Removing contaminants from tannery wastewater by chemical precipitation using CaO and Ca(OH)2

The removal of Cr, sulfates and Chemical Oxygen Demand (COD) from industrial tannery wastewater by chemical precipitation was carried out using Calcium oxide (CaO) and Calcium hydroxide (Ca(OH)₂). Different doses of alkalis, ranging from deficiency to excess hydroxyl species over the stoichiometric necessary, were used to remove theoretically the 100% of Cr [0.3-3.2 g alkali·(g Cr³⁺)^-1]. The precipitation was carried out at room temperature, 10 min of vigorous stirring, 200 r·min^-1 and a settling time of 24 h, followed by separation and characterization of liquid product. As result of addition of alkalis, pH underwent increase as did the alkali concentration. The removal of Cr, and sulfates also increased as alkali concentration did, although for first species the changes at higher alkali contents were less evident. COD removal on the other hand, did not follow a unique trend, instead exhibited a maximum. Based on our results, selection of a specific dose of alkali was carried out taking as reference the efficiency on total chromium removal, keeping the pH in the range 7 to 9 to ensure Cr precipitation as chromium hydroxide. With those conditions, jar test was used to produce enough liquid product in order to determine the removal percent of several other species. The removal percent of the species was as follows: Cr, SO₄^2-, ZnSO₄, FeSO₄, CN^-1, NiSO₄, Fe₂[Fe(CN)₆] at 99.8%, 66.9%, 99.6%, 21.4%, 70.9%, 52.8% and 76.4% with CaO, and 99.8%, 61.6%, 99.9%, 7.1%, 84.0%, 54.4% and 90.5% with Ca(OH)₂, respectively.