Wet removal of elemental mercury by acid-assisted electrochemical oxidation method

ZHANG Qian-qian ，ZHANG An-chao ，MENG Fan-mao ，LIU Yan-wen ，SUN Zhi-jun ，LI Hai-xia ，ZHENG Hai-kun

（School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo 454003, China）

Abstract: As a global pollutant, mercury emission is increasingly restricted in recent years. It is urgent to explore a new and efficient mercury removal technology for coal-fired power plants. A new acid-assisted electrochemical oxidation (AEO)technique for mercury removal was proposed using platinum plate as cathode and fluorine-doped tin dioxide (FTO) glass as anode. The effects of acid type, acid concentration, applied direct current (DC) voltage, electrolyte type, SO2, NO and O2 on the Hg0 removal efficiency were carried out. The results indicated that the mercury removal efficiency increased with the increase of DC voltage and nitric acid concentration. When the concentration of nitric acid increased to 0.15 mol/L, the mercury removal efficiency remained unchanged. SO2 and NO inhibited the removal of Hg0 in AEO system, but the inhibition was reversible. Compared with the mercury removal efficiency under single experimental conditions, the mercury removal efficiency of electrochemical oxidation can reach 96% under the experimental conditions of 0.1 mol/L nitric acid and 4V DC voltage, suggesting that the synergistic effect of nitric acid and DC voltage plays a key role. According to the experimental results, the mechanism of Hg0 removal in AEO system was analyzed. At the anode, Hg0 was oxidized by hydroxyl radical(?OH) generated by the oxidation reaction on the anode surface. At the cathode, dissolved oxygen or O2 adsorbed on the surface of Pt is reduced to form anionic superoxide radicals. Moreover, parts of would produce ?OH with the aid of electron at acidic condition. Free radicals capture experiments showed that and ?OH were the main active substances for the removal of Hg0 by acid-assisted electrochemical method. The research is helpful for the development of effective electrochemical techniques for industrial mercury removal and recycling of industrial acid waste.

Key words: elemental mercury；wet removal；waste acid；electrochemical oxidation.

Awful mercury environmental troubles have drawn the worldwide attentions on account of their potential threats to humanity[1]. Coal-fired flue gas is a major anthropogenic source of Hg0pollution,accounting for around 35% of the general emission[2].Mercury pollution has already turned into a terrible social matter owing to its toxicity and high volatility extremely[3]. Consequently, it is hanging over our head to remove the highly recalcitrant mercury from environment. It is known that mercury normally appears in three forms in tail flue gas: elemental mercury (Hg0), oxidized mercury (Hg2+) and particulate-bound mercury (Hgp)[4,5]. Hg2+and Hgpcould be simply captured using conventional equipment by means of wet flue gas desulfurization (WFGD) and electrostatic precipitators (ESP) or fabric filter (FF)[6-9],respectively. However, Hg0is difficult to be removed because of its water insoluble and chemical inertness.

In recent decades, numerous researchers have dedicated significant energies to explore diverse methods to remove Hg0, for instance physical adsorption, catalysis oxidation, photocatalytic oxidation, and Fenton-like reagent oxidation[10-14]. Xiao et al.[15]reported that MnOx-CeO2/γ-Al2O3catalyst exhibited higher Hg0oxidation efficiency. Zhou et al.[16]synthesized heterogeneous Fenton-like catalyst Fe3-xCuxO4by chemical co-precipitation method to remove Hg0, and found that about 96.6% of Hg0was removed under the optimum conditions. Comparing with the physical adsorption, oxidation method showed a greater capacity on Hg0removal. In general, the oxidation technologies were classified further into nonradical oxidation and active radical oxidation?OH) on the basis of different reaction principles[2,17]. It was reported that non-radical oxidation was preferable to remove Hg0, but its activity is lower than the active radical oxidation[18,19]. Zhou et al.[4]confirmed that Hg0could be removed efficiently by reactive species ofand h+in the visible light irradiation system. Cheng et al.[20]proved that the active species?OH was significant on the removal of Hg0in the TiO2/Bi5O7I composite photocatalytic oxidation process. The results exhibited that the active radical oxidation had significant advantages and prospects for Hg0removal compared with conventional oxidation. Based on the above considerations, how to remove Hg0economically by a simple active radical oxidation method has become a focus problem.

The electrochemical oxidation (EO) technology,being one of advanced oxidation processes (AOPs), has been widely employed to solve contaminants as a result of its convenience, operability, outstanding oxidation ability and rich active substance?OH[21]. The EO method could yield oxidants via below mechanisms:electrons transfer from the anode surface to the pollutants (P) (see Eq. (1)), that is direct electron transfer, and anode surface (A) can generate hydroxyl radicals A (?OH) using water oxidation (see Eq. (2))[22,23],after that A (?OH) desorb into aqueous solution in the form of?OH then oxidize contaminants.

Nidheesh et al.[24]employed graphite electrodes to degrade wastewater by electrochemical oxidation method, and the color abatement efficiency was up to 99.8%. In comparison with graphite felt as cathode,Rajasekhar et al.[23]synthesized Ti/Sb-SnO2/PbO2anode to degrade petroleum hydrocarbons, and identified out that?OH radical was vital under the situation of treating petroleum-contaminated waters. However, rarely literatures were reported on the electrochemical oxidation removal of gaseous Hg0. Recently, reactive?OH was utilized to remove gaseous Hg0. For instance,Zhou et al.[25]studied the Hg0removal performance using H2O2over Fe3O4, which confirmed a significant role of?OH in the Hg0oxidation process. Liu et al.[26]found that the yield?OH radicals from Fe2+activating H2O2in acid solution could oxidize Hg0to Hg2+. Thus it is conjectured the method of EO would be beneficial to Hg0removal under a specific environment.

To date, large quantities of waste acid were discharged by the industries such as pharmaceutical,hydrometallurgy, titanium dioxide production process and so on[27]. The direct discharge of waste acid was prohibited according to the environmental legislation of China[28]. Therefore, it is urgent to develop an economic and effective method to resolve the waste acid.Fortunately, some researchers found that acid condition would benefit to the treatment of pollutions. Govindan et al.[29]reported that acidic solution was more advantageous for the PCP degradation. Midassi et al.[30]performed a chloroquine degradation experiment and found 92% of total organic carbon removal and the fully chloroquine depletion in the case of pH = 3.0.Consequently, considering acid condition could enhance reaction activity, it was expected to obtain an EO method by combining acid solution for mercury removal from coal fired flue gas.

In view of the above discussion, a novel acidassisted electrochemical oxidation technology (AEO)system was designed to remove Hg0using a Pt sheet as cathode, a conductive fluorine-doped tin dioxide (FTO)glass as anode in Na2SO4electrolyte solution. The electrochemical performance was evaluated comprehensively at different operating conditions such as acid types, voltage, the kind of electrolyte, O2, SO2and NO. According to the above experiment analysis,the mechanism of mercury removal was proposed.

1 Materials and methods

1.1 Chemicals

The electrolytes (Na2SO4, NaNO3, Na2CO3, NaCl,Na3PO4·12H2O), acid reagents (HNO3(65%-68%),H2SO4(97%-98%), acetic acid (CH3COOH) (99.2%),citric acid (C6H8O7), oxalic acid (H2C2O4)),benzoquinone (BQ), and isopropanol (IPA) were used without further purification. Both of them are analytical reagents. Distilled water was used throughout the experiments.

1.2 Electrochemical oxidization test for Hg0 removal

The electrochemical oxidation experiment for Hg0removal was conducted on an electrochemical system[31], as shown in Figure 1. The flue gas simulation system was comprised of 12% CO2, 6% O2,balanced N2carrying around 50 μg/m3of Hg0,150 mg/m3SO2(when used), and 70 mg/m3of NO(when used) with a total gas flow of 1.3 L/min. The electrochemical oxidation reaction of Hg0proceeded in an electrochemical reactor (500 mL), in which the anode was a fluorine-doped tin dioxide (FTO, 20 mm ×20 mm × 2 mm) glass, a platinum sheet (Pt, 25 mm ×25 mm × 0.1 mm) as the cathode, and the distance between the anode and the cathode was 25 mm[31]. In the experiment, if there was no special specification,0.1 mol/L of HNO3, 4 V of direct current (DC) voltage,0.05 mol/L of Na2SO4electrolyte, and 300 mL of reaction solution were employed to remove Hg0at ambient temperature. The real-time concentration of Hg0was recorded by an online RA-915 M mercury analyzer (LUMEX Mercury Instruments, Russia). The efficiency (η) of Hg0removal was calculated by the following formula (3):

Figure 1 Schematic diagram of the electrochemical system for Hg0 removal

whereCinstands for the initial Hg0concentration(μg/m3) of the simulated flue gas andCterdenotes the termination concentration (μg/m3) of Hg0after electrochemical reaction.

2 Results and discussion

2.1 Synergistic effect between acid and electrochemical on Hg0 removal

Figure 2 shows that the performance of Hg0removal under different experimental conditions. It was observed that about 3% of Hg0removal efficiency appeared in the presence of 0.1 mol/L nitric acid(HNO3) alone as well as 4 V of DC voltage alone,demonstrating that independent nitric acid and individual DC voltage showed a faintly effect on the demercuration reaction.

Figure 2 Hg0 removal efficiencies under different experimental conditions (a), the corresponding linear fitted pseudo-first order plots(b), and the synergistic effect of nitric acid and DC voltage on Hg0 removal (c)

However, almost 96% of Hg0was removed when 4 V of DC voltage and 0.1 mol/L of HNO3were employed simultaneously, implying an excellent synergistic effect between acid and electrochemical in the process of Hg0removal[32]. The pseudo-first-order kinetics of -ln(Cout/Cin) vs. reaction time (t) were calculated in the initial reaction stage (Figure 2(b)). It was shown that the Hg0removal rate constant (k) under cooperative condition was 0.266 min-1, greatly higher than that under sole nitric acid or electrochemical condition. To validate the synergistic effect between nitric acid and electrochemical, two experiments were carefully designed as depicted in Figure 2(c). In the first 30 min, Hg0removal test was carried out in presence of 0.1 mol/L nitric acid, afterwards 4 V of DC power supply was turned on for another 30 min. A similar experiment was also implemented by changing the order of HNO3and DC power supply employed. It was obvious that superior Hg0removal performance appeared in the process of HNO3-assisted electrochemical oxidation.

2.2 Effects of acid types and the concentration of nitric acid

As is well-known, waste acid solution usually contained various types[27]. Thus, the effectiveness of different types of acid on Hg0removal was examined.From Figure 3(a), it was evident that oxalic acid, citric acid and acetic acid possessed little effect on Hg0removal in the AEO process, and their Hg0removal efficiencies were all below 5%. Under the same environment (0.1 mol/L hydrogen ion concentration),nitric acid, sulfuric acid could remove around 96% and 79% of Hg0in the AEO process, respectively. The curves of pseudo-first-order kinetics for different types of acid were exhibited in Figure 3(b). Thekvalues of nitric acid and sulfuric acid system were 0.266 and 0.208 min-1, respectively, which were bigger than those of other acid reaction processes. It could be seen clearly that strong acid solution was conducive to enhancing performance of electrochemical oxidation on Hg0removal[33].

Figure 3 Hg0 removal efficiencies ((a), (c)) and linear fitted pseudo-first order plots ((b), (d)) under various types of acid and different nitric acid concentrations

Nitric acid imposed a noteworthy influence on Hg0removal as described above, therefore, in what follows the effect of the amount of HNO3on the activity of Hg0removal was conducted. As shown in Figure 3(c), there was a slight Hg0removal efficiency in the condition of 0.025 mol/L HNO3. The performance of Hg0removal improved greatly from 21% to 77% as the amount of HNO3increased from 0.025 to 0.05 mol/L. The mercury removal efficiency was relatively low as the concentration of HNO3was 0.025 mol/L, which might be attributed to the following reasons: (i) The low concentration of acid led to an obvious decline of hydrogen ion, being unfavorable for production of H2O2and?OH radicals[33,34]. (ii) Conductive ions in the solution were less when the concentration of HNO3was lower, thereby leading to a lower conductivity. When the HNO3concentrations increased to 0.1 and 0.15 mol/L, Hg0removal performance climbed to about 96%, indicating that acidifying solution could contribute to generate more reactive oxidant to remove Hg0and a small amount of nitric acid could possess superior performance on producing reactive radical.However, the efficiency of Hg0removal did not continually raise when HNO3concentrations was 0.15 mol/L, which was probably because the reaction solution with strong acidity was not beneficial to the production of H2O2[35]. Accordingly, thekfor Hg0removal process accelerated gradually from 0.067 to 0.278 min-1with the HNO3concentration shifting from 0.025 to 0.15 mol/L (Figure 3(d)).

2.3 Effects of DC voltage and the types of electrolyte

The operating DC voltage was another critical factor in the EO system because the magnitude of electric potential would determine the speed of electron transfer and the generation of active substances.Figure 4(a) illustrates the correlation between Hg0removal efficiency and reaction time at different DC voltages. The performance of Hg0removal was almost zero at the applied voltage of 1 V. At the DC voltage of 2 and 3 V, the Hg0removal performances significantly increased and presented concave shape. With the extension of reaction time, Hg0removal efficiency gradually improved. With the DC voltage increasing to 4 and 5 V, the concave shape disappeared and the Hg0removal efficiency reached 95% and 97%,respectively[31,36]. Tavan et al.[37]also reported a similar phenomenon that the efficiency curve by applying fixed potential of 7.6 V was concave shape, which disappeared at 9.3 V of applied potential. It was true that low voltage would produce small current density,thereby reducing transfer of electron in the solution and reaction rate[36]. While the electrode reaction and generation of radical greatly depended on gain or loss of electrons, so that mercury removal capacity was weak at low voltage. And with time increasing and the accumulation of free radicals, the Hg0removal efficiency increased gradually. At higher voltage, the reactive species would increase quickly, which diffused from electrode to reaction solution[38], therefore leading to a higher activity.

Figure 4 Hg0 removal efficiencies under different DC voltages(a) and different electrolytes (b)

Figure 4(b) shows the performance of Hg0removal in the presences of 0.1 mol/L of HNO3and different electrolytes (Na3PO4, Na2CO3, NaNO3, Na2SO4and NaCl). When Na3PO4, Na2CO3and NaNO3were employed as electrolyte, it was clear that all the Hg0removal efficiencies were lower than 75%. In comparison, nearly 96% of Hg0was removed in the Na2SO4electrolyte solution. Because Na2SO4was strong electrolyte, the electrolytic ability of which was better than NaNO3, Na3PO4and Na2CO3[39-41]. Moreover,the electrochemical system of NaCl could remove almost 100% of Hg0in the absence of nitric acid(Figure 4(b)), which could be correlated to the electrogeneration of active chlorine species (ClO?, Cl2) based on the Volmer-Heyrovsky mechanism, as well as its chain reaction on the anode during the process of the electrochemical oxidation, as following equations(4)-(8)[31,42-44]. The strong oxidizing Cl2(aq)/ClO?species could achieve higher Hg0oxidation removal and the occurred reaction was Eq. (9)[31].

2.4 Effects of SO2, NO and O2

Figure 5 exhibits the variations of Hg0removal efficiency by turning SO2, NO or O2on and off in AEO system under the optimal reaction condition of 0.1 mol/L nitric acid and 4V DC voltage. When pouring into 150 mg/m3SO2at the period of 30-60 min, Hg0removal efficiency severely declined from 94% to 63%, while it gradually increased to 93% in the absence of SO2for 60-90 min (Figure 5(a)).Subsequently, a same phenomenon emerged under the second input of SO2at the time-span of 90-120 min.Moreover, Hg0removal efficiency still increased again to above 95% after turning off SO2gas, indicating that the effect of SO2on Hg0removal in AEO system was inhibitory (Eqs. (10)-(11)) but reversible.

Figure 5 Effects of SO2 (a), NO (b) and O2 (c) on Hg0 removal efficiency

Figure 5(b) shows that the Hg0removal efficiency decreased to 78% and 48% in presence of 70 mg/m3NO, respectively. Fortunately, Hg0removal efficiency gradually ascended to 89% and 91% when the NO was cut off, suggesting that the suppression of NO was also reversible. The inhibitory reason could be due to the reaction of reactive species with NO as follows Eqs.(12)-(16)[31].

Moreover, the effect of O2was also conducted to further explore the AEO system mechanism on Hg0removal. As described in Figure 5(c), the efficiency of Hg0removal could achieve about 70% in absence of external O2. While when 6% of O2was injected to the simulated flue gas, Hg0removal efficiency increased by around 14.7%. The enhancement of Hg0removal efficiency might be ascribed to the increased O2import,which could produce moreto oxidize mercury (Eq.(17))[45].

2.5 Energy consumption

Herein, the electric energy consumption (EEA) was utilized to estimate the energy consumption of electrochemical process.EEAis defined as the electrical energy (in kilowatt hours) consumed to decrease the concentration of pollutant by one order of magnitude(90%) in 1 m3contaminated water, expressed as the following Eq. (18)[46]:

whereUandIstand for the voltage (V) and current(A),tdenotes the electrochemical oxidization time (h),Vdenotes the solution volume (L),CinandCterare the initial and terminal Hg0concentration, respectively.

In the AEO system, theEEAof Hg0removal was calculated in the condition of 4 V of operating voltage,5 mA of working current, 1 h of electrochemical time,0.3 L of solution volume, 50 μg/m3of initial Hg0concentration and 2 μg/m3of terminal Hg0concentration. By calculation, theEEAof AEO system was 4.76×10-2kW·h/m3energy consumption for removing 96% of Hg0. Therefore, the method of electrochemical oxidization could efficiently remove Hg0, and the associated electrical energy consumption was acceptable for its potential commercial application.

2.6 Proposed acid-assisted electrochemical oxidation mechanism

For a better comprehending of the active radicals from AEO system on Hg0removal, the quenching experiments were conducted using BQ (0.2 g), IPA(10 mL) to scavenge the possible reactive radicals such asand?OH, respectively. It was exhibited from Figure 6(a) that the addition of BQ and IPA significantly inhibited the removal of Hg0, revealing that the electrochemical oxidation of Hg0removal were dominantly governed byand?OH species (Eqs.(19)-(20))[15].

Figure 6 (a) Effects of different scavengers on the Hg0 removal, (b) Reaction mechanism of AEO process for Hg0 removal

Based on the above discussion, a reasonable pathway of acid-assisted electrochemical oxidation for Hg0removal was proposed (Figure 6(b)). There were two potential paths for higher demercuration performance. On the anode, Hg0was removed by?OH produced via the oxidation reaction of H2O adsorbed on the anode surface (see Eq. (2)). On the cathode, it was anticipated that the dissolved oxygen in water were reduced on the surface of Pt sheet to generate radicalby single electron reduction reaction (see Eq. (17)).In the pathway, a part ofwas consumed via oxidation process for removing Hg0. Other parts ofwas adopted to produce?OH with the aid of electron at acidic condition (see Eqs. (21)-(22)), in which O2as product was released. It could account for the effect experiment of O2(Figure 5(c)), demonstrating the most ofradical was generated from dissolved oxygen of the reaction solution rather than gaseous oxygen.Therefore, the acid-assisted electrochemical oxidation system has a superior ability to oxidize and remove Hg0from the simulated flue gas.

3 Conclusions

In this study, a novel method to remove gaseous Hg0using electrochemical method coupled with acid solution was proposed via Pt sheet cathode and FTO anode. The optimal efficiency of Hg0removal could achieve 96% under the conditions of 0.1 mol/L of nitric acid and 4 V of DC voltage, the rate constant of which was 22 and 65 times that of the independently presence of acid and electrochemical reaction, respectively. The Hg0removal efficiency was accordingly raised with the increase of nitric acid concentration and the applied DC voltage, respectively. However, the excess of nitric acid or potential could not enhance further the performance of electrochemical oxidation. Addition of SO2or NO would inhibit Hg0removal in the AEO system, while the inhibition was reversible. Moreover,the electrical energy consumption of electrochemical oxidization method on Hg0removal was calculated,which is acceptable for its potential commercial application. The active radicals scavenger experiments revealed thatand?OH governed the electrochemical process of Hg0removal. The AEO system provided a promising development prospective for practical coalfired power plant mercury treatment and effectively utilization of waste acid.