Abstract
A laboratory method was designed and verified that allows for the testing of alkaline, magnesite-based reactive materials for permeable reactive barriers (PRBs) to remove heavy metals from contaminated groundwater. It was found that caustic calcined magnesia (CCM) with high reactivity and low solubility to remove Cu2+, Zn2+, Ni2+, and Mn2+ cations from mixed aqueous solutions can be prepared by calcination at a suitable temperature and residence time. Regarding the solubility of both the reactive material itself and the precipitates formed, the CCM should contain just a limited content of lime. One way is the calcination of a ferroan magnesite at temperatures above 1000 °C. However, the decrease in pH is accompanied by lower efficiency, attributed to the solid-phase reactions of free lime. A different way is the calcination of magnesite under the conditions when CaCO3 is not thermally decomposed. The virtually complete removal of the heavy metals from the model solution was achieved using the CCM characterised by the fraction of carbonates decomposed of approximately 80% and with the highest specific surface area. CCM calcined at higher temperatures could also be used, but this would be associated with higher consumption of crude magnesite. Under the conditions considered in the present work, the product obtained by the calcination at 750 °C for 3 h appeared to be optimal. The full heavy metal removal was observed in this case using less magnesite, and, moreover, at a lower temperature (resulting, therefore, in a lower consumption of energy for the calcination and material handling).
Highlights
Groundwater is the main source of drinking water worldwide [1,2]
The present work reports on how the tailor-made calcined magnesia (CCM) for use in the permeable reactive barriers (PRBs) can be prepared by the thermal activation of the natural magnesite
It was found out that the CCM with the required reactivity for the removal of the Cu2+, Zn2+, Ni2+, and Mn2+ cations from a model mixed aqueous solution can be obtained by a suitable adjustment of the calcination temperature and time
Summary
There are numerous sites all over the world characterised by groundwater contaminated with organic and/or inorganic compounds, including toxic metals. Groundwater contamination with heavy metals as a consequence of mining, refining, and industrial operations, such as galvanic processing [3], scrubbing of the flue gases in power plants [4], leather tanning [5], improper waste disposal, or accidents in the production of chemicals, represents an important challenge for the authorities responsible for the environmental protection [6,7]. Some heavy metals, such as copper (Cu), manganese (Mn), and zinc (Zn), are essential for living organisms at specific concentrations, significant amounts can result in shortness of breath, central nervous system disorders, reproductive failure, gastrointestinal problems, and several types of cancers [8,9,10,11,12]. The presence and accumulation of nickel has a toxic or carcinogenic effect on living organisms [14]
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