Iron-based persulfate activation process for environmental decontamination in water and soil

Highlights

• Iron is an effective activator of persulfate.

• Homogeneous, heterogeneous and electrochemical activation of persulfate is effective for pollutant degradation.

• Iron activated persulfate is effective for water, wastewater and soil decontamination.

Abstract

Sulfate radical based advanced oxidation processes have been extensively studied for the degradation of environmental contaminants. Iron-based materials such as ferrous, ferric, ZVI, iron oxides, sulfides etc., and various natural iron minerals have been explored for activating persulfate to generate sulfate radicals. In this review, an overview of different iron activated persulfate systems and their application in the removal of organic pollutants and metals in water and soil are summarised. The chemistry behind the activation of persulfate by homogenous and heterogeneous iron-based materials with/without the assistance of electrochemical techniques are also discussed. Besides, the soil decontamination by iron persulfate system and a brief discussion on the ability of the persulfate system to reduce metals presence in wastewater are also summarised. Finally, future research prospects, believed to be useful for all researchers in this field, based on up to date research progress is also given.

Introduction

The increasing occurrence of organic pollutants in the environment due to industrialisation is a major concern to living organisms and poses significant risks to the ecosystem. Several methods are available for the removal of organic compounds from environmental matrices. The chemical oxidation of organic compounds using oxidants such as H₂O₂, KMnO_4, and ozone have been extensively studied both in the laboratory and field. Poor chemical stability and shorter life span of H₂O₂, inability to oxidise unsaturated chlorinated compounds by KMnO_4, and low solubility of ozone in water limit their practical applicability in organic pollutant removal (Oh et al., 2009; Wei et al., 2016). Water and wastewater treatment using advanced oxidation processes (AOPs) have received considerable attention in the last few decades as these processes can mineralise persistent organic pollutants present in the aqueous medium. Photo-catalysis, Fenton process, ozonation, electro-Fenton, photo-electro-Fenton, anodic oxidation, peroxicoagulation, sonochemical method, sono-Fenton, sono-photo, sono-photo-Fenton, and sono-electro-Fenton have been investigated for the degradation of various organic pollutants (Divyapriya et al., 2018; Patidar and Chandra Srivastava, 2020; Ren et al., 2018; Sharma et al., 2019; Zhang et al., 2020a, 2020b, 2020c). Hydroxyl radical is the second powerful oxidant after fluorine which can mineralise organic pollutants to CO₂, H₂O, and other inorganic minerals via hydroxylation, redox reaction, and ipso-substitution reactions (Brillas et al., 2009; Mousset et al., 2018; Nidheesh et al., 2018).

Sulfate radical (SO₄^−•) based AOPs are also found as an effective treatment method for degrading organic pollutants present in the water medium (Sarath et al., 2016; Zhu et al., 2020). Peroximonosulfate and peroxidisulfate (persulfate) are the two main precursors used for generating sulfate radicals in the water medium (Wang et al., 2020a, 2020b). They are commonly used as in-situ chemical oxidants, which can be activated thermally or chemically to produce powerful sulfate radicals (Liang and Guo, 2010). Even though the oxidation potential is slightly lower than that of ^•OH, superior performance of SO₄^−• over ^•OH has been reported (Nidheesh and Rajan, 2016). High solubility and stability in aqueous solution in addition to the non-selective reactive nature of SO₄^−• make it suitable for organic pollutant degradation (Hussain et al., 2012; Nachiappan and Gopinath, 2015). Longer lifetime of sulfate radicals (30–40 μs) compared with hydroxyl radicals (20 ns) is another advantage of sulfate radicals over hydroxyl radicals to degrade organic pollutants (Wang et al., 2017). Apart from water and wastewater treatment, sulfate radicals are effective for the decontamination of soil and heavy metal removal from aqueous solution (Ma et al., 2018; Qu et al., 2020).

Activation of persulfate (PS) or peroximonosulfate (PMS) is essential for the effective generation of SO₄^−• in the water medium. Heat, alkali medium, metal ions, ultrasound, UV radiation, microwave radiation, electrochemical method, activated carbon, and hydrogen peroxide are found to be useful sources that can generate ^•OH by activating PS or PMS (Chen et al., 2019; Hu et al., 2020; Huang et al., 2019a, 2019b; Pan et al., 2019; Wang et al., 2020a, 2020b). Among the different activation techniques, thermal activations systems are practically undesirable due to higher energy consumption, microwave radiation due to higher cost, higher chemical requirement for hydrogen peroxide etc. (Nie et al., 2015; Rodriguez et al., 2017). In this regard, metal activation of PS for the degradation of environmental contaminants has high capability and adaptability. PS can be activated using metal ions such as Cu, Fe, Al, Fe, etc. Iron is found as an effective, less expensive, and most widely used activator of PS (Guerra-Rodríguez et al., 2018; Kaur et al., 2019). It is the second abundant metal as well as the fifth abundant element on earth. Compared to other heavy metals, the toxicity of iron is negligible and can be removed from water medium by simple aeration or deposition. Thus, iron is a promising activator for PS in water and wastewater treatment. The high reduction potential of Fe²⁺ results in faster reactions with PS to form SO₄^−• and further increases the oxidation of organic compounds (Liu et al., 2012). Similar to soluble Fe²⁺ and Fe³⁺ species, insoluble forms of iron like magnetite, zero-valent iron (ZVI), and hematite are also found as effective catalysts for PS decomposition (Xiao et al., 2020). Furthermore, iron activated PS in the subsurface conditions has the benefits of treating contaminants without injection of stronger oxidants, reducing the treatment time, cost-effectiveness, and carrying out of remediation without disturbing the above-ground structures (Lu et al., 2013; Yan and Lo, 2013). Sequential addition or gradual release of Fe²⁺ as in the case of ZVI into the system can minimise the non-desired termination of the process and enhance its longevity (Rastogi et al., 2009; Romero et al., 2010). However, metal leaching, pH-dependence, and the subsequent higher concentration of Fe²⁺ ions hinder the interactions with organic pollutants (Hussain et al., 2012).

Comprehensive reviews are already reported on Fe based homogeneous and heterogeneous for activation of PS or PMS to generate SO₄^−• (Gao et al., 2020a, 2020b; Xiao et al., 2020). However, limited studies focused on Fe based homogeneous, heterogeneous and electrochemical PS activation processes for organic pollutant and metal removal. Given these importance, the manuscript is prepared to provide a cohesive understanding of the state of art in the research field of Fe based PS systems. This review article describes the possibilities of PS activation by using iron in water and soil. Initially, PS properties and the chemistry behind its activation mechanism were highlighted. Further, the homogenous and heterogeneous activation of PS using different Fe sources and some significant factors influencing the degradation of compounds were discussed. Following this, special attention was paid on the power of Fe-mediated electrochemical activation of the PS system. Further, a brief discussion on soil remediation for organic pollutants by PS oxidation was given to provide a broad indication of SO₄^−• radical-based decontamination technology. Finally, the review turns to a discussion of the efficiency of the Fe-PS system for metal removal which makes the article distinct from current review articles where this area is seldom considered. The article ends with a quick discussion on future research prospects.