Influence of pH and temperature on the adsorption/desorption capacity of microplastics

, Godoy V., Martín-Lara M.a., Blázquez G., Calero M..

Microplastics have a demonstrated capacity to adsorb chemical substances (metals, pharmaceuticals and others) that are frequent in seawater, freshwater or wastewater. This poses a high risk for fauna that ingest microplastics accidentally, due to the fact that, under acidic pH and high temperature conditions, microplastics can release the adsorbed contaminants into the organism. In the present study, kinetic and equilibrium tests were carried out with lead and amoxicillin in Milli-Q water, using microplastics of the most common polymers (polypropylene and polyethylene). The selected concentration for kinetic tests was 1 mg/l, whereas in equilibrium tests concentrations of 0.5, 1, 2, 4, 16 and 32 mg/l were used. All results were fitted to Langmuir and Freundlich models. Also, desorption tests were carried out in Milli-Q water at three pH values (2, 4, 6) and two temperatures (25 and 40ºC). The results showed large differences in adsorption behavior between metal and pharmaceutical, and also between polymers. In the case of lead with polypropylene, the equilibrium was reached after 5 days of contact with the microplastics, whereas in the case of amoxicillin with polyethylene, equilibrium was not reached after 28 days of contact. The desorption tests with amoxicillin and lead showed, in general, a great release at the lowest tested temperature (25ºC) and pH (2). In desorption tests with lead, the greatest desorption occurred at pH 4 at both temperatures. The temperature of 40ºC and pH 6 showed the lowest rate of desorption. In desorption tests with amoxicillin, the greatest desorption ocurred at pH 6 and temperature of 40ºC, just the opposite of the results obtained for lead. However, a combined phenomenon of adsorption-desorption was observed, which indicate the influence of several factors in this case.

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