Adsorption of malachite green from aqueous solution by nylon microplastics: Reaction mechanism and the optimum conditions by response surface methodology

Abstract Nylon microplastics and malachite green (MG) coexist ubiquitously in the coastal aquaculture areas. Factors controlling MG adsorption to aged and virgin nylon microplastics were identified. The microplastics were characterized by scanning electron microscope equipped with the energy dispersive X-ray spectroscope (SEM-EDAX) and Fourier transform-infrared (FTIR) spectroscopy to elucidate the possible interaction mechanism. The MG adsorption closely conformed to the pseudo-second-order model and the Langmuir isotherm. The MG uptake by the aged nylon microplastics significantly decreased with an increase in solution Pb2+ concentration while the effect of Pb2+ on MG uptake by the virgin microplastics was negligible. The optimum conditions for MG uptake by aged nylon microplastics obtained were 33.51 mg g−1 at initial MG concentration of 57 mg L−1, initial solution pH of 6.84 and aged nylon microplastics dosage of 1.66 g L−1 by use of response surface methodology (RSM). Desorption efficiencies (%) of MG/nylon microplastics combinations in gut surfactant were significantly higher than those in the water, which indicates MG with nylon microplastics has the higher potential for transport to organisms. Overall, the results of this study highlight the role of surface carboxyl functional groups and H-bonding, which both jointly regulate the adsorption of MG on aged nylon microplastics.