Removal of chromium from Cr(VI) polluted wastewaters by reduction with scrap iron and subsequent precipitation of resulted cations
Highlights
► We examine total removal of chromium by Cr(VI) reduction with scrap iron and subsequent precipitation of resulted cations. ► The process was detrimentally affected by a compactly passivation film occurred at scrap iron surface. ► 96.8% from the initial scrap iron still remained unreacted in the column at Cr(VI) breakthrough. ► The optimum precipitation pH range which resulted from this study is 7.6–8.0.
Introduction
Chromium compounds are used in a wide variety of industrial processes such as: metallurgy, chemical and refractory industries, textile dying, tanneries, metal electroplating, wood preserving, and preparation of chromate compounds. Therefore, chromium contamination has been often reported in many industrial sites, due to accidental leakages or improper disposals measures [1], [2], [3], [4]. In aquatic environments chromium is present mainly as hexavalent and trivalent species, characterized by markedly different chemical behavior and toxicity [5]. While Cr(VI) exists mainly as highly soluble oxyanions [6], Cr(III) is less soluble and readily precipitates as Cr(OH)3 [7]. Cr(III) has a low toxicity, being considered an essential nutrient for many organisms [8]. In contrast, Cr(VI) is up to 1000-fold more toxic than Cr(III) [9] and a well-established carcinogen by the inhalation route of exposure [5]. Therefore, Cr(VI) must be removed from wastewaters before their disposal to natural aquatic environments.
During last two decades there has been important interest in finding new materials with high removal efficiency or/and low cost, for the removal of Cr(VI) from contaminated waters [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Reduction to Cr(III) may be considered a satisfactory solution in eliminating the toxicity of Cr(VI). Scrap iron is a cheap waste material that has been successfully tested for the removal of Cr(VI) via reduction to Cr(III) according to [24]:2Cr2O72−(aq) + 6Fe0(s) + 28H+(aq) → 4Cr3+(aq) + 6Fe2+(aq) + 14H2OSubsequently, Cr(VI) may be reduced in the solution (homogeneously) by Fe(II):Cr2O72−(aq) + 6Fe2+(aq) + 14H+(aq) → 2Cr3+(aq) + 6Fe3+(aq) + 7H2OThe two equations can be added together to yield the net reaction for the reduction process:Cr2O72−(aq) + 2Fe0(s) + 14H+(aq) → 2Cr3+(aq) + 2Fe3+(aq) + 7H2OGould [25] reported that 1.33 mol of Fe(0) dissolved for each mol of Cr(VI) reduced. Such a high efficiency suggested that hydrogen generated during iron corrosion acts as a reducing agent for the Cr(VI) (see Eq. (4)). Recent theoretical analysis by Noubactep [26], [27] supports this view. In fact, contaminants are demonstrated to be removed by adsorption and co-precipitation, while contaminant reduction, when occurs, mainly results from indirect reducing agents (Fe(II) and H/H2). In other words, Fe0 should be regarded as generator of reducing agents [26].
Wastewater treatment systems based only on Cr(VI) reduction at pH < 6.0 cannot remove chromium from the aqueous phase because resulted Cr(III) is still soluble [28]. Since the efficiency of Cr(VI) reduction with Fe(0) is very low under circumneutral conditions, the process must be conducted at acidic pH values (2.5–3.0) [29], [30]. Therefore, most of the resulted species (Cr(III), Fe(II), and Fe(III)) will remain dissolved. All these species must be removed from the wastewater in a final step, in order to complete the treatment process.
To the best of our knowledge, no continuous-flow studies concerning both Cr(VI) reduction and removal of resulted chromium and iron species have been reported. As a continuation of our previous work [30], [31], [32], the present study describes the treatment of Cr(VI) polluted wastewater in continuous system, by reduction with scrap iron and subsequent precipitation of the resulted cations. This work will present data regarding the mechanism of Cr(VI) reduction inside the column, and of Cr(total) and Fe(total) removal inside the clarifier. Additionally, the optimum pH for the precipitation of cationic species resulted from the reduction process will also be established.
Section snippets
Scrap iron
Scrap iron spirals (5 mm < spiral diameter < 10 mm; 5 mm < spiral length < 20 mm) used in this study originated from “SPM” metals processing laboratory, at the “Politehnica” University of Timisoara. The scrap iron was washed several times with warm distilled water to assure the complete removal of all impurities, and air dried.
Background electrolyte
The used background electrolyte was made up of: 50 ppm Ca2+; 20 ppm Mg2+; 128 ppm Cl−; 104 ppm Na+; and 293 ppm HCO3−. The mixture was chosen to maintain a constant ionic strength.
Experimental procedure
A
Continuous reduction of Cr(VI)
Fig. 2 summarizes the results of Cr(VI) breakthrough. It is shown that, during the first 48 h, Cr(VI) concentration in the column pore water decreased from the input value to below the detection limit at the front as the Cr(VI) front passes through the column. High concentrations of Cr(VI) were present in the pore water of P1 from the 6th hour onward. This observation is attributed to two facts: (1) limited extent of Cr(VI) reduction by limited mass of scrap iron available below P1, and (2) low
Conclusions
Long-term column experiment performed in this work confirmed the possibility of Cr(VI) conversion to Cr(III) by using scrap iron. Although 96.8% from the initial metallic scrap iron still remained unreacted in the column, Cr(VI) breakthrough occurred after 48 h, due to a compactly Cr–Fe composed passivation film formed on the scrap iron. Cr(VI) breakthrough seems to may be predicted by the maximum peak of Fe(III) and Cr(III) concentrations in column effluent. The precipitation of Cr–Fe secondary
Acknowledgments
This research was conducted under CNCSIS-UEFISCDI PN II IDEI Exploratory Research Project No. 647/19.01.2009 “Innovative technologies for the removal of hexavalent chromium from wastewaters by reuse of scrap iron”, CNCSIS code 1031/2008. The manuscript was improved by the insightful comments of two anonymous reviewers.
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