Removal of chromium in electroplating wastewater by nanoscale zero-valent metal with synergistic effect of reduction and immobilization
Graphical abstract
Highlights
► Steel pickling waste liquor was employed to obtain nanoscale zero-valent metal. ► Effects of factors were investigated to lay foundations for in situ remediation. ► The nZVM had a removal capacity of 182 mg/g. ► Mechanism was proposed to understand transport pathways of Cr(VI) in mediums.
Introduction
Chromium is a major heavy metal contaminant, which is widely found in ambient medium. For instance, concentration of Cr(VI) in groundwater in Chennai City of India ranges from 32.50 to 200.30 μg L−1 [1]; the average content of chromium in topsoil in Zhangjiagang city of China is 57.9 mg kg−1 [2]. Cr(VI) pollution is mainly caused by excessive discharging of wastewater from metals and electroplating industries, etc., in which Cr(VI) concentrations are ranged from 42.8 to 3950.0 mg L−1 [3]. Because of its high toxicity and mobility, Cr(VI) is known for mutagenesis and carcinogenesis to humanity. Whereas Cr(III) with less toxicity was easily precipitated and adsorbed by clay minerals [4]. Additionally, the World Health Organization (WHO) regulates that maximum allowable limit of total chromium in drinking water is 0.05 mg L−1.
The traditional treatment processes of Cr(VI) include chemical reduction, physicochemical adsorption, bioremediation, etc. [3], which are mostly limited by their small scope of application and unsatisfactory removal efficiency. Meanwhile, zero-valent iron (ZVI) has been used in pollution control for its environmental benignity and reducibility, such as reducing halogenated organic compounds, remediation of soils and groundwater [5], [6], [7]. However, various environmental factors including H+, hardness and natural organic matter (NOM) may negatively affect its activity. Several studies concluded that bimetal particles could alleviate the passivation caused by negative factors during removal process [8], [9]. However, preparation cost of bimetal particles was further raised with the consumption of noble metals. Even so, micron-sized bimetal particles were not strong enough in detoxifying capacity and demanded long contact time in treating Cr(VI).
Nanoscale zero-valent iron (nZVI) is promising in pollution abatement for its high reactivity and catalytic capability [10], [11], [12], [13]. For in-situ remediation, nZVI presents better mobility and reactivity than micron-sized materials, yielding much higher efficiency. H.S. Cao and W.X. Zhang concluded that removal capacity of nZVI towards Cr(VI) in groundwater was 50–70 times greater than that of ZVI [14]. For nZVI preparation, liquid-phase reduction method is usually adopted for its simplicity and productivity [15]. However, the consumption of borohydride and iron chemical reagent [16], [17] make it unaffordable for wide application. In order to reduce the cost, as well as make good use of waste material, nanoscale zero-valent metal (nZVM) was prepared from steel pickling waste liquor and applied to treating electroplating wastewater in this work. Previously [18], it was observed that steel pickling waste liquor was highly reclaimable resource containing iron, nickel and zinc ions, whose contents were about 122 g L−1, 17 mg L−1 and 3 mg L−1, respectively. So the nZVM contained elements of Fe, Ni and Zn, whose composition was in favor of chromium removal.
Practical remediation for Cr-polluted medium is usually in-situ reduction and immobilization. Therefore, reductive reaction and adsorption process between Cr and Fe are important transport pathways. Recent researches indicated that Cr(VI) was reduced and then formed stable Cr–Fe (oxy)hydroxide [19], and the predominant removal pathway of Cr(VI) was reductive precipitation [20]. But other study suggested that Cr(VI) removal might be adsorption-dominated, and partial Cr(VI) was reduced to Cr(III), existing as Cr2O3 or Cr(OH)3 on the surface of nanoparticles [21]. In view of above arguments, it is worthy to clarify the ambiguities concerning removal mechanism. Through investigating chromium mass distribution and X-ray photoelectron spectroscopy analysis of reaction products, the removal mechanism of Cr(VI) by nZVM can be elucidated.
In this study, evaluations of impacts of environmental factors on Cr(VI) removal were described. The removal capacity of nZVM in treating electroplating wastewater was investigated, as well as the mechanism of synergistic reduction and immobilization.
Section snippets
Materials and chemicals
Potassium dichromate, sodium borohydride, polyvinylpyrrolidone(K30), diphenylcarbazide, acid potassium permanganate, ethanol, acetone, sulfuric acid and phosphoric acid (Tianjin Damao Chemical Reagent Factory, China) were all analytical reagent grade. Buffer solutions were: acetic acid–lithium salt buffer (pH = 3.00), phosphate buffer (pH = 5.00 and 7.00), boric acid–potassium chloride buffer (pH = 9.00), borax-sodium carbonate (pH = 10.95). Standard solutions (Aladdin) of Fe, Ni, Zn were used as
Nanoscale zero-valent metal characterization
Steel pickling waste liquor was composed of strong acids and metal ions (Table 1), and the contents of Fe, Ni and Zn were 121.86 g L−1, 17 mg L−1 and 3 mg L−1, respectively. From the analyses of metallic contents in nZVM, the ratios of Fe, Ni and Zn in the synthesized nZVM were 99.987%, 0.011% and 0.002%, respectively.
TEM image indicated that aged nZVM particles were spherical nanoparticles with sizes in a range of 20–40 nm and linked or aggregated together (Fig. 1a), so as to remain in
Conclusions
Cr(VI) in wastewater could be rapidly reduced and immobilized by nZVM prepared from steel pickling waste liquor, and nZVM had the potential for remediation of Cr(VI)-polluted watersheds. Experimental results indicated that removal efficiencies enhanced with increasing nZVM doses, as well as with diminishing initial Cr(VI) concentrations and pH values. Buffer substances alleviated the raise of pH values, resulting improvement of removal efficiency. In acidic condition (pH = 4.82), NOM principally
Acknowledgments
This work was supported by National Science and Technology Major Projects of Water Pollution Control and Management of China (2009ZX07011).
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