Formation and transformation of schwertmannite in the classic Fenton process

Abstract

The massive amount of sludge generated by the classic Fenton process, which has often been hypothesized to consist of ferric hydroxide, remains a major obstacle to its large-scale application. Therefore, reutilization of Fenton sludge has recently gained more attention. Understanding the formation, transformation, and properties of Fenton sludge combined with the stages of the Fenton reaction is pivotal, but not well illustrated yet. In this study, SEM-EDS, FT-IR, XRD, and XPS were applied to study the morphology, crystallinity, elemental composition, and valence state of Fenton sludge. The authors report that schwertmannite and 2-line ferrihydrite were generated and transformed in the oxidation phase and the neutralization phase of the Fenton process. SO₄²⁻ in the solution decreased by 8.7%–26.0% at different molar ratios of Fe(II) to H₂O₂; meanwhile, iron ion precipitated completely at pH 3.70 with the formation of schwertmannite containing sulfate groups in the Fenton sludge. The structural sulfate (Fe-SO₄) in schwertmannite was released from the precipitate with the addition of OH⁻, and the production of Fenton sludge decreased with increasing pH when pH > 3.70. Goethite was found to form when the final pH was adjusted to 12 or at a reaction temperature of 80°C. Moreover, the possible thermal transformation to goethite and hematite indicated that Fenton sludge can be reused as a raw material for synthesizing more stable iron (hydro)oxides. The results provide useful insights into the formation and transformation of Fenton sludge, with implications for regulating the crystal type of Fenton sludge for further reuse.

Introduction

With the improvement of emission standards, the Fenton process has been widely applied in advanced treatment of industrial wastewater in China. The Fenton process, with a load of 600,000 tons per day, has been put into operation and has generated 0.4 kg/m³ Fenton sludge over a period of several years (These data were collected from a sewage treatment plant in Zhejiang Province, China and is not yet published.). Practical application of the Fenton process in wastewater treatment is mainly limited by the massive iron sludge production in the process (Babuponnusami and Muthukumar, 2014, Bautista et al., 2008, Neyens and Baeyens, 2003). Fenton sludge is characterized by low heating value and generated from industrial wastewater, which means it cannot be disposed by conventional incineration and landfilling. Further treatment is required to prevent secondary pollution by Fenton sludge, which makes the process uneconomical.

Hence, reutilization of Fenton sludge has been gaining the interest of researchers. Fenton sludge normally contains iron hydroxide (Cao et al., 2009, Gamaralalage et al., 2018, Kattel et al., 2016), which has been widely studied as an adsorbent for the removal of organics, phosphate, arsenic, chromium, and lead from water and hydrogen sulfide from biogas (Banerjee et al., 2008, Gypser et al., 2018, Kartashevsky et al., 2015, Magnone et al., 2018, Mahmood et al., 2014, Mezenner and Bensmaili, 2009, Xiong et al., 2017, Zhang et al., 2010, Zhao et al., 2018). It has been reported that the surface morphology and crystallinity of adsorbents influences the adsorption performance a great deal (Gypser et al., 2018, Kartashevsky et al., 2015, Zhao et al., 2018). Considering the massive amounts generated, reutilizing Fenton sludge in the construction material field could be a more effective disposal method. The presence of a high content of iron minerals in Fenton sludge gives it the possibility to be used as a raw material in cement production (Amin et al., 2018, Rezaee et al., 2019, Świerczek et al., 2018). However, it was reported that the use of sulfate-contaminated aggregates in concrete increased the concrete carbonation rate and depth, which is quite harmful for the durability of concrete (Elmoaty, 2018).The effects of internal attack by sulfate on the mechanical properties and durability of concretes have been well studied (Abid et al., 2018, Campos et al., 2018, Debieb et al., 2010). Hence, reutilization methods are strongly related to the composition and crystallinity of Fenton sludge.

Before disposal can take place, understanding the properties and formation process of Fenton sludge is of great importance. Precipitates of Fe(OH)₃ have often been hypothesized to comprise the sole content in Fenton sludge. Moreover, the footprint of sulfate ions has never been considered, which are introduced into the Fenton process by pH regulation and the addition of ferrous sulfate (Cao et al., 2009, Gamaralalage et al., 2018, Kattel et al., 2016).

Recently, schwertmannite has drawn increasing attention due to its potential application in the fields of adsorption and catalysis (Li et al., 2018, Liao et al., 2011). It is a poorly crystallized ferric oxyhydroxysulfate mineral with the general formula Fe₈O₈(OH)_x(SO₄)_y·nH₂O, where x = 8 − 2y and 0.52 ≤ y ≤ 2.12 (Bigham et al., 1990, Bigham et al., 1990, Caraballo et al., 2013). Schwertmannite is widely present in acidic (2.5–5.5) high-sulfate (1000–3000 mg/L) and iron-rich waters (Bigham et al., 1996, Fitzpatrick et al., 2017, Regenspurg et al., 2004). Acidithiobacillus ferrooxidans is considered to play an important role in schwertmannite biotic formation. Schwertmannite formation in geochemical environments is very common and well-studied (Bigham et al., 1990, Burton et al., 2006, Caraballo et al., 2013, Collins et al., 2010, Fitzpatrick et al., 2017). However, the occurrence of schwertmannite in wastewater treatment processes has been rarely reported. Recently, it was reported for the first time that biogenic schwertmannite occurred as the sole secondary Fe mineral in a dissolved organic matter-rich tannery sludge bio-leaching system (Liao et al., 2009).

The classic Fenton process should be divided into two phases: oxidation and neutralization. The Fenton reaction conditions in the oxidation phase are characterized by low pH and high content of Fe and sulfate ion, which are similar to the formation conditions of schwertmannite. To the best of our knowledge, few studies have reported on the formation and transformation of secondary Fe minerals during the Fenton reaction process. However, the variation of pH during the neutralization phase could also influence the properties of Fenton sludge.

Herein, this study aimed to investigate the formation, transformation, and properties of Fenton sludge along the stages of the Fenton reaction. The Fenton sludge was collected from a simulated Fenton reaction for practical wastewater and a wastewater treatment plant. Then the variation of sulfate group content was determined in the ionic state in the solution and structural state in the sludge during the Fenton process to identify the generation of schwertmannite. Moreover, the influence of solution pH, temperature, and excess alkali on the generation of schwertmannite and transformation to 2-line ferrihydrite, goethite, and hematite was investigated. These results can help researchers to thoroughly understand the Fenton reaction process and the generation of Fenton sludge. The study also provides a new pathway for the reduction and stabilization of Fenton sludge.