Adsorption and chemical precipitation of lead and zinc from contaminated solutions in porous rocks: Possible application in environmental protection

• Lead (II) and zinc (II) were chosen for testing heavy metal adsorption.
• Two different rocks were used as test materials: natural rhyolite tuff and a porous limestone from Hungary.
• Lead (II) and zinc (II) ions could be removed from aqueous solutions by bulk and powdered rhyolite tuff and porous limestone.
• Lithologies are recommended to be used for removing heavy metals in environmental processes, such as treating of groundwater.

Natural porous rocks, like limestone and rhyolite tuff are able to reduce heavy metal pollution by adsorbing or precipitating them from heavy metal containing solutions due to the favourable physical and chemical properties of these rocks.In our experiment, two porous rocks, a porous limestone and rhyolite tuff were used. Petrophysical parameters namely apparent density, real density, capillary water absorption, ultrasonic pulse velocity, total porosity and open porosity of the two porous rocks were determined in water-saturated and dried conditions.Powdered rock samples and cylindrical specimens were placed in lead-nitrate and zinc-sulphate solutions (initial concentration: 1000 ppm) and the amount of lead (II) and zinc (II) ions were identified by titration (chelatometry) of the residual solution. According to the experiments, powdered rocks and rock specimens of limestone and rhyolite tuff reduced the lead (II) and zinc (II) ion concentrations in aqueous solution. The results were cross-checked by ICP-MS. Heavy metal removal capacity was relatively high, 92–99% in each case. The treated powdered rocks and rock specimens were also studied by scanning electron microscope (SEM-EDS) and new heavy metal precipitates were identified.According to the tests result, it could be confirmed that these types of lithologies are capable of removing heavy metals and can be used in environmental protection technologies in a form of permeable reactive barrier.