New type of highly selective microcapsules for the removal of mercury from surface polluted waters

•Microcapsules containing trioctylmethylammonium chloride from poly(styrene-co-divinylbenzene) were developed.•Equilibrium experiments for mercury uptake at pH=1 and natural pH were studied.•A mathematical model using the ideal mass action law was proposed.•Different natural waters containing mercury in ngL−1 levels from the mining district of Almadén were studied.•Treatment of natural waters with these microcapsules would prevent risks to human health. A new class of particulate material for the selective removal of mercury from polluted aqueous streams has been developed. The material consists in core–shell microcapsules containing a highly selective extractant agent for mercury. For this purpose, trioctylmethylammonium chloride (TOMAC) has been encapsulated within a poly(styrene-co-divinylbenzene) – P(St-DVB) – shell by suspension-like polymerisation technique. This process gave rise to a high product yield with an encapsulation efficiency of 95%. Microcapsules presented an average particle size lower than 40μm and a TOMAC content of 37.70wt.%. Different equilibrium experiments for mercury uptake were conducted at pH=1 and natural pH, using synthetic solutions and natural waters from the mining district of Almadén (Spain). A mathematical model that takes into account the dissociation of mercuric chloride in solution was proposed and the solid equilibrium parameters were obtained. A good fitting between the experimental data and the model was achieved, obtaining a maximum useful capacity of 1.009±0.018eqkg−1 with a value of equilibrium constant of the monovalent specie (HgCl3-) two orders of magnitude smaller than those of the divalent HgCl42-. The mercury uptake was favoured at acid pH and low chloride concentration. Finally, it was demonstrated that using this material is possible to remove more than 92% and 99% of the metal content from the natural waters containing mercury in ngL−1 levels at natural and acid pH, respectively. The use of this new type of extractive microcapsules allows to get mercury removal levels in surface polluted waters that satisfy the mercury standard for aquatic life proposed by the USEPA (12ngL−1) without additional pretreatments for the adjustment of the pH value.