process and exothermic nature of adsorption, respectively. The calculated sticking probability (S*) showed the high probability of sticking to the surface. The proposed adsorbent showed very good adsorption behavior and showed reusability more than six cycles without significant reduction in its performance that makes it as a proper adsorbent for paraquat removal.