The enhanced removal of highly toxic Cr(VI) by the synergy of uniform fiber ball loaded with Fe(OH)3 and oxalate acid
Graphical abstract
Introduction
With the development of industry, a large amount of chromium-containing wastewater was discharged into the environment, which caused serious chromium pollution in water body (Barrera-Diaz et al., 2012; Zhang et al., 2018). Chromium in water is mainly present in the form of Cr(VI) and Cr(III) (Li et al., 2019; Zhang et al., 2014). Cr(VI) is a highly toxic substance with carcinogenic and teratogenic properties, and its accumulation in the food chain will pose a great threat to human health (Xu et al., 2019; Zhang et al., 2011). Therefore, the removal of Cr(VI) is the key to control chromium pollution. The most commonly used chemical reduction method plays a very important role in the detoxification of Cr(VI) from water. But this method usually requires a large amount of reducing agent (such as SO2, NaHSO3 and FeSO4), which not only increases the cost but also generates large volume of sludge and causes secondary pollution (Dhal et al., 2013).
Oxalate acid (Ox), mainly from the decomposition of organic matter and plant root exudates, seems to be an ideal reducing agent for detoxifying Cr(VI): Ox is one of the most widely natural existed organic acids. Ox shows reductive ability, the product is non-toxic and harmless carbon dioxide (Wei et al., 2014). However, the rate of direct reduction of Cr (VI) by Ox is relatively slow. Under the excitation of UV, the Cr(VI) reduction by Ox was accelerated to a certain extent (Wang et al., 2008). But this did not achieve the purpose of rapidly removal of Cr(VI). Although the presence of Fe(III) significantly promoted the reduction of Cr(VI) by Ox in homogeneous systems (Hug et al., 1997; Wang et al., 2008), the used excess Fe(III) will result in a waste of iron resources and not conducive to recycling. Recently, zero-valent iron (ZVI) as a source of Fe(III) had been widely used to enhance the removal of Cr(VI) by Ox (Fan et al., 2020; Fang et al., 2020; Fu et al., 2013; Hu et al., 2020; Wei et al., 2013), but ZVI was unstable and easy to be oxidized and deactivated.
If Fe(III) was supported on a substrate to prepare a stable composite material, it could not only achieve the purpose of enhancing photoreduction of Cr(VI) by Ox through quantitative dissolution of Fe(III) from the composite, but also avoid the waste of iron resources. Meanwhile, through multiple use of the composite in the Cr(VI) photoreduction by Ox, it could further reduce the cost and save the iron resources. The key is to find a reliable, economically and suitable substrate.
Uniform fiber ball (UFB) had been successfully prepared from waste paper in our previous research (Zhang et al., 2020). UFB has the advantages of simple preparation, low cost, convenience in recycling, and easiness in modification.
In this paper, in the view of green and efficient removal of chromium pollution, using UFB as the substrate, UFB-Fe(OH)3 was firstly synthesized and successfully used in the system of Cr(VI) photoreduction by Ox. The effect of factors (initial pH of the solution, concentration of Ox, initial Cr(VI) concentration, dosage of UFB-Fe(OH)3 and the reusability of UFB-Fe(OH)3) in UFB-Fe(OH)3 + Ox + UV system were investigated. Finally, by studying the impact of the initial pH on the Cr(VI) photoreduction, the relationship between the change in pH and the Cr(VI) removal during the reaction, the effect of NO3−, and the free radicals quenching tests in the system, the Cr(VI) removal mechanism in UFB-Fe(OH)3 + Ox + UV was further discussed.
Section snippets
Materials
Waste papers were collected from abandoned papers. Sodium chloroacetate (ClCH2COONa), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), ammonium hydroxide (NH3·H2O), hydrochloric acid (HCl), sulfuric acid (H2SO4), sodium hydroxide (NaOH), potassium dichromate (K2Cr2O7), naphthoquinone (NQ), isopropanol (IPA) and diphenylcarbazide were obtained from Sinopharm Chemical Reagent Co., Ltd.
Synthesis of UFB-Fe(OH)3
1 g UFB (prepared on the basis of our previous studies (Zhang et al., 2020) from waste papers) was put into 20 mL
UFB-Fe(OH)3 prepared at different Fe(NO3)3 concentrations
The appearance of the prepared UFB-Fe(OH)3(UFB–Fe(OH)3-1 to UFB-Fe(OH)3-6) at different Fe(NO3)3 concentrations were shown in Fig. 1a, and the performance of UFB-Fe(OH)3 on enhancing photoreduction of Cr(VI) by Ox was shown in Fig. 1b ([Cr(VI)]0 = 1.5 and 2.0 mM, [Ox]0 = 5 mM, pH = 3, dosage of UFB-Fe(OH)3 = 1.0 g L−1).
Under dark, the Cr(VI) removal in UFB, UFB-Fe(OH)3, Ox, and UFB-Fe(OH)3 + Ox system, were compared: the removal efficiency of Cr(VI) by UFB and UFB-Fe(OH)3 were 3.0% and 4.3%,
Conclusion
In this paper, the synthesized UFB-Fe(OH)3, as an efficient and low-cost iron source, could not only greatly improve the photoreduction of Cr(VI) by Ox, but also save the iron resources, which followed the concept of “waste control by waste”. The possible mechanism of Cr(VI) removal in UFB-Fe(OH)3 + Ox + UV system consisted of three parts, and the mainly part was attributed to the production of large amount of CO2·- and Fe(II) from photoactive Fe(III)Ox species formed by the dissolved Fe(III)
Credit Author Statement
Author contributions: Weiya Niu: Investigation, Data Curation, Writing - Original Draft, Formal analysis. Jie Sun: Investigation, Data Curation. Ling Zhang: Conceptualization, Resources, Writing - Review & Editing, Supervision. Fengming Cao: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors would like to thank anonymous reviewers and questionnaire survey exercises.
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