English

• 1. Introduction

  The massive discharge of wastewater from industrial and domestic sources has made water pollution an urgent environmental challenge that we face [1-3]. Among the various issues associated with water pollution, the presence of antibiotics, halogenated hydrocarbons, heavy metals, and bromate is particularly concerning [4-7]. Prolonged exposure to these wastewaters may compromise the human immune system, thereby elevating the risk of developing chronic illnesses and cancer [8-12]. Moreover, the accumulation and enrichment of pollutants in nature have led to a series of ecological issues [13-19]. The urgent need to address these multifaceted and diverse water pollution challenges necessitates the development of effective and sustainable water treatment technologies to safeguard the safety and sustainable utilization of water resources.

  Low-valent sulfur oxy-acid salts (LVSOs), such as sodium sulfite and sodium thiosulfate, are defined as sulfur oxides characterized by sulfur valence state lower than +6. Due to the absence of sulfur in its highest valence state, these compounds typically exhibit strong reducing properties. Common examples of stable reducing LVSOs include sulfite, dithionite, and thiosulfate (Table S1 in Supporting information) [20-23]. Basic research has validated that wastewater treatment processes associated with LVSOs demonstrate exceptional treatment efficiency [24-27]. To further explore the potential applications of LVSOs in wastewater treatment, we conducted a comprehensive literature search in the science citation index and social science citation index databases for research articles on sulfite, dithionite, and thiosulfate published in the past five years, and utilized a Venn diagram for visual analysis of the search results. As shown in Fig. 1, recent research hotspots have mainly focused on sulfites and thiosulfates, while the treatment processes related to LVSOs are advanced oxidation processes (AOPs) and advanced reduction process (ARPs). This research trend not only reveals the immense potential of LVSOs in the field of wastewater treatment but also indicates their broad developmental prospects.

  Figure 1

  

  Figure 1.  Current status of research on LVSOs in the last five years.

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  Traditional AOPs primarily rely on strong oxidants such as hydrogen peroxide and persulfates, generating free radicals through various activation methods to achieve effective removal of pollutants [28-35]. Although LVSOs possess strong reducing properties, they can also participate in the formation of advanced oxidation systems, and avoiding the disadvantages of secondary pollution and excessive costs inherent in traditional AOPs [36-39]. Notably, sulfite can generate sulfate radicals (SO₄^•−) and hydroxyl radicals (^•OH) under relatively mild conditions, and its environmentally friendly characteristics render it a promising alternative to persulfate [40-42]. Moreover, thiosulfate and dithionite are gaining attention in the realm of AOPs, with evidence supporting their role in enhancing the oxidation capacity of such systems [43-46]. The excess of LVSOs can be effectively managed through aeration, which converts them into stable and low-toxicity sulfate, thereby mitigating the risk of secondary pollution associated with residual reagents [40,42,47]. This treatment method does not generate more toxic bromates or higher-valent heavy metal ions when applied to wastewater containing bromine or heavy metals. In light of the limitations of current technological systems, LVSOs are anticipated to offer solutions in several key area: (1) Serving as an environmentally benign alternative to traditional sources of reactive oxygen species (ROS) to minimize secondary pollution; (2) acting as a catalyst to expedite the redox cycle and decrease dependence on metal ions; (3) functioning as an effective activator to improve the economic efficiency of the treatment process.

  Furthermore, sulfite, thiosulfate, and dithionite demonstrate considerable ultraviolet absorption characteristics [48-53]. When subjected to ultraviolet (UV) irradiation, these compounds can be activated to generate a range of potent reducing agents including sulfite radicals (SO₃^•−), hydrated electrons (e_aq⁻), and hydrogen radicals (^•H) [49,51,54,55]. The interaction of these LVSOs with UV light facilitates an ARPs that holds significant potential for the degradation pollutants that are susceptible to reduction treatment, such as heavy metals and bromates [4,38,56-59]. For example, the UV/sulfite process method has been demonstrated to effectively reduce heavy metal ions and decrease the formation of bromate by-products during disinfection processes [38,60-62]. Notably, the e_aq⁻ generated by this system possesses the capability to cleave the robust carbon-fluorine (C-F) bonds found in per- and polyfluoroalkyl substances, thereby facilitating the degradation of perfluorinated compounds [8,63-65]. Additionally, the incorporation of iodides and photocatalytic materials can further augment the reduction capacity of the system.

  The application of LVSOs to augment conventional wastewater treatment technologies presents significant potential for pollutant removal through tailored activation strategies aimed at either oxidation or reduction processes. Nonetheless, a comprehensive review of AOPs/ARPs that incorporate LVSOs has yet to be conducted. To address this gap in the literature, the present study systematically examines recent advancements in the utilization of LVSOs in AOPs/ARPs for the remediation of water contaminants. This review emphasizes the various activation methods and mechanisms associated with LVSOs, as well as the critical factors influencing their removal efficiency. Furthermore, the paper highlights existing knowledge deficiencies and proposes avenues for future endeavors.

  2. Overview of recent progresses in LVSOs-based AOPs

  2.1 LVSOs as a source of ROS in AOPs systems

  The reaction mechanism of ROS production by the three LVSOs as shown in Fig. 2. Sulfite, as a widely studied source of ROS in AOPs, is involved in the most extensive range of research fields. Current findings indicate that metal ions such as iron (Fe), cobalt (Co), copper (Cu), manganese (Mn), and chromium (Cr) can effectively activate sulfite, leading to the generation of strong oxidizing free radicals [66-73]. Particularly, high-valent metal ions, including Fe(Ⅲ) and Cu(Ⅱ), form complexes with sulfite and facilitate the transfer of electrons from SO₃²⁻ due to their inherent oxidizing properties, resulting in the formation of SO₃^•−, which possesses an unpaired electron (Eqs. S1-S7 in Table S2 in Supporting information) [71,74-76]. Conversely, weak-oxidizing metal ions such as Co(Ⅱ), Fe(Ⅱ), and Mn(Ⅱ) necessitate the presence of O₂ as an electron acceptor during their interaction with SO₃²⁻ (Eqs. S8-S15 in Table S2) [41,70,77]. In this reaction mechanism, the changes in the valence states of the metal ions are not directly linked to the free radical production chain reaction; rather, their primary function is to form more reactive complexes with SO₃²⁻. Recently, leveraging the interaction characteristics of the reaction between high-valent metal ions and S(Ⅳ), researchers have innovated a technology that employs the coupling of high-valent chromium in wastewater with sulfite for treatment purposes (Eqs. S16-S18 in Table S2) [72]. This approach not only facilitates the removal of pollutants but also concurrently reduces high-valent heavy metals, thereby promoting resource recycling and the effective waste management [72].

  Figure 2

  

  Figure 2.  The mechanism of free radical generation by LVSOs as ROS sources (M = Fe, Cu, Mn, Co).

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  The activation of sulfite systems by transition metal ions has been shown to facilitate the rapid degradation of pollutants. However, the potential toxicity associated with metal ions, coupled with the low efficiency of ROS utilization, has limited their practical application in engineering contexts [78,79]. To address these challenges, researchers have developed solid materials characterized by high reactivity, transitioning from homogeneous to heterogeneous systems based on sulfite [78-83]. Among these materials, those modified with zero-valent iron (ZVI) as a primary component exhibit a gradual release of metal ions, thereby minimizing metal ion waste while simultaneously enhancing the cycling of ferrous and ferric ions within the system [82-84]. Notably, the electron paramagnetic resonance (EPR) spectra of ZVI/ potassium pyrosulfite (PPS) indicate that the types of ROS present in the system evolve as the reaction progresses (Figs. 3a and b) [68]. Furthermore, regulating the release of metal ions mitigates the quenching effect that potent oxidizing free radicals [85-87]. In addition to serving as activators, the leachable transition metal materials themselves possess inherent capabilities to oxidize pollutants. For example, in the FeMnO/S(Ⅳ) system utilized for the degradation of trivalent arsenic (As), the iron manganese oxide material not only provides Fe(Ⅲ) but also facilitates the participation of Mn(Ⅲ) to in the oxidation of As(Ⅲ) [83]. Another category of materials, primarily composed of transition metals, activates sulfite by creating highly reactive sites on their surfaces [88-92]. The activation process involves valence changes in the bound states of transition metals, which, under specific conditions, can revert to their original valence state through electron transfer, thereby maintaining their functionality (Eqs. S19-S27 in Table S2) [88,90,91]. These materials are characterized by minimal release of metal ions, which helps prevent secondary contamination, and exhibit structural and functional stability, ensuring reliable long-term performance in practical applications. For example, the composite material CaCu₃Ti₄O₁₂ activates sulfite directly through Cu and Ti sites on its surface, thereby efficient and environmentally sustainable pollutant treatment [93].

  Figure 3

  

  Figure 3.  EPR spectra of (a) ^•OH and SO₃^•−, (b) O₂^•− and ^•OH in ZVI/PPS system. Reprinted with permission [68]. Copyright 2022, Elsevier. (c) EPR spectra of Fe³⁺/O₂/dithionite system. Reprinted with permission [124]. Copyright 2022, Elsevier. (d) EPR spectra of UV/g-C₃N₄/thiosulfate system. Reprinted with permission [128]. Copyright 2022, Elsevier. ♦DMPO-^•OH, *DMPO-SO₃^•−, ♣DMPO—O₂^•−, ▲mixing of DMPO-^•OH and DMPO-SO₃^•−, •DMPO-SO₄^•−.

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  In the course of examining the alterations in sulfite during photolysis, researchers have determined that UV radiation can excite SO₃²⁻, leading to the release of e_aq⁻ and the formation of SO₃^•− as indicated in Eqs. S28-S30 (Table S2) [94-96]. It is noteworthy that SO₃^•− can swiftly convert into the more reactive SO₄^•− in environments rich in oxygen. Consequently, researchers have successfully developed a homogeneous AOPs system utilizing UV light and sulfate S(Ⅳ) by meticulously regulating the levels of dissolved oxygen (DO). Furthermore, the activation of sulfite through external energy sources can be achieved via electro-activation and sonic-activation methods [97-100]. In a homogeneous electrocatalytic system, the electron transfer mechanism facilitates the efficient removal of electrons from SO₃²⁻ resulting in the generation of SO₃^•− [99-102]. By selecting various electrode materials, the production efficiency of SO₄^•− production can be significantly improved, thereby improving the degradation of pollutants. Similarly, sono-activation employs ultrasound energy to excite sulfite, which can also lead to the decomposition water molecules, yielding additional ^•OH as described in Eqs. S31 and S32 (Table S2) [96,97].

  In homogeneous UV/S(Ⅳ) systems, the introduction of photocatalysts significantly enhances the oxidative potential of the system. These catalysts effectively reduce the interference caused by strong reducing agents, such as the e_aq⁻, which are produced during the UV activation of sulfite [103,104]. Moreover, they facilitate the transformation of the less reactive SO₃^•− into the more potent SO₄^•−, thereby augmenting the overall oxidative efficiency of the system [94,105]. Additionally, certain photocatalysts are capable of facilitating the generation of other ROS within the system [106-111]. For example, upon photoexcitation, the electrons generated by the C₃N₄ photocatalyst can reduce O₂ to form superoxide radicals (O₂^•−) [106,112]. The development of hetero-structured composite materials, such as C₃N₄/Fe₃O₄, enhances the system's ability to absorb visible light and effectively minimizes the recombination of charge carriers. This dual functionality optimizes the utilization of available electrons and holes [113]. Moreover, the electrons generated through the photoexcitation of C₃N₄ can facilitate the cycling of Fe(Ⅲ) and Fe(Ⅱ), thereby enhancing activation efficiency [114]. In electrocatalytic sulfite systems, the incorporation of auxiliary materials can also improve the activation efficiency of S(Ⅳ) [115-118]. For instance, Co-loaded silicon-based materials activate sulfite via surface-bound Co to produce SO₃^•−. In this system, the anode generates O₂ by reducing water molecules, while the cathode provides a substantial number of electrons to promote the cycling of Co valence states [119]. Similarly, iron-carbon materials, ferromanganese materials and TiO₂ can also enhance the oxidative capacity of the system through the abundant oxygen and electrons supplied by the electrodes [117,120,121].

  Dithionite is classified as an unable LVSOs that readily undergoes disproportionation reactions in acidic environments, resulting in the formation of sulfite and thiosulfate (Eqs. S33-S35 in Table S3 in Supporting information) [122]. The instability of S-S bond present in the S₂O₄²⁻ ion facilitates its cleavage into two SO₂^•− [123]. Consequently, the activation of dithionite can be delineated into two distinct processes: the activation of sulfite within the system and the subsequent generation of the cleavage product, SO₂^•− [124-126]. In AOPs utilizing dithionite as a ROS source, Fe(Ⅲ) is the most frequently employed activator. The predominant reaction in dithionite-based AOPs involves the formation of a complex between Fe(Ⅲ) and S₂O₄²⁻, which promotes the generation of SO₂^•−, and subsequently catalyzes its conversion to SO₃^•− (Eqs. S36-S40 in Table S3) [123]. The primary ROS within the system is illustrated in Fig. 3c [124]. However, the system is characterized by numerous and intricate chain reactions, alongside various rate-limiting side reactions that are challenging to regulate, leading to suboptimal yields potent oxidizing radicals [127]. Song et al. sought to augment the oxidizing capacity of the Fe(Ⅲ)/S₂O₄²⁻ system through the incorporation electrodes [127]. This approach facilitated the acceleration of the redox cycle, thereby promoting increased ROS production. Processes employing dithionite as a ROS source have demonstrated the capability to rapidly eliminate brominated pollutants, achieving both dehalogenation and mineralization [125,126]. This technology has also exhibited effective degradation efficiency for hazardous and recalcitrant pollutants, such as atrazine (ATZ) [94]. Furthermore, with the aid of electrocatalysis, the effective removal of contaminants from high-salinity wastewater has been successfully accomplished [127].

  Thiosulfate, akin to other LVSOs, demonstrates considerable photosensitivity. Upon exposure to UV irradiation, is capable of generating a range of free radicals, including S^•−, S₂O₃^•−, SO₃^•−, and ^•OH [51]. Nevertheless, the UV/thiosulfate system is not ideally suited for direct application in AOPs due to the limited oxidizing potential of the radicals produced. In efforts to enhance the efficacy of the UV/thiosulfate system, researchers have identified that the incorporation of the photocatalyst g-C₃N₄ significantly elevates the yield of oxidative radicals. This enhancement is attributed to the photoelectrons generated by the UV-excited material, which can reduce O₂ to form O₂^•− and facilitate the transformation of reductive radicals into oxidative radicals, thereby generating ^•OH and SO₄^•− (Eqs. S41-S48 in Table S3) [51,128,129]. The signal of SO₃^•− can be detected in the system (Fig. 3d) [128]. Additionally, Zhang et al. have reported that high-valent iron can activate thiosulfate, resulting in the production of SO₄^•− and ^•OH [130]. The catalytic mechanism underlying this system is currently understood from two principal perspectives: firstly, it is posited that high-valent iron directly interacts with thiosulfate, leading to the formation of SO₄^•−; secondly, it is suggested that high-valent iron initially oxidizes thiosulfate to sulfite, which subsequently reacts with high-valent iron or Fe(Ⅲ) to generate ROS (Eqs. S49-S54 in Table S3) [130-132]. Although the system generates the highly oxidative species such as Fe(Ⅴ)/(Ⅳ) from Fe(Ⅵ), the predominant reactive species remain serves as SO₄^•− and ^•OH. Therefore, we believe that thiosulfate remains an effective source of ROS, with Fe(Ⅵ) primarily functioning as an activator. Based on these findings, thiosulfate exhibits notable potential as a source of ROS in AOPs.

  2.2 LVSOs as activators in AOPs systems

  Sulfite, while generally recognized as an electron donor, does not typically function as an activator in most AOPs. However, research conducted by Sun et al. has demonstrated that sulfite can facilitate the transformation of the less reactive Fe(Ⅵ) into the more reactive Fe(Ⅴ) and Fe(Ⅳ) species (Eqs. S55-S57 in Table S4 in Supporting information) [133]. Notably, S(Ⅳ) within the system can also serve as a source of ROS and can be activated by the generated Fe(Ⅲ), akin to the behavior observed in sulfite-based homogeneous AOP systems, leading to the production of SO₄^•−. The introduction of sulfite significantly improves the oxidative efficacy of the Fe(Ⅵ) process for the removal of pollutant across a broad pH spectrum [75,134]. It is important to note that variations in pH influence the reaction mechanism of the sulfite/Fe(Ⅵ) system. Under acidic conditions, the reactivity of high-valent iron is heightened, with pollutant removal predominantly facilitated by Fe(Ⅴ) and Fe(Ⅳ), while thiosulfate acts as an activator within the system [135]. Conversely, in alkaline conditions, the activity of high-valent iron diminishes, and the resultant SO₃^•− is more likely to combine with O₂, ultimately resulting in the generation of quantities of SO₄^•−, at which point high-valent iron assumes the role of the activator [135]. Furthermore, the sulfite-activated Fe(Ⅵ)/(Ⅴ)/(Ⅳ) system is capable of oxidizing over 50% of persistent pollutants, achieving a reduction of pathogens by more than 4 logarithmic units, while generating relatively minimal bromine by-products [75,136]. This system also demonstrates effective removal of iodine, successfully eliminating 41.2% of I⁻ and 53.8% of IO₃⁻ when addressing iodine-containing pollutants [137].

  In recent years, researchers have employed dithionite as an activator for peroxodisulfate (PDS), demonstrating its efficacy in enhancing the removal of pollutants. The method has garnered attention due to its cost-effectiveness and straightforward operational requirements. Within the dithionite/PDS system, the S₂O₄²⁻ assumes the role traditionally held by low-valent transition metals, thereby facilitating the generation of the highly reactive SO₄^•− through the transfer of a single electron to S₂O₈²⁻ (Eq. S58 in Table S4) [138]. Concurrently, S₂O₄²⁻ undergoes a disproportionation-like reaction upon interaction with PDS, resulting in the formation of S₂O₃²⁻ and SO₃^•−. This system, effectively optimizes the utilization of oxidants and activators, exhibiting robust resistance to interference and mitigating the risk of secondary pollution associated with excessive PDS application [44,139].

  Furthermore, thiosulfate has been demonstrated to facilitate the activation of PDS, operating on the principle that S₂O₃²⁻ donates an electron to activate S₂O₈²⁻ (Eq. S59 in Table S4) [140]. Thiosulfate exhibits a synergistic effect when combined with P-doped biochar materials to activate PDS, resulting in the efficient degradation of pollutants [141]. In addition to its role in PMS (peroxymonosulfate) activation, thiosulfate can also activate ozone, leading to the formation of adducts with ozone that subsequently decompose to yield ¹O₂, O₂^•−, and ^•OH. This process enhances the degradation and mineralization of pollutants (Eqs. S60-S69 in Table S4) [46].

  2.3 LVSOs as auxiliary agents in AOPs systems

  The cycling of ions with varying valence states within AOPs is frequently a critical factor in mitigating the production of ROS [142]. LVSOs, characterized by their strong reducing properties, function effectively as electron donors; when utilized in appropriate quantities, they can significantly enhance redox cycling within AOPs [143-145]. Specifically, in persulfate-based AOPs, sulfite has been shown to augment the oxidizing potential and accelerate the degradation of pollutants [146,147]. In systems activated by ultraviolet light and persulfate, the incorporation of sulfite not only acts as a precursor for free radicals, thereby increasing the overall yield of potent ROS, but also mitigates the presence of excess persulfate, thus preventing secondary pollution [143,148]. In systems that combine Fe(Ⅲ) or Fe(Ⅲ)-based materials with persulfate, sulfite functions as a reducing agent, facilitating the cycling of Fe(Ⅱ) and Fe(Ⅲ) and enhancing the efficiency of ferric ion utilization [146,147,149,150]. Concurrently, sulfite also acts as an oxidant, generating free radicals and consuming oxygen within the system, which helps to diminish the natural oxidation of ferrous ions [146,149]. Furthermore, in wastewater containing high concentrations of bromide ions, the addition of sulfite effectively reduces bromate, a toxic byproduct commonly generated in AOPs [148].

  Dithionite is frequently employed used as an auxiliary agent within the Fe(Ⅱ) activation persulfate system to assist in the reduction processes [144,151,152]. For instance, in the context of sulfamethoxazole degradation, the removal efficiency observed in the conventional Fe(Ⅱ)/persulfate system was 50.4%, while the Fe(Ⅱ)/O₂/dithionite system and achieved a removal rate of 41.3%. In contrast, the degradation rate in the Fe(Ⅱ)/persulfate system assisted by dithionite reached an impressive 84% [144]. Notably, dithionite has the capacity to expedite the cycling of metal ions in the solution thereby improving the activation efficacy of iron-based metal materials [151]. For example, the reducing free radical SO₂^•−, generated from dithionite decomposition, can reduce Fe(Ⅲ) and V(Ⅴ) produced during the activation of PMS by FeVO₄, which significantly extends the operational lifespan of the material [151]. Furthermore, dithionite markedly enhances the efficiency of ^•OH generate in the Fe(Ⅱ)-activated H₂O₂ system, operating on a similar principle of accelerating the Fe(Ⅱ)/Fe(Ⅲ) cycling [153].

  Thiosulfate plays a crucial role in modulating the valence cycle of transition metal ions within Fenton or similar Fenton systems [145,154]. It exhibits a dual function by facilitating the regeneration of Fe(Ⅱ) and forming complexes with Fe(Ⅲ), which consequently diminishes the formation of Fe(OH)₃ and broadens the pH range applicable to the system [154]. Notably, during the reduction of metal ions, thiosulfate generates sulfite, which, under the catalytic influence of high-valent metal ions, can further yield a certain quantity of SO₄^•− [154]. In the context of a Cu(Ⅱ)-activated H₂O₂ system, the incorporation of thiosulfate resulted in a 5.8-fold increase in the degradation rate of p-benzoic acid, while simultaneously promoting the Cu(Ⅰ)/Cu(Ⅱ) cycling and indirectly enhancing the production of Cu(Ⅲ) [144]. Furthermore, in the Fe(Ⅱ) activation persulfate system, thiosulfate effectively reduces the required dosage of iron and enhances the resource recovery from waste activated sludge [155,156].

  3. Overview of recent progresses in low-valent sulfur-based ARPs

  3.1 Mechanism and application of ARPs for sulfite

  In neutral and alkaline conditions, sulfite can generate a potent reducing agent, specifically the e_aq⁻ (E = −2.9 V), when exposed 275 nm UV [53,57,157-160]. Conversely, in acidic conditions, e_aq⁻ reacts with H⁺ to yield another strong reducing agent, the ^•H (E = −2.3 V) [161-165]. Typically, these reducing agents are rapidly consumed by O₂ or by the oxidizing free radicals produced in an oxygen-rich environments [99,100]. Nevertheless, through the meticulous regulation of critical parameters such as DO and sulfite concentration across various systems, it is possible to effectively channel these reducing agents towards specific target pollutants, thereby enhancing treatment efficiency [163-165].

  Numerous hazardous substances exist within aquatic environments that necessitate removal through reduction processes [56,57,63]. Among these, widely used disinfectants such as chlorates and bromates exhibit potent oxidizing and corrosive characteristics, posing a risk of acute poisoning upon human ingestion [53,58,166-170]. In the UV/sulfite system, the reductive species such as e_aq⁻ are generated, which can effectively reduce chlorate and bromate, thereby mitigating their potential toxicity, and promoting the safety of the aquatic ecosystems [58,60,169]. The fundamental mechanism for perchlorate removal involves the gradual reduction of the valence state of Cl⁻ by e_aq⁻ and SO₃^•− generated within the UV/sulfite system, ultimately resulting in its conversion to Cl⁻ [167]. The reduction process of bromates follows a process analogous to that of chlorates. However, it is comparatively more intricate (Eqs. S70-S78 in Table S5 in Supporting information) [57]. It is important to note that during the reduction of bromates, e_aq⁻ can also produce a minor quantity of oxidizing agents [170]. This observation has led some researchers to propose that the degradation process of bromates does not adhere strictly to a first-order kinetic model, but rather aligns more closely with a second-order kinetic model [60]. Furthermore, the degradation of bromates is subject to the influence of operational conditions and reaction parameters, rendering the mechanism of bromate degradation a topic of ongoing debate [60,168].

  The toxicity of heavy metals, such as chromium (Cr) and selenium (Se), is generally more pronounced in their higher valence states compared to their lower valence states, and the latter are more amenable to recovery [56,171]. In the case of chromium in its hexavalent form (Cr(Ⅵ)), the robust reducing capability of the UV/sulfite system under alkaline conditions can facilitate its reduction to trivalent chromium (Cr(Ⅲ)), which subsequently precipitates as Cr(OH)₃, allowing for recovery [56]. Both SO₃^•− and e_aq⁻ produced in the system participated in the reduction of Cr(Ⅵ), contributing about 25% and 75%, respectively (Eqs. S79-S84 in Table S5) [56]. For the removal and recovery of Se(Ⅵ), it is reduced to Se(Ⅳ) by the e_aq⁻ generated in the UV/sulfite system, followed by coagulation with Fe(Ⅲ) (Eqs. S85-S88 in Table S5) [171]. This method has demonstrated the capability to eliminate 99% of Se(Ⅵ) even in environments with high sulfate concentrations, with minimal interference from other coexisting anions present in the aqueous medium [171]. Additionally, for heavy metals like As, where the toxicity is higher in the lower valence state than in the higher valence state, the system can also utilize its strong reducing power to reduce As(Ⅴ) to As(0) for recovery (Eqs. S89-S95 in Table S5) [62,161].

  Currently, UV/sulfite-based ARPs have emerged as a prominent research area in the field of dehalogenation technology, showcasing remarkable effectiveness in treating a variety of organohalide compounds [162,172-174]. The carbon-halogen bond in halogenated hydrocarbons, due to its strong electrophilicity, is prone to cleavage under attack by e_aq⁻ or ^•H generated in the UV/sulfite system [163,165,173]. In ARPs, the detached halogen ions do not produce toxic halogenates and have minimal side effects on the environment [174,175]. Additionally, I⁻ can also be activated by UV light to produce e_aq⁻, and the addition of I⁻ to the UV/sulfite system significantly increased the yield and lifetime of e_aq⁻ [56,58,164,176]. The UV/sulfite system is one of the most commonly used processes for the detoxification of perfluorinated compounds [63,64,177]. As a special type of halogenated hydrocarbon where all H are replaced by F, perfluorinated compounds are difficult to decompose harmlessly with conventional techniques [178-180]. However, the e_aq⁻ produced in the UV/sulfite system are capable of breaking the robust C-F bonds, ultimately transforming them into harmless common organic molecules and F⁻ [63,177,181].

  3.2 Mechanism and application of ARPs for dithionite

  Dithionite is a potent reducing agent, and its unstable molecular structure makes it prone to releasing electrons in aqueous solutions [182]. Under the action of UV, dithionite can release e_aq⁻, SO₂^•−, and S₂O₄^•−. The process of activating S₂O₄²⁻ mainly involves two mechanisms. The first is the breaking of the S-S bond in S₂O₄²⁻ through energy input, resulting in the formation of two SO₂^•−. These radicals contain an unpaired electron that is not coordinated, giving SO₂^•− a significant reducing property and making it easy to be released, subsequently forming SO₂ (Eqs. S96-S98 in Table S5) [48,49,183,184]. The second mechanism involves UV light exciting S₂O₄²⁻, causing it to lose an electron overall, thus forming an oxidizing radical S₂O₄^•− and e_aq⁻. Although this activation method produces the highly reducing e_aq⁻, the accompanying oxidizing radicals may also accelerate the quenching process of electrons (Eqs. S99 and S100 in Table S5) [55,125,185,186]. The reaction mechanism of UV/dithionite as shown in Fig. 4.

  Figure 4

  

  Figure 4.  The mechanism of UV/dithionite.

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  The UV/dithionite system has been proven effective in treating halogenated organic wastewater and heavy metal wastewater [55,125,183]. The treatment of organic wastewater primarily involves the e_aq⁻ generated in the system attacking electrophilic sites in organic molecules to facilitate dehalogenation. In the process of reducing and removing As, the system efficiently converts dissolved As(Ⅲ) to insoluble As(0) or arsenic sulfide precipitates [183,184]. It is worth noting that the reducing power of dithionite alone is insufficient to reduce and recover As, and once the UV light or S₂O₄²⁻ is depleted, the precipitated arsenic may redissolve [183,184]. The UV/dithionite system is also commonly used for the reduction of nitrates, with the mechanism involving the reduction of NO₃⁻ to NH₃ by the reducing substances produced after S₂O₄²⁻ is exposed to UV light [186]. In this process, UV plays a reinforcing role, enhancing the conversion rate of nitrates [187]. Furthermore, the system's production of SO₂^•− can reduce nitrobenzene to aniline with nearly 100% conversion efficiency [48].

  3.3 Mechanism and application of ARPs for thiosulfate

  Thiosulfate has a more unique structure compared to sulfite and dithionite. Although the average oxidation state of sulfur in S₂O₃²⁻ is +2, it is actually composed of a sulfur atom with an oxidation state of +6 and another with an oxidation state of −2 [187]. This special structure means that it does not splitting uniformly upon activation, unlike dithionite. Notably, the decomposition of S₂O₃²⁻ varies with different wavelengths of UV. When the wavelength of UV is close to 375 nm, S₂O₃²⁻ will decompose into S₂O₃^•− and e_aq⁻; at a wavelength of 280 nm, S₂O₃²⁻ may decompose into S₂O₂^•− and ^•OH, or S^•− and SO₃^•− (Eqs. S101-S103 in Table S5) [51]. The reaction mechanism of UV/thiosulfate as shown in Fig. 5.

  Figure 5

  

  Figure 5.  The mechanism of UV/thiosulfate.

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  According to the photolysis mechanism of thiosulfate, the production of strong reducing substances is relatively low, which limits its application in ARPs. However, under specific wavelengths of light, thiosulfate can produce S^•−, providing a new method for generating H₂S [50]. Since As and some heavy metals can form precipitates with S²⁻, this provides a feasible way for the recovery of As. However, under acidic conditions, S²⁻ is easily converted into H₂S and escapes into the air. This is a waste of sulfur resources and may also lead to the leakage of toxic gases. The method of photoactivating thiosulfate to produce H₂S can effectively prevent gas leakage, as in the absence of heavy metals, the generated H₂S is consumed by sulfite or its photolysis products produced from the decomposition of thiosulfate [50]. The system selectively recovers heavy metal elements because the reaction rate of S²⁻ with these metals is significantly higher than their decomposition rate by thiosulfate. Moreover, the reducing active species produced in the UV/thiosulfate system can effectively reduce heavy metals in high valence states. Thiosulfate is ultimately oxidized to sulfate, thus avoiding the issue of secondary pollution [50].

  4. Conclusions and outlook

  This article reviews three types of LVSOs: Sulfite, dithionite, and thiosulfate. Compared to other complex polysulfides, these LVSOs exhibit greater stability in their existence forms and are more amenable to industrial production. Their potential applications in AOPs and ARPs have been explored. In AOPs, they can avoid the harm of secondary pollution and toxic by-products caused by excessive oxidation capacity. Furthermore, serving as an electron donor to facilitate the redox cycle of persulfate-based AOPs, or acting as an activator to provide the necessary electrons for persulfate and the like, reflect the diversity of functions of LVSOs. In the field of ARPs, e_aq⁻ and ^•H generated by UV activation of LVSOs has excellent effects on the removal and detoxification of heavy metal ions, bromates, and haloalkanes, and is a terrific environmentally friendly process.

  Although the application of LVSOs has shown promising advancements in economic efficiency and versatility, there are still some challenges that need to be faced and overcome in future exploration. Based on our review, several key points are proposed as follow:

  (1) The process by which LVSOs generate free radicals is complex and susceptible to interference from coexisting substances and environmental conditions. Consequently, these interfering factors should be mitigated or regulated to enhance the system's oxidative capacity.

  (2) Current research into the factors influencing the production of ROS is still primarily focused on the individual impact of potential factors. To better reflect actual applications, it is essential to explore the synergistic effects of multiple factors.

  (3) LVSOs-based ARPs exhibit potent reductive capabilities, but most processes require an alkaline environment to operate properly. Achieving equivalent reductive effects under acidic conditions is an area that deserves further research and development.

  (4) In ARPs, the study of the activation method of LVSOs by different wavelengths of UV is not sufficient, and a deeper study of the relationship between wavelength and activated products can help control the active species.

  (5) In the actual wastewater treatment process, wastewater with high colority and high suspended solids can hinder the UV activation of LVSOs. Therefore, the issue of UV obstruction in practical applications necessitates additional research and exploration.

  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.

  CRediT authorship contribution statement

  Xin Zhou: Writing – original draft, Visualization, Methodology, Conceptualization. Xuejia Li: Visualization, Methodology, Conceptualization. Yujia Xiang: Methodology, Conceptualization. Heng Zhang: Resources, Methodology, Conceptualization. Chuanshu He: Resources, Methodology, Conceptualization. Zhaokun Xiong: Methodology, Conceptualization. Wei Li: Visualization, Methodology. Peng Zhou: Resources, Methodology, Conceptualization. Hongyu Zhou: Methodology, Conceptualization. Yang Liu: Writing – review & editing, Resources, Methodology, Conceptualization. Bo Lai: Visualization, Resources, Methodology, Conceptualization.

  Acknowledgments

  This work was supported by Natural Science Foundation of China (Nos. 52070133, 42107073, 42477075), Natural Science Foundation of Sichuan Province (No. 2024NSFSC0130), the Sichuan Science and Technology Program (No. 2024NSFTD0014), Key Laboratory of Jiangxi Province for Persistent Pollutants Prevention Control and Resource Reuse (No. 2023SSY02061), and Key R & D Program of Heilongjiang Province (No. 2023ZX02C01).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110664.

  
  
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  Figure 1  Current status of research on LVSOs in the last five years.

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  Figure 2  The mechanism of free radical generation by LVSOs as ROS sources (M = Fe, Cu, Mn, Co).

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  Figure 3  EPR spectra of (a) ^•OH and SO₃^•−, (b) O₂^•− and ^•OH in ZVI/PPS system. Reprinted with permission [68]. Copyright 2022, Elsevier. (c) EPR spectra of Fe³⁺/O₂/dithionite system. Reprinted with permission [124]. Copyright 2022, Elsevier. (d) EPR spectra of UV/g-C₃N₄/thiosulfate system. Reprinted with permission [128]. Copyright 2022, Elsevier. ♦DMPO-^•OH, *DMPO-SO₃^•−, ♣DMPO—O₂^•−, ▲mixing of DMPO-^•OH and DMPO-SO₃^•−, •DMPO-SO₄^•−.

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  Figure 4  The mechanism of UV/dithionite.

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  Figure 5  The mechanism of UV/thiosulfate.

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