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

https://doi.org/10.1016/j.cjche.2019.12.023Get rights and content

Abstract

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, SO42−, 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.

Introduction

The increasing generation of wastewater by tanneries demands urgent actions to remediate the contaminated effluents from those industries [1]. Tannery process to produce leather involves several stages, including addition of alkalis and acids, each one of those contributing with different pollutants in residuals. Thus, the resulting wastewater mixture to be treated is very complex in terms of composition and interactions among those species [2]. Cr is the main studied contaminant in tannery wastewater remediation and in recent years, attention has been paid in removing other substances of tannery wastewater in addition to Cr, for example the nitrogen, phosphorous, chloride ions, sulfur compounds, dye and metallic ions [3]. In this regard, several treatment processes have been employed; among them absorption by inorganic solids, biological treatments, and removal by methods based on electrochemical are widely studied [[4], [5], [6]]. Nowadays, there is not a single technology capable of removing simultaneously the main contaminants of tannery wastewater at desired levels. However, combinationatorial methods are often applied to satisfy the environmental regulations, and to attain the specifications to reintegrate those treated streams in the tanning process, mainly Cr as liquor [7].

Chemical precipitation is an alternative to remove Cr3+ as coming from effluents, using an alkaline metal oxide for such a purpose; also, it is feasible to promote other chemical reactions in the complex tannery wastewater by using alkalis [8]. The following chemical reaction, using Ca(OH)2, involves not only removal of Cr but also sulfates [9]:CrSO43+3CaOH22CrOH3+3CaSO4

It is reported that tannery wastewater contains, among others, the following ions: Cr3+, SO42−, PO43−, Fe, Cl, As y NH3-N [4,5,8], due to the addition of trace organic chemicals and the increasing use of synthetic chemicals such as pesticides, dyes and finishing agents [10]. Some of those metallic ions can be removed at proper conditions as metal hydroxides, as reported in previous contributions [11,12].

The following are the chemical reactions that can occur with the different ions contained in tannery wastewater in basic media, by using calcium hydroxide [13]:NiSO4+CaOH2NiOH2+CaSO4ZnSO4+CaOH2ZnOH2+CaSO4FeSO4+CaOH2FeOH2+CaSO4

In similar manner, (1), (2), (3), (4) occur in presence of CaO and in that case, an additional step is necessary, which is:CaO+H2OCa0H2

In the literature, it is common to find studies on the utilization of CaO and Ca(OH)2 to remove Cr in wastewater, and the excess alkali concentrations promote the reactions [[14], [15], [16], [17]]. However, utilization of a range of concentrations of alkali is not widely explored and understood for the same purpose [18]. Although there are evidences for the removal of other chemical species by addition of alkalis in tannery wastewaters, still the discussions on the efficiencies in this regard are lacking in the scientific literature.

In the present contribution, we performed a small scale experiment utilizing CaO and Ca(OH)2 to study the removal of Cr3+, SO42− and Chemical Oxygen Demand (COD) in wastewater coming from industrial tannery by using a range of doses of the alkalis, covering from deficiency to excess of calcium reagent for reaction of Eq. (1). Once almost the complete removal of Cr was attained ensuring its precipitation as amorphous solid by controlling pH, jar test was carried out with enough sample to determine the conversion of other contaminants. Furthermore, we discussed about implications of such chemical reactions in the wastewater remediation by chemical precipitation.

Section snippets

Reagents

Calcium oxide (98.6 wt%, J.T. Baker) and calcium hydroxide (98.6 wt%, J.T. Baker) were used as reagents for Cr removal. Also, NH4Cl (36 wt%, J.T. Baker), HCl (99.9 wt%, J.T. Baker) and deionized water (resistivity of 0.8 M Ω at 23 °C) were employed. For sulfates determination, distillated water and BaCl2 (99 wt%, J.T. Baker) were used. COD determination was carried out using professional tube test; also concentrated sulfuric acid (96.9%, Fermont) was required for such a purpose.

Removal procedure at small scale

150 ml of

Results and Discussion

Apparent characteristics of sample wastewater were dark blue color, with a pungent smell and suspended particles. Prior chemical removal of contaminant species, the liquid was filtered to retain fine particles. The characterization results of sample wastewater, obtained after chemical treatment, are shown in Table 1. According to Mexican legislation, Cr, Na, Fe, and sulfate values exceeded largely the limits suggesting discharges and arises questions over potable water quality, so that some

Conclusions

Treatment of wastewater coming from a Mexican tannery was carried out using different concentrations of CaO and Ca(OH)2 at small scale (150 ml), 10 min, room temperature and 200 r⋅min−1, and some differences were observed between the doses of both alkalis in the removal of Cr, sulfates and COD, mainly at lower pH values. It was shown that increasing alkali dose, pH also was increased, provoking better Cr removal and sulfates, but COD presented a maximum after that a diminution in removal

Acknowledgements

A. Reyes and J.E. López acknowledge CONACyT-México by MSc Fellowships numbers 483224 and 495008, respectively.

Authors thank financial support from IPN-México through grants 20170292, 20180385, and 20195583. Also, the partial financial support from CONACyT-México through grant 246176 is acknowledged.

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