Sulfate removal mechanism by internal circulation iron-carbon micro-electrolysis

doi:10.1016/j.seppur.2021.119762

Separation and Purification Technology
分离纯化技术

Volume 279, 15 December 2021, 119762
第 279 卷，2021 年 12 月 15 日，119762

https://doi.org/10.1016/j.seppur.2021.119762 Get rights and content 获取权利和内容

Highlights 突出

•
Internal circulation iron-carbon micro-electrolysis was used to remove sulfate.
采用内循环铁碳微电解去除硫酸盐。
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We investigated the effects of different conditions on the sulfate removal rate.
我们研究了不同条件对硫酸盐去除率的影响。
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The maximum sulfate removal efficiency was 76.6% under specific conditions.
特定条件下硫酸盐去除效率最高，为 76.6%。
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The sulfate removal mechanism was investigated using four different techniques.
使用四种不同的技术研究了硫酸盐去除机制。

Abstract 抽象

Internal circulation iron-carbon micro-electrolysis (ICE) was used to treat sulfate in wastewater. The influences of the reaction time, aeration rate, pH, iron-carbon ratio, and the initial sulfate concentration on sulfate removal by ICE were investigated. The maximum sulfate removal efficiency was 76.6% under the following conditions: reaction time = 120 min, aeration rate = 0.5 m³/h, pH = 2, iron-carbon ratio = 1:1, and initial sulfate concentration = 500 mg/L. Scanning electron microscopy, energy-dispersive spectrometry, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy analyses were performed to study the sulfate removal mechanism. The results showed that sulfate was mainly removed by two mechanisms: 1) sulfate may initially be reduced to SO₃^2-, and then to S^2-, which combines with H⁺ and Fe²⁺ to form H₂S and FeS, which can be removed; 2) sulfate can be removed by flocculation and precipitation of Fe(OH)₂ and Fe(OH)₃. Therefore, the ICE process can be used as an effective method to remove sulfate from wastewater.
采用内循环铁碳微电解（ICE）处理废水中的硫酸盐。研究了反应时间、曝气速率、pH 值、铁碳比和初始硫酸盐浓度对 ICE 去除硫酸盐的影响。在反应时间 = 120 min、曝气速率 = 0.5 m³/h、pH = 2、铁碳比 = 1：1 和初始硫酸盐浓度 = 500 mg/L 的条件下，硫酸盐去除效率最高为 76.6%。结果表明，硫酸盐的去除主要通过两种机制：1）硫酸盐最初可还原为SO₃^2-，然后还原为S^2-，与H⁺和Fe²⁺结合形成H₂S和FeS，可以被去除;2）可以通过 Fe（OH）₂ 和 Fe（OH）₃ 的絮凝沉淀去除硫酸盐。因此，ICE 工艺可以作为从废水中去除硫酸盐的有效方法。

Graphical abstract 图形摘要

Keywords 关键字

Internal circulation iron-carbon micro-electrolysis (ICE)

Sulfate

Removal mechanism

Flocculation

内循环铁碳微电解（ICE）

硫酸盐

去除机制

絮凝

1. Introduction 1. 引言

With the rapid industrial development, sulfur and sulfur compounds have been widely used in industrial production as important raw materials [1]. The waste waters released by various industries (e.g., petrochemicals and steel metallurgy plants) contain a lot of sulfate, which has significant impacts on the soil environment and aquatic ecosystems [2], [3], [4]. Sulfate wastewater has a wide range of sources, and the water quality changes significantly with different production processes. Sulfate wastewater is mainly derived from petrochemicals, leather, pharmaceuticals, papermaking, molasses, printing and dyeing, landfill leachate, mining, sulfonation, and other industries [5], [6], [7], [8], [9]. The sulfate concentration in such wastewaters can often reach hundreds or thousands of milligrams per liter [10].
随着工业的快速发展，硫磺和硫化合物作为重要的原料在工业生产中得到了广泛的应用[1]。各个行业（如石化和钢铁冶金厂）排放的废水中含有大量的硫酸盐，对土壤环境和水生生态系统有重大影响 [2]， [3]， [4]。硫酸盐废水的来源范围很广，水质会随着生产工艺的不同而变化。硫酸盐废水主要来源于石油化工、皮革、制药、造纸、糖蜜、印染、垃圾渗滤液、采矿、磺化等行业 [5]， [6]， [7]， [8]， [9].此类废水中的硫酸盐浓度通常可达数百或数千毫克/升 [10]。

Sulfate can cause serious damage to the water quality and aquatic ecological environment [11]. For example, sulfate can change the pH and salinity of water and lead to disorder of the sulfur cycle [12]. Sulfate can also precipitate with some metal ions, resulting in soil hardening and destruction of the soil structure [13]. Under anaerobic conditions, sulfate can be reduced to H₂S under the action of sulfate-reducing bacteria [14], [15]. H₂S is a serious threat to the lives of humans, animals, and plants [2], [16]. Therefore, sulfate wastewater should be effectively treated prior to discharge.
硫酸盐会对水质和水生生态环境造成严重破坏 [11]。例如，硫酸盐会改变水的 pH 值和盐度，导致硫循环紊乱 [12]。硫酸盐还会与一些金属离子沉淀，导致土壤硬化和土壤结构破坏 [13]。在厌氧条件下，硫酸盐在硫酸盐还原细菌的作用下可还原为 H ₂S [14]、[15]。H₂S 对人类、动物和植物的生命构成严重威胁 [2]，[16]。因此，硫酸盐废水在排放前应进行有效处理。

At present, commonly used treatment methods for sulfate wastewater include physical, chemical, and biological approaches. Among them, physical methods mainly include ion exchange [17], adsorption [18], and multi-effect evaporation crystallization [19], primarily including calcium salt, barium salt, and double salt methods [20]. Although they have been widely used, some technical and economic limitations exist; for example, physical and chemical methods can easily cause secondary pollution and are expensive [21], [22], and have the disadvantages a long treatment cycle and being unsuitable for treating water with high sulfate concentrations [23], [24]. Therefore, alternative methods have been developed for sulfate removal, including nanofiltration, electrodialysis, electrochemical, and reverse osmosis methods [25], [26], [27]. Our research group found that sulfide could be removed by iron-carbon micro-electrolysis during industrial wastewater treatment. According to a literature review, to our knowledge, there is no previous report on the use of iron-carbon micro-electrolysis to treat sulfate.
目前，硫酸盐废水常用的处理方法包括物理、化学和生物方法。其中，物理方法主要包括离子交换[17]、吸附[18]和多效蒸发结晶[19]，主要包括钙盐、钡盐和复盐方法[20]。尽管它们已被广泛使用，但仍然存在一些技术和经济限制;例如，物理和化学方法容易造成二次污染，价格昂贵[21]，[22]，缺点是处理周期长，不适合处理硫酸盐浓度高的水[23]，[24]。因此，已经开发了去除硫酸盐的替代方法，包括纳滤、电渗析、电化学和反渗透法 [25]、[26]、[27]。我们的研究小组发现，在工业废水处理过程中，可以通过铁碳微电解去除硫化物。根据文献综述，据我们所知，以前没有关于使用铁碳微电解处理硫酸盐的报道。

Iron is combined with activated carbon to form a micro battery, in which iron is the anode, activated carbon is the cathode, and wastewater is the electrolyte; this process is referred to as iron-carbon micro-electrolysis (IE). The reactions can be expressed by equations (1), (2) [28], [29]:(1)

Anodic : F e - 2 e^{-} \to {Fe}^{2 +} E^{θ} (F e / {Fe}^{2 +}) = - 0.44 V

(2)

Cathodic (a c t i v a t e d c a r b o n) : 2 H^{+} + 2 e^{-} \to 2 [H] \to H_{2} E^{θ} (H^{+} / H_{2}) = 0 V

铁与活性炭结合形成微型电池，其中铁是阳极，活性炭是阴极，废水是电解质;这个过程被称为铁碳微电解（IE）。反应可以用方程（1）、（2）[28]、[29] 表示： (1)

Anodic : F e - 2 e^{-} \to {Fe}^{2 +} E^{θ} (F e / {Fe}^{2 +}) = - 0.44 V

(2)

Cathodic (a c t i v a t e d c a r b o n) : 2 H^{+} + 2 e^{-} \to 2 [H] \to H_{2} E^{θ} (H^{+} / H_{2}) = 0 V

When oxygen is present, the cathodic reactions are expressed by equations (3)-(5):(3)

O_{2} + 4 H^{+} + 4 e^{-} \to 2 H_{2} O E^{θ} {(O}_{2}) = + 1.23 V

(4)

O_{2} + 2 H_{2} O + 4 e^{-} \to 2 {OH}^{-} E^{θ} (O_{2} / {OH}^{-}) = + 0.41 V

(5)

O_{2} + 2 H^{+} + 2 e^{-} \to 2 H_{2} O_{2} E^{θ} {(O}_{2} / H_{2} O_{2}) = + 0.68 V

当存在氧气时，阴极反应由方程（3）-（5）表示： (3)

O_{2} + 4 H^{+} + 4 e^{-} \to 2 H_{2} O E^{θ} {(O}_{2}) = + 1.23 V

(4)

O_{2} + 2 H_{2} O + 4 e^{-} \to 2 {OH}^{-} E^{θ} (O_{2} / {OH}^{-}) = + 0.41 V

(5)

O_{2} + 2 H^{+} + 2 e^{-} \to 2 H_{2} O_{2} E^{θ} {(O}_{2} / H_{2} O_{2}) = + 0.68 V

The IE process can remove pollutants by redox, micro battery, flocculation, micro-electric field attachment, and adsorption [30], [31]. The IE process is widely used to treat industrial wastewater (e.g., in coking, pharmaceutical, printing and dyeing, papermaking, and petrochemical industries) because of its advantages of simple processing, low cost, long service life, simple operation, and good treatment effect [32], [33], [34], [35], [36]. The IE process has a good removal effect on color, chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) [37], [38], [39]. For example, when IE was used to treat pharmaceutical wastewater, it provided COD, color, NH₃-N, and TP removals of 99.2%, 97.5%, 95.3%, and 78.9%, respectively [40]. Furthermore, when IE was combined with an oxic/anoxic process to treatment low-carbon greywater, removals of NH₄⁺-N, TN, TP, and COD were 94.3%, 86.2%, 98.0%, and 92.7%, respectively [41]. In some studies, the IE reaction was carried out in a fixed reactor, resulting in hardening and passivation [42], [43]. In order to overcome these shortcomings, an internal circulation iron-carbon micro-electrolysis (ICE) reactor (Fig. 1) was designed [37]. The reactor demonstrated excellent performance in industrial wastewater treatment and solved the problem of filler passivation [37]. Experimental results have shown that it has both oxidizing and reducing ability [37], [44]. To date, extensive research has been done on color, COD, TN, and TP treatments by the IE process [37], [38], [39]; however, minimal work has focused on sulfate treatment.
IE工艺可以通过氧化还原、微电池、絮凝、微电场附着和吸附来去除污染物[30]、[31]。IE工艺因其处理简单、成本低、使用寿命长、操作简单、处理效果好等优点，被广泛用于处理工业废水（如焦化、制药、印染、造纸和石化等行业）[32]、[33]、[34]、[35]、[36].IE工艺对颜色、化学需氧量（COD）、总氮（TN）和总磷（TP）具有良好的去除效果[37]、[38]、[39]。例如，当 IE 用于处理制药废水时，它的 COD、颜色、NH 3-N 和 TP 去除率分别为 99.2%、97.5%、95.3% 和 78.9% [40]。此外，当 IE 与氧化/缺氧工艺相结合处理低碳灰水时，NH₄⁺-N、TN、TP 和 COD 的去除率分别为 94.3%、86.2%、98.0% 和 92.7% [41]。在一些研究中，IE 反应是在固定反应器中进行的，导致硬化和钝化 [42]、[43]。为了克服这些缺点，一种内循环铁碳微电解（ICE）反应器（图 D）。 1）被设计 [37]。该反应器在工业废水处理中表现出优异的性能，解决了填料钝化问题 [37]。实验结果表明，它同时具有氧化和还原能力 [37]， [44]。迄今为止，已经对 IE 工艺的颜色、COD、TN 和 TP 处理进行了广泛的研究 [37]、[38]、[39];然而，很少的工作集中在硫酸盐处理上。

In this study, the ICE process was used to remove sulfate from wastewater. A static test was used to study the effects of different factors, including reaction time, aeration rate, pH, iron-carbon ratio, and initial sulfate concentration on sulfate removal by the ICE process. Scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to study the sulfate removal mechanism by the ICE process, and the main sulfate removal pathways were determined.
在这项研究中，使用 ICE 工艺去除废水中的硫酸盐。采用静态测试研究反应时间、曝气速率、pH 值、铁碳比和初始硫酸盐浓度等不同因素对 ICE 工艺去除硫酸盐的影响。采用扫描电子显微镜（SEM）、能量色散光谱（EDS）、X 射线衍射（XRD）、X 射线光电子能谱（XPS）和拉曼光谱研究 ICE 工艺去除硫酸盐的机理，并确定了主要的硫酸盐去除途径。

2. Materials and methods 2. 材料和方法

2.1. Reagents and materials
2.1. 试剂和材料

Sodium sulfate (Na₂SO₄) was obtained from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were obtained from Beijing Chemical Plant (Beijing, China). Ultrapure water was produced using an ultrapure water mechanism by the Beijing Liyuan Electronic Instruments Company (Beijing, China).
硫酸钠（Na₂SO₄）购自麦克林生化技术有限公司（中国上海）。氢氧化钠（NaOH）和盐酸（HCl）购自北京化工厂（中国北京）。超纯水由北京力源电子仪器公司（中国北京）使用超纯水机构生产。

Iron filings are taken from the metal processing workshop of our institute and passed through a 20-mesh sieve. The iron filings after sieving were cleaned with tap water and acetone to remove dust and grease from the surface, reacted in dilute hydrochloric acid solution for 30 min to eliminate iron oxide on the surface, and then rinsed with tap water to a neutral pH value [37], [38]. Granular activated carbon was obtained from Jiangsu Fuhuitong Environmental Protection Technology Co., Ltd. The granular activated carbon was first washed with tap water to remove dust on the surface of granular activated carbon, and then soaked in a high-concentration sodium sulfate solution until the activated carbon adsorption reached saturation [38]. The SO₄^2- concentration and adsorption amount were analyzed, and the results are showed in Fig. 2(a). In addition, in order to investigate the effect of adsorbed sulfate on the process, the change in desorption amount with reaction time was also investigated in the 800 mL solution without SO₄^2-. Meanwhile, the influence of pH on desorption amount was also studied. These results are shown in Fig. 2(b) and (c). Even in the solution without SO₄^2-, the maximum concentration of SO₄^2- after desorption was only 42 mg/L, and had little effect on the process of this study.
铁屑取自我所金属加工车间，通过 20 目筛。筛分后的铁屑用自来水和丙酮清洗，去除表面的灰尘和油脂，在稀盐酸溶液中反应30 分钟，去除表面的氧化铁，然后用自来水冲洗至中性pH值[37]、[38]。颗粒活性炭购自江苏富汇通环保科技有限公司。颗粒活性炭首先用自来水洗涤以去除颗粒活性炭表面的灰尘，然后在高浓度硫酸钠溶液中浸泡至活性炭吸附达到饱和[38]。分析了 SO₄^2- 的浓度和吸附量，结果如图 2（a）所示。此外，为了研究吸附硫酸盐对工艺的影响，还研究了在不含 SO₄^2- 的 800 mL 溶液中解吸量随反应时间的变化。同时，还研究了 pH 值对解吸量的影响。这些结果如图 2（b）和（c）所示。即使在没有 SO₄^2- 的溶液中，解吸后 SO₄^2- 的最大浓度也仅为 42 mg/L，对本研究过程影响不大。

2.2. Reactor and experimental procedure
2.2. 反应器和实验程序

Several identical reactors were used in the experiment, as shown in Fig. 1. The reactor with an internal circulation tube is made of glass and has a volume of 1000 mL. The total volume of iron filings and granular activated carbon remained unchanged at 360 mL. The volume of the solution in the reactor was 800 mL. The effects of the reaction time (0, 2, 5, 10, 30, 60, 90, 120, 180, 240, and 300 min), aeration rate (0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 m³/h), iron-carbon volume ratio (1:2, 2:3, 1:1, 3:2, and 2:1), pH (2, 4, 6, 7, 8, and 10), and initial sulfate concentration (500, 1000, 2000, 5000, and 10000 mg/L) on sulfate removal were studied.
实验中使用了几个相同的反应器，如图 1 所示。带有内循环管的反应器由玻璃制成，体积为 1000 mL。铁屑和颗粒活性炭的总体积保持不变，为 360 mL。反应器中溶液的体积为 800 mL。反应时间（0、2、5、10、30、60、90、120、180、240 和 300 min）、曝气速率（0.2、0.3、0.4、0.5、0.6 和 0.7 m³/h）、铁碳体积比（1：2、2：3、1：1、3：2 和 2：1）、pH 值（2、4、6、7、8 和 10）和初始硫酸盐浓度（500、1000、2000、5000、和 10000 mg/L）对硫酸盐去除进行了研究。

2.3. Analytical procedure
2.3. 分析程序

The pH of the solution was determined using a portable pH meter (PHSJ-3F, Shanghai Instrument & Electrical Scientific Instrument Co., Ltd., China). SO₄^2- was determined by ion chromatography (ICS-1000, DIONEX, USA). The iron concentration in the solution was measured by inductively coupled plasma-mass spectrometry (ICP-MS 7800, Agilent, USA). S^2- was determined using a sulfide meter (CHYS-241, Shenzhen Changhong Technology Co., Ltd., China).
使用便携式pH计（PHSJ-3F，上海仪器电气科学仪器有限公司，中国）测定溶液的pH值。SO₄^2- 通过离子色谱法（ICS-1000， DIONEX， USA）测定。通过电感耦合等离子体质谱法（ICP-MS 7800，Agilent，USA）测量溶液中的铁浓度。S^2- 使用硫化物计（CHYS-241，Shenzhen Changhong Technology Co.， Ltd.， China）测定。

An SEM (SSX-550, Shimadzu, Japan) was used to observe the surface morphology of the iron mud at a voltage of 15 kV. EDS, XPS (ESCALAB 250Xi, Thermo Fisher Scientific, USA), and Raman spectroscopy were used to determine the composition of the iron sludge after the reaction. An XRD (D8 FOCUS, BRUKER, Germany) was used to scan the iron sludge produced in the experiment. The voltage, current, scanning speed, and scanning angle ranges were 40 kV, 40 mA, 4°/min, and 10°～90°, respectively.
使用 SEM（SSX-550，日本岛津）在 15 kV 电压下观察铁泥的表面形态。使用 EDS、XPS（ESCALAB 250Xi，Thermo Fisher Scientific，美国）和拉曼光谱法测定反应后铁泥的成分。XRD（D8 FOCUS，BRUKER，德国）用于扫描实验中产生的铁污泥。电压、电流、扫描速度和扫描角度范围分别为 40 kV、40 mA、4°/min 和 10°~90°。

3. Results and discussion
3. 结果和讨论

3.1. Comparison of iron scraps, activated carbon, and their mixture
3.1. 铁屑、活性炭及其混合物的比较

To determine the performance of the ICE process for sulfate treatment, iron scraps, activated carbon, and a mixture of iron filings and granular activated carbon were used to treat sulfate wastewater. The initial sulfate concentration was 1000 mg/L.
为了确定 ICE 工艺的硫酸盐处理性能，使用铁屑、活性炭以及铁屑和颗粒活性炭的混合物来处理硫酸盐废水。初始硫酸盐浓度为 1000 mg/L。

Fig. 3 shows the change in the sulfate removal rate with time in the three processes, and illustrates that the removal rate was only approximately 3% when iron scraps were used to treat sulfate. With increasing time, the sulfate removal rate with activated carbon increased gradually and then tended to be stable. The removal rate reached a maximum value of 10.89% after 10 min. After 10 min, the removal rate decreased slightly, which may have been due to desorption of activated carbon during the reaction [45]. The removal rate was low regardless of whether it was treated with iron scraps or activated carbon, which showed that iron scraps and activated carbon alone are not suitable for treating of high concentration sulfate wastewater. However, the removal rate reached 50.43% after 120 min with a mixture of iron filings and granular activated carbon, confirming a strong synergistic effect that enhances the sulfate removal effect. Therefore, it is feasible to treat sulfate wastewater using ICE.
图 3 显示了三个过程中硫酸盐去除率随时间的变化，并说明了当使用铁屑处理硫酸盐时，去除率仅为约 3%。随着时间的增加，活性炭的硫酸盐去除率逐渐增加，然后趋于稳定。10 min 后去除率达到最大值 10.89%。10 min后，去除率略有下降，这可能是由于反应过程中活性炭的解吸[45]。无论是用铁屑还是活性炭处理，去除率都很低，这表明单独的铁屑和活性炭并不适合处理高浓度硫酸盐废水。但铁屑和颗粒活性炭混合120 min后去除率达到50.43%，证实了增强硫酸盐去除效果的强烈协同作用。因此，使用 ICE 处理硫酸盐废水是可行的。

3.2. Optimization of process parameters
3.2. 工艺参数的优化

3.2.1. Reaction time 3.2.1. 反应时间

According to our previous research [37], [38], [46], aeration rate, Fe/C ratio, and pH are important influencing factors of the ICE process. In order to make the iron-carbon fillers flow, the aeration volume should not be<0.5 m³/h. The optimal Fe/C ratio and pH were 1:1 and 2, respectively. The temperature was usually controlled at room temperature. The sulfate concentration in general industrial wastewater is in the range of 200–6000 mg/L, with the highest value reaching 11000 mg/L [47]. The sulfate concentration in industrial wastewater is mainly around 1000 mg/L [48]. Therefore, the effect of reaction time on sulfate treatment by ICE was studied at an initial sulfate (SO₄^2-)₀ concentration of 1000 mg/L, T of 20–25 °C, aeration rate of 0.6 m³/h, Fe/C of 1:1, and pH of 2. The experimental results are presented in Fig. 4(a). The sulfate removal rate increased gradually and then tended to stabilize with increasing reaction time. The sulfate removal rate increased rapidly within 30 min and reached 23.16% at 30 min. This was because Fe²⁺ and [H] with strong reducing ability were rapidly produced at the initial stage of the reaction, which greatly accelerated the reaction [49]. In addition, new ferrous ions and iron ions could form Fe(OH)₂ and Fe(OH)₃, which have the functions of flocculation and sedimentation and play roles in promoting the reaction rate. From 30 to 120 min, the increase in the sulfate removal rate was relatively slow. This was mainly because the iron was gradually consumed as the reaction proceeded, resulting in a reduction in the number of micro batteries and a decrease in the [H] and Fe²⁺ production rates [50]. The removal rate was 50.43% and the sulfate concentration was < 500 mg/L. After 120 min, the change in removal rate was very small (i.e., 56.11% at 300 min) and only increased by 5.68% compared with the rate at 120 min. Therefore, 120 min was selected as the reaction time for subsequent experiments.
根据我们之前的研究[37]、[38]、[46]，曝气速率、Fe/C 比和 pH 值是 ICE 过程的重要影响因素。为了使铁碳填料流动，曝气量不应为 <0.5 m³/h。最佳 Fe/C 比和 pH 分别为 1：1 和 2。温度通常控制在室温下。一般工业废水中的硫酸盐浓度在200–6000 mg/L范围内，最高达到11000 mg/L [47]。工业废水中的硫酸盐浓度主要在1000 mg/L左右[48]。因此，在初始硫酸盐（SO₄^2-）₀ 浓度为 1000 mg/L、T 为 20–25 °C、曝气速率为 0.6 m³/h、Fe/C 为 1：1 和 pH 值为 2 时，研究了反应时间对 ICE 处理硫酸盐的影响。实验结果如图 4（a）所示。硫酸盐去除率逐渐增加，然后随着反应时间的增加趋于稳定。硫酸盐去除率在 30 min 内迅速增加，在 30 min 时达到 23.16%。这是因为在反应初期迅速产生具有强还原能力的Fe²⁺和[H]，大大加快了反应速度[49]。此外，新的亚铁离子和铁离子可以形成 Fe（OH）₂ 和 Fe（OH）₃，它们具有絮凝和沉降的功能，并起到促进反应速率的作用。从 30 到 120 min，硫酸盐去除率的增加相对较慢。这主要是因为随着反应的进行，铁逐渐被消耗掉，导致微型电池的数量减少，[H]和Fe²⁺的产生速率降低[50]。去除率为 50.43%，硫酸盐浓度< 500 mg/L。120 min后，去除率的变化非常小（即300 min时为56.11%），与120 min时相比仅增加了5.68%。因此，选择 120 min 作为后续实验的反应时间。

3.2.2. Aeration rate 3.2.2. 曝气率

Aeration is an important factor in the treatment of pollutants, and appropriate aeration can improve the pollutant removal rate and solve the hardening problem of iron carbon filler [37]. However, excessive aeration affects the formation of the micro battery [38]; therefore, the effect of aeration rate on sulfate removal efficiency was investigated in this study. The reaction time was 120 min and the other reaction conditions were the same as in the previous experiment. The experimental results are presented in Fig. 4(b), in which the sulfate removal rate gradually increases and then decreases as the aeration rate increases. At 0.3 m³/h, the sulfate removal rate reached its highest value (53.39%). When the aeration rate was < 0.3 m³/h, the sulfate removal rate gradually increased. This was mainly because the dissolved oxygen in the wastewater and mass transfer coefficient increased as the aeration rate increased, so that contact between the sulfate and the iron carbon filler became sufficient [50], [51]. However, excessive aeration aggravates the separation of iron and carbon fillers, which is harmful to the formation of micro battery reactions [52]; therefore, the sulfate removal rate gradually decreased. Although the sulfate removal efficiency was highest when the aeration rate was 0.3 m³/h, the filler was prone to hardening because it was at a standstill at this speed [37], which was solved when the aeration rate was 0.5 m³/h. Therefore, after comprehensive consideration, an aeration rate of 0.5 m³/h was used in subsequent experiments.
曝气是处理污染物的重要因素，适当的曝气可以提高污染物去除率，解决铁碳填料的硬化问题[37]。然而，过度曝气会影响微型电池的形成 [38];因此，本研究考察了曝气速率对硫酸盐去除效率的影响。反应时间为 120 min，其他反应条件与上一次实验相同。实验结果如图 4（b）所示，其中硫酸盐去除率随着曝气速率的增加而逐渐增加，然后降低。在 0.3 m³/h 时，硫酸盐去除率达到最高值（53.39%）。当曝气速率< 0.3 m³/h 时，硫酸盐去除率逐渐提高。这主要是因为废水中的溶解氧和传质系数随着曝气速率的增加而增加，因此硫酸盐和铁碳填料之间的接触变得足够[50]、[51]。然而，过度曝气会加剧铁和碳填料的分离，这对微电池反应的形成有害[52];因此，硫酸盐去除率逐渐降低。虽然在曝气速率为 0.3 m³/h 时硫酸盐去除效率最高，但填料在此速度下处于静止状态[37]，当通气速率为 0.5 m³/h 时，解决了这个问题。因此，经过综合考虑，曝气率为 0.在随后的实验中使用 5 m³/h。

3.2.3. pH 3.2.3. pH 值

pH is a common factor affecting the treatment of pollutants, and fluctuates frequently in wastewater [53]. Therefore, the effect of pH on the removal of sulfate by ICE was studied, and the experimental results are shown in Fig. 4(c). The other reaction conditions were the same as in the previous experiment. Fig. 4(c) shows that sulfate removal rate decreased with increasing initial pH value. When the pH of the solution was more than 7, the sulfate removal rate dropped abruptly and then tended to be flat. At pH 2, the sulfate removal rate reached 50.19%; it dropped to 42.46% at pH 7.0, whereas, the sulfate removal rate was only 33.13% at pH 10. It can be seen that pH had a considerable influence on the sulfate treatment effect. This is because when the pH was low, the solution was more acidic, the iron corrosion rate was more rapid, and the potential difference of the micro battery was larger, which greatly enhanced the treatment effect [54]. With an increase in pH, the potential difference of the micro battery decreased, and the reaction slowed, causing the sulfate removal efficiency to decrease [55]. When the pH is greater than 7, Fe²⁺ precipitates easily, which weakens its reducibility [56]. A pH of 2 was used in subsequent experiments.
pH 值是影响污染物处理的常见因素，在废水中经常波动 [53]。因此，研究了 pH 值对 ICE 去除硫酸盐的影响，实验结果如图 4（c）所示。其他反应条件与上一个实验相同。图 4（c）显示硫酸盐去除率随着初始 pH 值的增加而降低。当溶液的 pH 值大于 7 时，硫酸盐去除率急剧下降，然后趋于平稳。pH 2时，硫酸盐去除率达到50.19%;在 pH 7.0 时下降到 42.46%，而在 pH 10 时硫酸盐去除率仅为 33.13%。由此可见，pH 值对硫酸盐处理效果有相当大的影响。这是因为当 pH 值较低时，溶液的酸性更强，铁腐蚀速度更快，微电池的电位差更大，大大增强了处理效果 [54]。随着 pH 值的增加，微型电池的电位差减小，反应减慢，导致硫酸盐去除效率降低 [55]。当 pH 值大于 7 时，Fe²⁺ 容易沉淀，从而削弱其还原性 [56]。在随后的实验中使用 pH 值 2。

3.2.4. Iron-carbon ratio 3.2.4. 铁碳比

The influence of the iron-carbon ratio on sulfate treatment by ICE was carried out, and the experimental results are given in Fig. 4(d). The pH was 2, and the other reaction conditions were the same as in the previous experiment. As shown in Fig. 4(d), with an increase in the iron-carbon ratio, the sulfate removal rate gradually increased and then decreased. When the iron–carbon ratio was 1:1, the removal effect was the highest and the removal rate was 50.19%. This was because when the iron–carbon ratio was 1:1, the iron scraps and activated carbon could be in full contact, and the number of primary batteries formed in the reaction system reached a maximum, which promoted the reaction [42], [43]. However, excess iron scraps or activated carbon would lead to insufficient contact between iron scraps and activated carbon, and the number of primary batteries formed would be lower, resulting in a reduction in the removal rate [42]. The iron-carbon ratio was determined to be 1:1 in subsequent experiments.
进行了铁碳比对 ICE 硫酸盐处理的影响，实验结果如图 4（d）所示。pH 值为 2，其他反应条件与上一个实验相同。如图 4（d）所示，随着铁碳比的增加，硫酸盐去除率逐渐增加，然后降低。当铁碳比为 1：1 时，去除效果最高，去除率为 50.19%。这是因为当铁碳比为 1：1 时，铁屑和活性炭可以完全接触，反应体系中形成的一次电池数量达到最大值，从而促进了反应 [42]、[43]。然而，过多的铁屑或活性炭会导致铁屑和活性炭之间的接触不足，形成的一次电池数量会减少，从而导致去除率降低[42]。在随后的实验中，铁碳比被确定为 1：1。

3.2.5. Initial sulfate concentration
3.2.5. 初始硫酸盐浓度

Sulfate concentrations in different industrial wastewaters often differ. To determine the applicable conditions of the ICE process, it was necessary to study the effect of the initial sulfate concentration on the sulfate removal rate. The reaction conditions were: T = 20–25 °C, reaction time = 120 min, aeration rate = 0.5 m³/h, Fe/C = 1:1, and pH = 2; the results are presented in Fig. 4(e). The sulfate removal rate decreased with an increase in the initial sulfate concentration, and the removal rate was 76.61% at an initial sulfate concentration of 500 mg/L. Although the absolute amount of sulfate removal was up to 1072 mg/L, the removal rate was only 10.72% at an initial sulfate concentration of 10,000 mg/L. The removal rate could be maintained at greater than 50% at an initial sulfate concentration of 1,000 mg/L. Thus, it can be seen that the ICE process is more suitable for treating wastewater with sulfate concentrations of < 1 000 mg/L.
不同工业废水中的硫酸盐浓度通常不同。为了确定 ICE 工艺的适用条件，有必要研究初始硫酸盐浓度对硫酸盐去除率的影响。反应条件为：T = 20–25 °C，反应时间 = 120 min，曝气速率 = 0.5 m³/h，Fe/C = 1：1，pH = 2;结果如图 4（e）所示。硫酸盐去除率随初始硫酸盐浓度的增加而降低，在初始硫酸盐浓度为 500 mg/L 时，去除率为 76.61%。虽然硫酸盐去除的绝对量高达 1072 mg/L，但在初始硫酸盐浓度为 10,000 mg/L 时，去除率仅为 10.72%。在初始硫酸盐浓度为 1,000 mg/L 时，去除率可保持在 50% 以上。由此可见，ICE 工艺更适合处理硫酸盐浓度< 1 000 mg/L 的废水。

3.3. Operational stability of the ICE process
3.3. ICE 流程的运行稳定性

To investigate the reusability of iron-carbon fillers in the ICE process, a 20-day continuous operation test was carried out; the results are shown in Fig. 5. The removal rate of SO₄^2- for an initial concentration of 1000 mg/L at pH 2 gradually decreased as the running time increased. This may have been due to iron consumption at this low pH value, inducing a reduction in the number of micro-batteries. According to the measurement of dissolved iron ions, the amount of iron consumed in 5 days was ∼ 5 g. Therefore, 5 g of fresh iron was added into the reactor every 5 days. From the experimental results, the removal rate of SO₄^2- almost returns to the original level after adding new iron. To shorten the frequency of adding iron while investigating low concentration conditions, the initial concentration of SO₄^2- and the pH value were controlled at 500 mg/L and 2, respectively; other conditions were the same as the above experiment. The experimental results show that within the operating time of 20 days, the removal rate of SO₄^2- was maintained between 65% and 70% without adding iron. Therefore, when treating actual wastewater, appropriate operating conditions need to be selected according to the wastewater performance and treatment goals.
为了研究铁碳填料在 ICE 工艺中的可重复使用性，进行了 20 天的连续运行测试;结果如图 5 所示。在 pH 2 下，初始浓度为 1000 mg/L 时，SO₄^2- 的去除率随着运行时间的增加而逐渐降低。这可能是由于在如此低的 pH 值下消耗铁，导致微型电池的数量减少。根据溶解铁离子的测定，5 天内消耗的铁量为 ∼ 5 g。因此，每 5 天向反应器中加入 5 g 新鲜铁。从实验结果来看，加入新铁后，SO₄^2- 的去除率几乎恢复到原来的水平。为了在研究低浓度条件下缩短添加铁的频率，SO₄^2- 的初始浓度和 pH 值分别控制在 500 mg/L 和 2;其他条件与上述实验相同。实验结果表明，在20 d的操作时间内，SO₄^2-的去除率维持在65%-70%之间，不加铁。因此，在处理实际废水时，需要根据废水性能和处理目标选择合适的操作条件。

3.4. Mechanism experiment
3.4. 机制实验

3.4.1. Characterization of iron sludge generated in the reaction
3.4.1. 反应中产生的铁污泥的表征

SEM was used to visually observe the change in morphology of the iron sludge produced in the blank experiment (activated carbon does not adsorb sulfate in advance, the reaction solution does not contain sulfate, and the pH value was adjusted using hydrochloric acid; other conditions were the same as those for the sulfate treatment experiment) and the sulfate treatment experiment. From Fig. 6(a), the iron sludge had a regular sheet structure. However, the surface of the iron sludge (Fig. 6(b)) was rough, the shape was irregular, and the sheet structure was greatly reduced, indicating that the addition of sulfate affected the shape and structure of the iron sludge. In addition, Fig. 6(b) also shows that there were many small grains on the surface in some sheet structures, which may have been new products formed after sulfate treatment.
采用 SEM 目测观察空白实验（活性炭不预先吸附硫酸盐，反应液不含硫酸盐，用盐酸调节 pH 值;其他条件与硫酸盐处理实验相同）和硫酸盐处理实验产生的铁污泥的形态变化。从图 6（a）中可以看出，铁污泥具有规则的片状结构。然而，铁污泥的表面粗糙 [图 6（b）] 形状不规则，片状结构大大减少，表明硫酸盐的添加影响了铁污泥的形状和结构。此外，图 6（b）还显示，在一些片状结构中，表面有许多小颗粒，这可能是硫酸盐处理后形成的新产品。

To investigate the changes in elemental compositions, the two types of iron sludge were characterized by EDS. As shown in Fig. 7, the iron sludge mainly comprised oxygen, iron, carbon, and sulfur. Comparing Fig. 7(a) and 7(b), the peak height and area of oxygen decreased, iron remained basically unchanged, and sulfur increased. As shown in Table 1, the atomic percentage of sulfur only increased from 0.11% to 0.55%, which was not significant. Considering the error in EDS analysis, XPS, XRD, and Raman spectroscopy were used to further determine elemental changes.
为了研究元素组成的变化，采用 EDS 对两种类型的铁污泥进行了表征。如图 7 所示，铁污泥主要由氧、铁、碳和硫组成。对比图 7（a）和 7（b），氧的峰高和面积降低，铁基本保持不变，硫增加。如表 1 所示，硫的原子百分比仅从 0.11% 增加到 0.55%，这并不显着。考虑到 EDS 分析中的误差，使用 XPS、XRD 和拉曼光谱进一步确定元素变化。

Table 1. Elemental contents in iron sludge analyzed by energy-dispersive spectroscopy (EDS).
表 1.通过能量色散光谱（EDS）分析铁污泥中的元素含量。

Element 元素	Iron sludge produced in blank experiment 空白实验中产生的铁泥		Iron sludge produced in sulfate treatment experiment 硫酸盐处理实验产生的铁泥
Element 元素	Weight (%) 重量（%）	Atomic (%) 原子（%）	Weight (%) 重量（%）	Atomic (%) 原子（%）
O	53.27	79.52	48.65	77.87
Fe 铁	41.92	17.93	41.23	18.91
C	0.96	1.92	0.59	1.26
Pt 铂	3.57	0.44	8.41	1.1
S	0.15	0.11	0.69	0.55
Cl Cl （四）	0.13	0.08	0.43	0.31

Furthermore, XPS analysis was conducted to confirm the elements on the surfaces of the two types of iron sludge. The XPS analysis spectra, which were calibrated with C 1 s (284.6 eV), are shown in Fig. 8. As shown in Fig. 8(a), oxygen, iron, carbon, and sulfur were observed on the surfaces of both iron sludge samples by XPS. The atomic percentage of iron on the surface increased from 51.06% to 71.62% and that of sulfur increased from 0.09% to 1.14%, as listed in Table 2. This result is similar to the EDS analysis. Meanwhile, the high-resolution spectra of iron, oxygen, and sulfur were fitted, as shown in Fig. 8(b), Fig. 8(c), and Fig. 8(d), respectively. The iron 2p XPS spectra of the iron sludge produced in the blank experiment could be deconvoluted into four peaks (710.47, 712.43, 719.03, and 725.23 eV), and that of the of the iron sludge produced in the sulfate treatment experiment could be deconvoluted into three peaks (711.43, 719.29, and 725.41 eV), all of which were assigned to Fe²⁺ and Fe³⁺ [55], [57], [58]. This is basically the same as the binding energy of iron in lepidocrocite (FeOOH) [59]. In the oxygen 1 s XPS spectra, the binding energy was mainly located at 529.59 and 531.03 eV of the iron sludge produced in the blank experiment, and at 529.81 and 531.18 eV of the iron sludge produced in the sulfate treatment experiment, corresponding to Fe-O and O-H, respectively [60], [61]; this confirmed the existence of FeOOH in the iron sludge. As illustrated in Fig. 8(d), characteristic peaks of SO₃^2- (168.39 eV), SO₄^2- (165.06 eV), and S^2- (162.38 eV) were observed in the iron sludge produced in the blank experiment, and the characteristic peaks at 168.17, 164.08, and 161.67 eV also revealed the existence of SO₃^2-, SO₄^2-, and S^2-, respectively, in the iron sludge produced in the sulfate treatment experiment. The peak area increased after sulfate treatment, proving that sulfate would form a precipitate that could be removed by flocculation and oxidation–reduction.
此外，还进行了 XPS 分析以确认两种铁污泥表面的元素。使用 C 1 s （284.6 eV）校准的 XPS 分析谱如图 8 所示。如图 8（a）所示，XPS 在两个铁污泥样品的表面观察到氧、铁、碳和硫。表面铁的原子百分比从 51.06% 增加到 71.62%，硫的原子百分比从 0.09% 增加到 1.14%，如表 2 所示。此结果类似于 EDS 分析。同时，拟合了铁、氧和硫的高分辨率光谱，分别如图 8（b）、图 8（c）和图 8（d）所示。空白实验产生的铁污泥的2p XPS谱图可以解卷积为4个峰（710.47、712.43、719.03和725.23 eV），硫酸盐处理实验产生的铁污泥的2p XPS谱图可以解卷积为3个峰（711.43、719.29和725.41 eV），均被分配给Fe²⁺和Fe³⁺[55]， [57]，[58]。这与铁在鳞片石中的结合能（FeOOH）基本相同 [59]。在氧 1 s XPS 谱图中，结合能主要位于空白实验产生的铁泥的 529.59 和 531.03 eV，以及 529.81 和 531 eV。硫酸盐处理实验中产生的铁污泥为 18 eV，分别对应于 Fe-O 和 O-H [60]、[61];这证实了铁污泥中存在 FeOOH。如图 8（d）所示，在空白实验产生的铁泥中观察到 SO₃^2- （168.39 eV）、SO₄^2- （165.06 eV）和 S^2- （162.38 eV）的特征峰，在 168.17、164.08 和 161.67 eV 处的特征峰也揭示了 SO₃^2-、SO₄^2- 的存在。和 S^2-，分别在硫酸盐处理实验产生的铁泥中。硫酸盐处理后峰面积增加，证明硫酸盐会形成沉淀物，可通过絮凝和氧化还原去除。

Table 2. Elemental contents in iron sludge analyzed by X-ray photoelectron spectroscopy (XPS).
表 2.通过 X 射线光电子能谱（XPS）分析铁污泥中的元素含量。

Element 元素	Iron sludge produced in blank experiment 空白实验中产生的铁泥		Iron sludge produced in sulfate treatment experiment 硫酸盐处理实验产生的铁泥
Element 元素	Peak position 峰位置	Atomic (%) 原子（%）	Peak position 峰位置	Atomic (%) 原子（%）
Fe 铁	711.33	51.06	711.72	71.62
O	530.80	48.85	529.81	27.24
S	165.83	0.09	168.17	1.14

To further verify the formation of different substances in the two types of iron sludge, XRD was used to analyze the samples, as shown in Fig. 9. Both types of iron sludge had obvious peaks at 14.08° (2θ), 27.10° (2θ), 36.31° (2θ), and 46.90° (2θ), which are characteristic reflections of FeOOH (PDF08-0098 and PDF44-1415). This is because Fe²⁺ and Fe³⁺ would form Fe(OH)₂ and Fe(OH)₃, respectively, in the reaction, which would be oxidized to FeOOH. At the same time, Fe₂O₃, Fe(OH)₂, and Fe(OH)₃ peaks were observed. In addition, the peak intensity of iron sludge in the sulfate treatment experiment was lower than that in the blank experiment. This shows that sulfate affected FeOOH formation. In contrast, the characteristic peaks of FeS₂ (PDF42-1340) were observed in the sulfate treatment experiment [62] but not in the blank experiment. This indicates that sulfate may have been converted to FeS₂ and reduced to S^2-. Overall, the XRD results are consistent with the XPS results.
为了进一步验证两种铁泥中不同物质的形成，使用 XRD 对样品进行分析，如图 9 所示。两种类型的铁泥在 14.08° （2θ）、 27.10° （2θ）、 36.31° （2θ）和 46.90° （2θ）处都有明显的峰值，这是 FeOOH （PDF08-0098 和 PDF44-1415）的特征反射。这是因为 Fe²⁺ 和 Fe³⁺ 会在反应中分别形成 Fe（OH）₂ 和 Fe（OH）₃，它们将被氧化成 FeOOH。同时观察到 Fe₂O₃ 、 Fe（OH）₂ 和 Fe（OH）₃ 峰。此外，硫酸盐处理实验中铁泥的峰值强度低于空白实验。这表明硫酸盐影响了 FeOOH 的形成。相比之下，在硫酸盐处理实验中观察到 FeS₂ （PDF42-1340）的特征峰 [62]，但在空白实验中没有观察到。这表明硫酸盐可能已经转化为 FeS₂ 并还原为 S^2-。总体而言，XRD 结果与 XPS 结果一致。

Raman spectroscopy has been widely used to study the bonding modes and compositions of derivatives [63]. The Raman spectra of the two types of iron sludge are shown in Fig. 10. Four characteristic peaks at 210, 277, 390, and 587 cm⁻¹ were assigned to the typical position of FeOOH [64], [65]. The peak intensity of iron sludge in the sulfate treatment experiment was lower than that in the blank experiment, which was similar to the XRD results. Two characteristic peaks at 490 and 663 cm⁻¹ were identified as fougerite [66] and Fe₃O₄ [65], respectively, in the iron sludge produced in the blank experiment, while the peaks disappeared in the sulfate treatment experiment, which may have been because sulfate affected their formation.
拉曼光谱已被广泛用于研究衍生物的键合模式和组成[63]。两种铁污泥的拉曼光谱如图 10 所示。在 210、277、390 和 587 cm⁻¹ 处的四个特征峰被分配给 FeOOH 的典型位置 [64]、[65]。硫酸盐处理实验中铁污泥的峰值强度低于空白实验，与 XRD 结果相似。在空白实验产生的铁泥中，分别在 490 和 663 ^cm-1 处鉴定出两个特征峰，分别为辉石 [66] 和 Fe₃O₄[65]，而在硫酸盐处理实验中，这些峰消失了，这可能是因为硫酸盐影响了它们的形成。

3.4.2. Detection of reaction products in solution and gas
3.4.2. 溶液和气体中反应产物的检测

To further study the sulfate removal mechanism, the reaction products were detected and analyzed. First, the pH and iron concentration were determined, as shown in Fig. 11(a). The iron concentration first increased and then decreased as time increased. The iron concentration increased rapidly before 5 min (i.e., from 0.29 to 192.83 mg/L), which was mainly due to the low pH and high acidity at the initial stage of the reaction, which accelerated iron corrosion [54]. This trend also explains why the sulfate removal rate increased more rapidly during the initial stage of the reaction. After 5 min, the iron concentration decreased. At 5 min, the pH was 5.39, and Qc = C(Fe³⁺) × C(OH^–)³ ≈ 2.8 × 10⁻¹⁴ > K_sp(Fe[OH]₃) = 1.1 × 10⁻³⁶. Therefore, Fe(OH)₃ flocs was formed and the consumption rate was greater than the generation rate, leading to a decrease in the iron concentration.
为了进一步研究硫酸盐去除机制，对反应产物进行了检测和分析。首先，测定 pH 值和铁浓度，如图 11（a）所示。铁浓度随时间增加呈先升高后降低的趋势。铁浓度在 5 min 前迅速增加（即从 0.29 增加到 192.83 mg/L），这主要是由于反应初期的低 pH 值和高酸度，加速了铁的腐蚀 [54]。这一趋势也解释了为什么硫酸盐去除率在反应的初始阶段增加得更快。5 min后，铁浓度降低。5 分钟时，pH 值为 5.39，Qc = C（Fe³⁺） × C（OH^–）³ ≈ 2.8 × 10⁻¹⁴ > K_sp（Fe[OH]₃） = 1.1 × 10⁻³⁶。因此，形成了 Fe（OH）₃ 絮凝体，消耗速率大于生成速率，导致铁浓度降低。

Then, in addition to SO₄^2-, the SO₃^2- and S^2- concentrations in the solution were determined, as shown in Fig. 11(b). The SO₄^2- concentration in solution showed a gradually decreasing trend with increasing time. However, the S^2- and SO₃^2- concentrations in solution showed gradually increasing trends with increasing time. This indicates that sulfate could initially be reduced to SO₃^2- and then to S^2-. Since S^2- easily combines with H⁺ to form H₂S gas, after the tail gas was absorbed by the alkali solution, the S^2- concentration was determined. The mass balance of sulfur in gas phase, liquid phase, and mud was carried out, as shown in Fig. 11(c). The sulfur content in the mud was obtained from the total sulfur content minus the sulfur content in the gas and liquid phases. The sulfur content in the liquid phase included SO₄^2-, SO₃^2-, and S^2-. It can be seen from Fig. 11(c) that the removed sulfur mainly entered the mud and rarely entered the gas phase.
然后，除 SO₄^2- 外，还测定溶液中的 SO₃^2- 和 S^2- 浓度，如图 11（b）所示。溶液中 SO₄^2- 浓度随时间的增加呈逐渐降低的趋势。然而，溶液中 S^2- 和 SO₃^2- 浓度随时间的增加而逐渐增加。这表明硫酸盐最初可以还原为 SO₃^2-，然后还原为 S^2-。由于 S^2- 易与 H⁺ 结合形成 H₂S 气体，因此尾气被碱溶液吸收后，测定 S^2- 浓度。进行气相、液相和泥浆中硫的质量平衡，如图 11（c）所示。泥浆中的硫含量是由总硫含量减去气相和液相中的硫含量得出的。液相中的硫含量包括 SO₄^2-、SO₃^2- 和 S^2-。从图 11（c）中可以看出，脱除的硫主要进入泥浆，很少进入气相。

3.5. Sulfate removal pathways by the ICE process
3.5. ICE 工艺去除硫酸盐的途径

Based on the above experimental results and reactor performance, the main sulfate removal pathways by the ICE process were identified. The ICE reactor had been shown to have dual functions of oxidation and reduction in our previous studies [46]. Sulfate may initially be reduced to SO₃^2- (

E^{Î¸} ({SO}_{4}^{2-} / {SO}_{3}^{2-}) = - 0.93 V

), then to S^2- (

E^{Î¸} ({SO}_{3}^{2-} / S^{2-}) = - 0.59 V

) [67], and S^2- combines with H⁺ and Fe²⁺ to form H₂S and FeS, which can be removed [27], [68]. In addition, as the reaction proceeds, Fe(OH)₂ and Fe(OH)₃ flocs can be produced in the solution, and sulfate can be removed by flocculation and precipitation of Fe(OH)₂ and Fe(OH)₃ [69]. The possible reactions are shown in equations (6), (7), (8), (9), (10), (11), (12).(6)

Fe - 2 e^{-} \to {Fe}^{2 +}

(7)

2 H^{+} + 2 e^{-} \to [H] \to H_{2}

(8)

O_{2} + 2 H_{2} O + 4 e^{-} \to 4 {OH}^{-}

(9)

{SO}_{4}^{2 -} + 2 e^{-} + H_{2} O \to {SO}_{3}^{2 -} + 2 {OH}^{-} E^{θ} ({SO}_{4}^{2 -} / {SO}_{3}^{2 -}) = - 0.93 V

(10)

{SO}_{3}^{2 -} + 6 e^{-} + 6 H^{+} \to S^{2 -} + 3 H_{2} O E^{θ} ({SO}_{3}^{2 -} / S^{2 -}) = - 0.59 V

(11)

S^{2 -} + 2 H^{+} \to H_{2} S ↑

(12)

S^{2 -} + {Fe}^{2 +} \to F e S ↓

(13)

{4 F e}^{2 +} + 8 {OH}^{-} + O_{2} + 2 H_{2} O \to 4 F e {(O H)}_{3} (f l o c s)

基于上述实验结果和反应器性能，确定了 ICE 工艺去除硫酸盐的主要途径。在我们之前的研究中，ICE反应器已被证明具有氧化和还原的双重功能[46]。硫酸盐最初可还原为 SO₃^2- （

E^{Î¸} ({SO}_{4}^{2-} / {SO}_{3}^{2-}) = - 0.93 V

），然后还原为 S^2- （

E^{Î¸} ({SO}_{3}^{2-} / S^{2-}) = - 0.59 V

） [67]，S^2- 与 H⁺ 和 Fe²⁺ 结合形成 H₂S 和 FeS，可以被去除 [27]、[68]。此外，随着反应的进行，溶液中可以产生 Fe（OH）₂ 和 Fe（OH）₃ 絮凝体，并且可以通过 Fe（OH）₂ 和 Fe（OH）₃ 的絮凝沉淀去除硫酸盐[69]。可能的反应如方程（6）、（7）、（8）、（9）、（10）、（11）、（12）所示。 (6)

Fe - 2 e^{-} \to {Fe}^{2 +}

(7)

2 H^{+} + 2 e^{-} \to [H] \to H_{2}

(8)

O_{2} + 2 H_{2} O + 4 e^{-} \to 4 {OH}^{-}

(9)

{SO}_{4}^{2 -} + 2 e^{-} + H_{2} O \to {SO}_{3}^{2 -} + 2 {OH}^{-} E^{θ} ({SO}_{4}^{2 -} / {SO}_{3}^{2 -}) = - 0.93 V

(10)

{SO}_{3}^{2 -} + 6 e^{-} + 6 H^{+} \to S^{2 -} + 3 H_{2} O E^{θ} ({SO}_{3}^{2 -} / S^{2 -}) = - 0.59 V

(11)

S^{2 -} + 2 H^{+} \to H_{2} S ↑

(12)

S^{2 -} + {Fe}^{2 +} \to F e S ↓

(13)

{4 F e}^{2 +} + 8 {OH}^{-} + O_{2} + 2 H_{2} O \to 4 F e {(O H)}_{3} (f l o c s)

4. Conclusion 4. 结论

The ICE process is feasible for sulfate wastewater treatment, and the sulfate removal rate reaches greater than 50% when the initial sulfate concentration is 1000 mg/L. The effects of the reaction time, aeration rate, pH, iron-carbon ratio, and initial sulfate concentration on sulfate removal by ICE were investigated. The maximum sulfate removal efficiency was 76.6% with a reaction time of 120 min, aeration rate of 0.5 m³/h, pH of 2, iron-carbon ratio of 1:1, and initial sulfate concentration of 500 mg/L. Meanwhile, SEM, EDS, XPS, XRD, and Raman spectroscopy analysis results show that sulfate may have initially been reduced to SO₃^2-, and then to S^2-, which combined with H⁺ and Fe²⁺ to form H₂S and FeS, respectively. In addition, sulfate can be removed by flocculation and precipitation of Fe(OH)₂ and Fe(OH)₃. In conclusion, the ICE process may be an effective method to readily remove sulfate from wastewater.
ICE工艺对硫酸盐废水处理是可行的，当初始硫酸盐浓度为1000 mg/L时，硫酸盐去除率达到 50%以上。研究了反应时间、曝气速率、pH 值、铁碳比和初始硫酸盐浓度对 ICE 去除硫酸盐的影响。反应时间为 120 min，曝气速率为 0.5 m³/h，pH 值为 2，铁碳比为 1：1，初始硫酸盐浓度为 500 mg/L，硫酸盐去除效率最高为 76.6%。同时，SEM、EDS、XPS、XRD和拉曼光谱分析结果表明，硫酸盐最初可能被还原为SO₃^2-，然后还原为S^2-，与H⁺和Fe²⁺结合形成H₂S和FeS。此外，可以通过 Fe（OH）₂ 和 Fe（OH）₃ 的絮凝和沉淀去除硫酸盐。总之，ICE 工艺可能是从废水中轻松去除硫酸盐的有效方法。

CRediT authorship contribution statement
CRediT 作者贡献声明

Yanhe Han: Conceptualization, Writing – review & editing, Supervision, Funding acquisition. Chuantao Wu: Methodology, Investigation, Formal analysis, Writing – original draft. Xiaolu Fu: Investigation, Formal analysis. Zhimin Su: Investigation, Formal analysis. Meili Liu: Writing – review & editing.
韩燕和：概念化、写作 – 审查和编辑、监督、资金获取。吴传涛：方法论、调查、形式分析、写作 - 原稿。傅小璐：调查，正式分析。苏志敏：调查，正式分析。刘梅丽：写作 - 审查和编辑。

Declaration of Competing Interest
利益争夺声明

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.
作者声明，他们没有已知的竞争性经济利益或个人关系，这些利益或个人关系似乎可能会影响本文报告的工作。

Acknowledgments 确认

This work was supported by the National Natural Science Foundation of China [grant number 21677018, 21908008] and the Jointly Projects of Beijing Natural Science Foundation and Beijing Municipal Education Commission [grant number KZ201810017024].
这项工作得到了中国国家自然科学基金 [授权号 21677018， 21908008] 以及北京自然科学基金和北京市教育委员会联合项目 [授权号 KZ201810017024] 的支持。

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