On the critical use of zero valent iron nanoparticles and Fenton processes for the treatment of tannery wastewater

https://doi.org/10.1016/j.jwpe.2018.01.011Get rights and content

Highlights

  • Tannery wastewater nZVI-Fenton oxidation led to significant COD and phenols removal.

  • Sludge production and residual Cr(VI) was evaluated in function of initial [Fe]tot.

  • nZVI and Fe(II) catalysts combination led to the best treatment efficiency.

  • nZVI-Fe(II) Fenton required less catalyst amount with respect to nZVI-Fenton.

Abstract

The use of iron nanoparticles in the heterogeneous Fenton oxidation treatment of tannery wastewater was investigated. A comparison with the conventional oxidation process, involving Fe(II) addition was performed. The main operating parameters influence, such as pH, temperature and reagents amount, on total phenolic species, total organic compounds, Cr(VI) content and chemical oxygen demand reduction was investigated.

Heterogeneous Fenton oxidation resulted more efficient and fast with respect to the conventional process, but a higher amount of iron was required in the process. In this case, the optimal conditions were assessed at H2O2/COD (w/w) = 0.5, nZVI/H2O2 (w/w) = 0.75 and pH = 3, achieving a total Cr(VI) removal as well as a remarkable TOC, COD and phenols degradation efficiency (up to 70, 73 and 88%, respectively). The best results were obtained with the combination of the two processes, fixing the optimal conventional Fenton operating parameters (H2O2/COD (w/w) = 0.75, Fe(II)/H2O2 (w/w) = 0.15 and pH = 2.5), adopting a nZVI/H2O2 (w/w) ratio equal to 0.2. In such conditions, that also allowed to reduce the catalyst amount used with respect to the Heterogeneous Fenton process, a noticeable TOC, COD and phenols removal efficiency (81.15, 87.44 and 94.04%, respectively) was achieved. In addition, the iron sludge production of such combined process was close to that obtained in the conventional one.

Introduction

Leather tanning industry represents one of the most present activities in the Mediterranean area and it is well known that produces huge amount of liquid waste [1]. About 1218 tanneries are sited on the Italian territory contributing for 65% of the overall EU leather production, considering that nearly one out of three skins traded between international operators is from Italy [2]. About 2110 Mm2 of leather is produced annually in the world generating approximately 40 million L of Cr-polluted wastewater every year [3], since 100–110 L of water are necessary to tan 1 m2 of hides [2]. Chromium tanning process remains still the widest employed process with respect to vegetable one [4], because of better leather quality and stability, besides the less time required from the former tanning process [5]. The high concentrations of low-biodegradable compounds in tannery wastewater represents a serious technological and environmental challenge for the operators, because chrome tanning process, hide pre-treatment and post-treatment require the use of several chemicals, such as chromium salts, bicarbonate salts, inorganic and organic acids and surfactants [6]. Tannery wastewater (TW) is characterized by high values of chemical oxygen demand (COD 3–10 g L−1), biological oxygen demand (BOD 0.5–4 g L−1), total dissolved solids (TDS of 5–15 g L−1), total suspended solids (TSS of 500–300 mg L−1), Cr(VI) (5–20 mg L−1) and Cr(III) (50–300 mg L−1) content, and phenolic compounds concentration (100–500 mg L−1), with strong odor and dark brown color [[7], [8]]. The complex TW composition has led researchers to develop combined and integrated processes to remove the most toxic and stable compounds before the final treating in biological reactors [[7], [8], [9], [10], [11], [12]]. Chemical coagulation, flocculation, membrane filtration (MF) and advanced oxidation processes (AOPs) are the main solutions proposed and used [[13], [14], [15], [16], [17], [18], [19], [20]]. Membrane processes demonstrated to be noticeably effective towards chromium recovery from Tannery wastewaters [21], considering also the significant simultaneous COD and polyphenols removal efficiencies obtained in various studies [[22], [23]]. Regarding the latter processes, instead, they have raised notable interest because of their flexibility, effectiveness and relatively low-cost [[24], [25]]. AOPs consist of a wide class of methods capable of generating radical species such as OHradical dot, able to oxidize recalcitrant pollutants [26], generally through a radical mechanism characterized by large kinetic rate constant values (106–109 M−1 s−1), 106–1012 times faster than ozone use [27]. Fenton is an effective AOP for the degradation of toxic organic pollutants in wastewater [[28], [29], [30]], based on the production of highly reactive hydroxyl radical by reaction between H2O2 and Fe(II) [31]. The Fenton process main advantages are the limited inexpensive required reagents, easy to store, relatively safe to handle and readily available [32]. This process can be carried out at ambient pressure and temperature, though a relatively recent study has demonstrated that increasing temperature from 25 °C to 90 °C improves the Total Organic Carbon (TOC) removal from 28% to 80% [33]. On the contrary, the main drawbacks involve high iron sludge production, large hydrogen peroxide dosage and strongly acidic condition (pH in the range 2–3.5) [34]. To reduce sludge production, the use of iron based solid catalysts, such as magnetite (even in form of nanoparticles) or zero-valent iron nanoparticles (nZVI) have been proposed, thus conceiving a heterogeneous Fenton (HF) process [[35], [36]]. In particular, the nZVI use in wastewater treatment has already demonstrated remarkable removal efficiency towards inorganic [[37], [38]] and organic pollutants [39], ascribable to the high chemical activity induced by the peculiar physical properties of nZVI, such as the large specific surface area [40]. HF, using mesoporous activated carbon as heterogeneous catalyst, has already been tested for TW treatment by Sekaran et al. [41], whereas few studies in literature deal with the traditional Fenton oxidation in TW treatment [[42], [43]].

The aim of this study is to compare Conventional Fenton (CF) and HF oxidation processes in treating synthetic TW, investigating the influence of the main operating parameters pH, temperature, Fe(II)/H2O2 (w/w) H2O2/COD (w/w) and nZVI/H2O2 (w/w) ratios and testing the possibility to combine the use of both Fe(0) (as solid particles) and Fe(II) (as dissolved ions) as catalyst for the Fenton process development. The last set of experiments was necessary to investigate the possibility of enhancing the conventional Fenton process, without the significant increase of catalyst amount, typical of nZVI-Heterogeneous Fenton [[35], [44], [45]], and also to compare the iron sludge production with the CF and HF processes.

To the best of the Authors’ knowledge, no studies regarding synthetic TW treatment by means of nZVI-enhanced Fenton process are available in the literature and the present is the first study exploring this possibility.

Section snippets

Materials and methods

All the used reagents were of analytical grade and purchased from Sigma-Aldrich (Milan); all solutions were prepared with deionized water.

The synthetic tannery wastewater (TW) preparation was based on previous studies [[46], [47]] with some variations. Selected amounts from stock solutions of the following reagents were used: tannic acid (5 g L−1), NH4Cl (1 g L−1), K2Cr2O7 (12.26 g L−1), Cr2(SO4)3 (1 g L−1), Polyethylene glycol 300, CH2O2, NaHCO3 (1 g L−1), HCOONa (1 g L−1), C6H6O (5 g L−1). TW

Conventional Fenton test results

The results obtained as function of pH and Fe(II)/H2O2 ratio are displayed in Fig. 2, Fig. 3, in a series of 3D graphs where the fitting surfaces are highlighted.

The analysis of Fig. 2, Fig. 3 allows to draw various observations. First, the optimal H2O2/COD (w/w) value is 0.75 as the obtained results clearly show. In fact, the removal efficiencies adopting this ratio value, for each considered parameter, is always larger with respect to those obtained adopting a ratio equal to 1. This implies

Conclusions

This study reports the comparison among conventional and heterogeneous Fenton oxidation processes in the treatment of tannery wastewater. nZVI particles were produced and used as heterogeneous catalyzer in place of Fe(II) in Fenton oxidation. TOC, COD, TP and Cr(VI) removal efficiency were evaluated, as well as the reduction of 228 nm-absorbance value of UV spectra was monitored, varying the main operating parameters: H2O2/COD (w/w), Fe(II)/H2O2 (w/w), nZVI/H2O2 (w/w) and pH. Heterogeneous

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