Detection, quantification, and characterization of polystyrene microplastics and adsorbed bisphenol A contaminant using electroanalytical techniques

Abstract

The potential applications of electroanalytical techniques for the quantification and size characterization of nonelectroactive polystyrene microplastics is reported, in addition to characterizing the kinetics of adsorption of bisphenol A on these polystyrene microparticles. The individual adsorption events of very diluted polystyrene microparticles dispersions on glassy-carbon microelectrodes produce the blocking of the charge transfer of a mediator (ferrocene-methanol) thus decreasing the current of the recorded chronoamperogram in a stepwise manner. The magnitude of the current steps are in the order of pA values and can be related to the diameter of the plastic microparticles in the size range 0.1 to 10 µm. The frequency of the current steps in the domain time used (120 s) allows to quantify the number concentration of these microparticles in the range 0.005 to 0.500 pM. Electrochemical impedance spectroscopy confirms the adsorption of the polystyrene microplastics on carbon microelectrodes (and to a lesser extent on platinum microelectrodes) under the same experimental conditions as above. On the other hand, the adsorbed microplastics become concentrators of other pollutants found in the environment. The sensitive differential-pulse voltammetry determination of bisphenol A (linear range 0.80-15.00 µM; detection limit 0.24 µM) was used together with a simple separation procedure for studying the adsorption of bisphenol A on polystyrene microparticles. The adsorption capacity (mg of bisphenol A retained per g of the polystyrene microplastics) decreased from approximately 5.7 to 0.8 mg g^-1 with increasing dosages of polystyrene microparticles from 0.2 to 1.6 g l^-1. The adsorption isotherms were modeled resulting in a monolayer of bisphenol A adsorbed on the microplastics (i.e., best fitted to a Langmuir model).

Keywords: Adsorptive enrichment; Blocking nanoimpact electrochemistry; Chronoamperometry; Electrochemical impedance spectroscopy; Polystyrene microparticles.

Fig. 1

Typical changes in the steady-state currents of the chronoamperograms measured in A a blank solution (FcMeOH 5^.10^–3 M in 1^.10^–3 M KCl); B the same blank solution as above in the presence of PS-MPs 0.050 pM. Working electrode: µGCE. C Magnified view of a discrete current step measurement (∆I_s)

Fig. 2

Distributions of the current step values counted between 10 and 130 s. from current–time chronoamperograms of the mediator FcMeOH in concentrations of the following: A 2.5^.10^–3 M and B 5^.10^–3 M. In both cases, the concentration of PS-MPs (0.50 µm diameter) was 0.050 pM. Supporting electrolyte: KCl 1^.10^–3 M. The µGCE was held to a potential of E_a =  + 0.50 V. vs Ag/AgCl

Fig. 3

Equivalent circuits used for the fittings of the adsorbed polystyrene layers: A a series of capacitance and resistance in capacitative measurements (RC); B a parallel circuit in faradaic mode (R[CR])

Fig. 4

EIS measurements of A C_dl (without mediator); and B R_ct (with charge-transfer mediator, [FcMeOH] = 1^.10^–3 M) as a function of the adsorption time (t_ads) of PS-MPs 0.50 pM on µGCEs. Supporting electrolyte: KCl 1^.10^–3 M

Fig. 5

Representative plots of the adsorption efficiency (%EA) in function of the adsorption time (tads), for a dosage of 0.4 mg mL⁻¹ of PS-MPs (0.50 µm diameter). Fittings to a Langmuir (A) and Freundlich (B) isotherm adsorption models. Concentration of BPA: 1^.10^–4 M