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Investigation of the fate of heavy metals based on process regulation-chemical reaction-phase distribution in an A-O1-H-O2 biological coking wastewater treatment system
Journal of Environmental Management 247 (2019) 234–241
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Research article
Investigation of the fate of heavy metals based on process regulationchemical reaction-phase distribution in an A-O1-H-O2 biological coking wastewater treatment system
T
Qiaoping Konga, Zemin Lia, Yasi Zhaoa, Cong Weia, Guanglei Qiua,b, Chaohai Weia,b,∗ a
School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, South China University of Technology, Guangzhou, 510006, PR China
b
ARTICLE INFO
ABSTRACT
Keywords: Coking wastewater Heavy metals Regulation Distribution Waste sludge
Regulation mechanism of typical substances including OH−, CN−, SCN−, S2−, NH3 on the distribution of heavy metals was investigated in coking wastewater treatment plant with our self-designed Anaerobic-Oxic-Hydrolytic-Oxic (A-O1-HO2) system through engineering data exposure and computational density functional theory (DFT) verification. The results showed that coking sludge had superior enrichment ability for heavy metals, especially for the sludge from the A and H tanks. The enrichment ratio of the 8 heavy metals including Cd, Pb, Ni, Zn, Cu, Hg, Cr and As in coking waste sludge was found to be 6232 (comparing to these in the influent wastewater of A-O1-H-O2 system). The distribution of 8 heavy metals was closely related to their chemical (precipitation and/or complexation) and biochemical reaction potential with OH−, CN−, SCN−, S2−, NH3 in the A-O1-H-O2 system. The regulation mechanism of these precipitation and/or complexation agents on heavy metals was confirmed by DFT calculation. The stable energy of complexes formed between typical compounds and common heavy metal ions follow the order: OH: Cu2+ > Pb2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; S2−: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; CN−: Zn2+ > Cu2+ > Cd2+ > Hg2+ > Pb2+ > Ni2+; SCN−: Zn2+ > Cd2+ > Pb2+ > Hg2+ > Cu2+ > Ni2+; NH3: Cu2+ > Zn2+ > Cd2+ > Pb2+ > Hg2+ > Ni2+, providing reference for the judgement of which metal ions were preferentially combined with the typical compounds in coking wastewater. The results of this paper indicated that the enrichment of heavy metal ions in coking wastewater can be achieved by process design combined with the control of operating conditions (dissolved oxygen, hydraulic retention time, sludge retention time and pH), basing on the nature of heavy metal ions. Finally, the separation and differential management of heavy metals can be achieved.
1. Introduction Heavy metals including Cd, Cr, Cu, Pb, Hg, Ni and Zn are commonly founded in industrial wastewater resulting from mining, smelting, electroplating, battery manufacture and so on (Chiu et al., 2016; Moscatello et al., 2018). For example, in the process of coking and coking wastewater treatment, heavy metals in coal will enter into surrounding water in the form of evaporative gasification-nucleationcondensation (Zhuang and Biswas, 2001) or ash particles, posing serious threats to ecosystem. In general, the biological process as one of the traditional coking wastewater treatment technologies mainly targets on removing organic substance by the degradation of microorganisms in activated sludge, neglecting the removal of heavy metals due to the weak economic benefits. Because of the relatively low concentration of heavy metals in the raw coking wastewater (Diao and Wei, ∗
2017) and the effluent has been stable up to the standard, people have ignored their influence for a long time. According to our long-term monitoring of the full-scale biological treatment system with stable operation for more than 10 years, we found that the concentration of heavy metals in the sludge phase of coking wastewater was relatively high (Diao and Wei, 2017). The coking sludge containing heavy metals have a certain but not prominent environmental risk (Zhang et al., 2017). Up to now, rarely studies have been conducted about the characteristic of heavy metals in coke production process, and most of them are focused on the solid-air interface, such as dust, fly ash, PM2.5 (Clarke, 1993), but not wastewater treatment system. Mu et al. (2012) have studied the distribution of heavy metals (Cu, Zn, As, Pb, Cr, Ni, Co, Cd, Fe and V) during coking production process, but the reason for the distribution law was unclear. According to our previous publications (Yu et al., 2016; Zhang et al.,
Corresponding author. School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China. E-mail address: [email protected] (C. Wei).
https://doi.org/10.1016/j.jenvman.2019.06.061 Received 9 April 2019; Received in revised form 29 May 2019; Accepted 13 June 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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2015), some typical substances including OH−, CN−, SCN−, S2− and NH3 existed in coking wastewater. These substances may also influence the distribution of heavy metals in the coking wastewater treatment process. For example, pH, a representative of OH− concentration, plays an important role in determining the efficiency of wastewater treatment and it will also influence the physical and chemical performance of heavy metals (Alwan et al., 2010). As we all know, the concentration of typical compounds in coking wastewater treatment system including OH−, CN−, SCN−, S2− and NH3 varies greatly in different unit, so it will be better to analyze the distribution of heavy metals in each separate reaction unit so as to investigate the interaction between typical compounds and heavy metals. But sludge recirculation is generally used in wastewater treatment process (AO (Anaerobic-Oxic), AAO (Anaerobic-Anoxic-Oxic) and AOO (Anoxic-Oxic-Oxic)), among which the components in each unit of coking wastewater will be uniformity mixed and heavy metals will be equally distributed in each unit of coking wastewater treatment system. Because of this, it would be difficult for the investigation of regulation mechanism of typical compounds on heavy metals in each unit of coking wastewater treatment system. In order to solve the equal distribution of heavy metals caused by sludge recirculation, we use our self-designed Anaerobic-Oxic-Hydrolytic-Oxic (A-O1-H-O2) system to operate without sludge reflux during our test period (06/2018-12/2018). A is an anaerobic biofilm reactor, O1 is a high-load aerobic reactor, H is a hydrolytic denitrification reactor, and O2 is a complete nitrification/mineralization reactor. This A-O1-H-O2 system has been running steadily for more than 10 years and this A-O1H-O2 has been mentioned many times in our group's previous publications (Kong et al., 2018; Zhang et al., 2012). If the regulation mechanism of typical compound on the distribution of heavy metals in coking wastewater really exist, the heavy metals can be effectively controlled in the wastewater treatment process, the environmental risks of sludge can be well identified and the directional enrichment and resource utilization of heavy metals in wastewater can be also realized. To our best knowledge, no publication to date has focused on the regulation mechanism of typical compound on the distribution of heavy metals in coking wastewater. In China, there are currently more than 800 coking wastewater treatment projects. For example, Guangdong Shaogang iron and steel group company has three coking plant supporting wastewater treatment projects and the amount of coking wastewater produced is approximately 3600–3800 m3/d. China has an annual coke production of 480 million tons. Given that the discharge of wastewater per ton of coke is around 0.6 m3, the coking wastewater generation and treatment capacity in China is about 7.9 × 105 m3/d. Meanwhile, the waste sludge (with 75% water content) generated from these coking wastewater treatment plants (i.e. coking sludge) of China is estimated to be 3200 t/ d. The contents of Cd, Cr, Hg, Cu, Zn, Pb, Ni and As in coking sludge vary from 3.34 to 788 mg/kg (Diao and Wei, 2017). The heavy metals resources in coking sludge are very rich considering the large amount of coking sludge generated around the world, especially in China. If we can use the regulation mechanism of typical compound on the distribution of heavy metals in coking wastewater to realize the directional enrichment of heavy metals in specific units of wastewater treatment, it will reduce the risk of heavy metals disposal, and facilitate the recovery and reuse of heavy metals in coking sludge. The same concept may be applied to the treatment and resource recovery of other heavy metalcontaining waste sludges. The objectives of this study include: (1) the investigation of the distribution of 8 kinds of heavy metals including Cd, Pb, Ni, Zn, Cu, Hg, Cr and As in each unit of A-O1-H-O2 coking wastewater treatment system; (2) the investigation about the interaction between OH−, CN−, SCN−, S2−, NH3 on the distribution of heavy metals through engineering data analysis. For explicit understand the regulation mechanism of OH−, CN−, SCN−, S2−, NH3 on the distribution of heavy metals, DFT calculation was also used to calculate the binding energies of M2+ (divalent heavy metal ions)-OH-, M2+-CN-, M2+-SCN-, M2+-S2-,
M2+-NH3 in coking wastewater. The results of this study aim to clarify the distribution law of heavy metals in coking wastewater from the aspects of process control, chemical reaction and phase distribution, so as to obtain the reasons for the accumulation of heavy metals in coking sludge, and to provide new ideas for the regulation, separation and resource utilization of heavy metals in coking wastewater. 2. Materials and methods 2.1. Chemicals and materials Hydrochloric acid, nitric acid, perchloric acid, acetic acid, potassium permanganate and hydroxylamine hydrochloride were all purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). H2O2 (30%), sodium borohydride, and ethylene were purchased from Chemical Reagent Co., Ltd. (Laiyang, China). Stock solutions of Cd, Pb, Ni, Zn, Cu, Hg, Cr and As (100 mg/L) were purchased from Chinese National Standard Sample Center. Calibration standard solutions were prepared by further diluting heavy metal stock solution with 2% HNO3 for the determination of total heavy metals. All the glassware used in this study was soaked with 20% HNO3 and then washed by deionized water in order to eliminate interference from foreign impurities. 2.2. Sampling of coking wastewater The performance of coking wastewater treatment plant, the subsidiary of Shaogang iron and steel group company which has been running stably for more than 10 years, located in Shaoguan city, Guangdong Province, China, applying high efficiency biological fluidized bed technology with A-O1-H-O2 wastewater treatment systems, was studied (Fig. S1). The hydraulic retention time (HRT) of A, O1, H and O2 were 28–32.5 h, 12–14 h, 20–23.3 h and 16–18.6 h, respectively. The sludge retention time (SRT) of A, O1, H and O2 were 180–200 d, 10–15 d, 150–180 d and 50–60 d, respectively. No sludge reflux model was adopted during our test period (06/2018-12/2018). The characteristic of raw coking wastewater, operational information and main parameter of A-O1-H-O2 system were shown in Tables S1, S2, S3 and S4, respectively. The coking wastewater samples were collected from the influent, A, O1, H, O2 tank and effluent. The sludge samples were collected from A, O1, H, O2 and sewage sludge. The sampling points of the sludge sample were selected at the sediment discharge port of each unit and the sludge outlet of the sludge pressure filtration locomotives. The detailed sample point distribution was shown in Fig. 1. The sampling time was selected during the continuous and stable operation period of entire coking wastewater treatment system. The wastewater and sludge samples were collected once every 0.5 h for 3 times to ensure that the collected wastewater and sludge samples were representative. All the samples were stored in brown volumetric bottles washed with acetone and Milli-Q water. In pre-process, sludge samples with mixed liquor suspended solids (MLSS) concentration ranging from 4 to 6 g/L were first freeze-dried, then crushed through 20 mesh sieve, and stored at −20 °C before use (Kong et al., 2018). 2.3. Analytical methods The concentration of 8 kinds of heavy metals, which are the common heavy metal components mentioned above in aqueous and sludge phase of coking wastewater, was determined using an AA-6300C flame atomic absorption spectrometer (FAAS, Shimadzu, Japan). As for the determination of heavy metals content in coking wastewater and coking sludge, pretreatment of the aqueous and sludge samples was performed according to Diao et al. (Diao and Wei, 2017). The concentration of heavy metals in aqueous and sludge phase of coking wastewater were recorded as C (μg/L) and Q (mg/kg), respectively. The quality control substance of heavy metals used in the experiment was 235
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Fig. 1. The sampling point location distribution in A-O1-H-O2 system.
GBW08303 (the standard substance for analyzing the soil composition of the polluted farmland). The concentration of NH3, CN−, SCN−, and S2− was determined according to the method described in the previous publication of our group (Pan et al., 2018).
N and S atom. The solvation effects were simulated by using a conductor polarizable continuum model (CPCM). ΔE(au) = ELUMO(au)EHOMO(au). When the value of ΔE is larger which means that the value of EHOMO is smaller and the value of ELUMO is larger, the molecular system will less likely to participate in the chemical reaction and the molecular system will be more stable. All calculations were carried out with the Gaussian 16 software package.
2.4. The enrichment ratio of heavy metals in coking wastewater by coking sludge The enrichment ratio (Ks/w(L/g)) reflects the enrichment ability of coking sludge for heavy metals and it can be calculated by Eq. (1) (Karvelas et al., 2003). Theoretically speaking, the input and output loads of total heavy metals in the A-O1-H-O2 system should be equal because heavy metals can't be degraded but settle out in the sludge or remain in the water stream. Based on mass balance, we can obtain the content of heavy metals in the waste sludge (Q) by using the values of known variables including C1, V1, C2, V2, mA, mO1, mH and mO2 (Eqs. (2) and (3)).
Ks / w =
Q C1
3. Results and discussion 3.1. Distribution of heavy metals in biological reactors with A-O1-H-O2 system The concentration of heavy metals in raw coking wastewater were mainly depending on the content of heavy metals in the coal. Besides, the coking production technology will also influence the distribution of heavy metals. The heavy metals that symbiotic with coal mines includes Pb, Hg, Zn, Cu, Ca, As, Cr, Ni, etc. (Jiao et al., 2012). These heavy metals can be transformed into the air, water and soil in coal combustion and coking production process, endangering humans and the ecological environment. In order to find out the distribution law of heavy metals in coking wastewater, we monitored the total concentration of 8 heavy metals in A, O1, H and O2 reactors of coking wastewater treatment plant with A-O1-H-O2 system. The operating parameters of A-O1-H-O2 system is shown in Table S2. The concentration of heavy metals in the aqueous and sludge phase within different unit of our self-designed A-O1-H-O2 system was investigated and the results was shown in Fig. 2. The total amount of 8 heavy metals in the sludge phase of A, O1, H and O2 tank were in the range of 21.3–774.1 mg/kg, 1.5–170 mg/kg, 4.6–2400 mg/kg and 2.1–130 mg/kg, respectively, which were much higher than that of aqueous phase. Specially, the concentration of Cu, Pb, Ni and Zn in tank A aqueous phase were much higher than others. The content of Ni, Cu and Zn occupied a larger proportion in H tank. The concentration of As, Cd and Hg were quite low in the aqueous phase of A-O1-H-O2 system. The coking sludge of A, O1, H and O2 tanks showed different enrichment ability for the above 8 heavy metals. In sludge phase, the major components in tank A were corresponding to Pb and Cu, while Zn appeared to be the most abundant metal in O1, H and O2 tanks. The phase distribution of the studied heavy metals appeared different from each other, indicating that heavy metals had formed different chemical
(1)
C1 V1 C V = 2 2 + Qm 1000 1000
(2)
m = mA + mO1 + mH + mO2
(3)
where C1 (μg/L) and C2 (μg/L) are the total concentration of the 8 heavy metals in A influent and O2 effluent, respectively; V1 (L) and V2 (L) are the volume of A influent and O2 effluent, respectively; Q, is the total concentration of the 8 heavy metals in the sewage sludge, mg/kg; mA, mO1, mH and mO2 are the mass of A tank sludge, O1 tank sludge, H tank sludge and O2 tank sludge, respectively. m was the total amount of sludge with the mixed of A, O1, H and O2 sludge due to the fact that the coking sludge from different stage of biological reactors was centralized processed in most case. 2.5. DFT calculation To fully understand the interaction between OH−, S2−, NH3, CN−, SCN− and heavy metals, the Perdew-Burke-Erzenrhof exchange-correlation functional (PBE1PBE) of DFT widely applied in geometrical optimization was used (Sun et al., 2017). The basis set by the LANL2DZ were used for heavy metal atom, whereas the 6-31G(d) was used for C, 236
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Fig. 2. The aqueous (a) and sludge (b) concentration of heavy metals in different unit of coking wastewater treatment A-O1-H-O2 system.
forms with different water solubility substance within pH ranging from 7.2 to 8.5 in A-O1-H-O2 coking wastewater treatment system. The analysis of the total amount of heavy metals in the aqueous phase and sludge phase of coking wastewater treatment plant is conducive to the treatment of heavy metals in aqueous phase and the classified disposal of coking sludge containing heavy metals. As shown in Fig. 3a, the total amount of heavy metals in aqueous phase of A-O1H-O2 system followed the sequence of A > O1 > H > O2, indicating that each step of A-O1-H-O2 biological reactors was effective to remove a certain amount of heavy metals. The total amount of heavy metals in sludge phase of A-O1-H-O2 was quite different from these in the aqueous phase. The 8 heavy metals were mainly accumulated in A and H tanks. By comparing the sludge retention time in A, O1, H and O2, it can be found that the accumulation rule of the total amount of heavy metals is consistent with the sludge residence time (SRT) of each pool and the SRT of A, O1, H and O2 in our self-designed A-O1-H-O2 system were 180–200 d, 10–15 d, 150–180 d and 50–60 d, respectively. A-O1-H-O2 seemed to have a special ability in the phase distribution of examined heavy metals. Heavy metals directionally accumulated in A and H tanks would be beneficial for the splitting and disposal of coking sludge containing heavy metals. Besides, it would help control the flow of suspended sludge, reducing the re-release of heavy metals in the sludge. The heavy metals accumulated in sludge phase might be also the reason for the decrease in heavy metals concentrations in the aqueous phase of coking wastewater. Based on the fact that the amount of heavy metals accumulated in the sludge of different unit were different (Fig. 3b), it was recommended to deal with the sludge based on the different process stage so as to better prevent the environmental risk caused by heavy metal pollution. In addition, heavy metal extraction and reuse can be considered, basing on the distribution of heavy metals in sludge at different stages of coking wastewater treatment process.
To better understand the enrichment ratio of coking sludge, we used the following simplified model (Fig. 4) to calculate Ks/w. In the actual production process, the value of m was about 600 t per month. The sludge residence time (SRT) of A, O1, H and O2 in our self-designed AO1-H-O2 system were 180–200 d, 10–15 d, 150–180 d, 50–60 d, respectively. To facilitate the calculation, we set the SRT values of A, O1, H and O2 tank as 200 d, 15 d, 180 d and 60 d, respectively. According to Eq. (1), the value of Ks/w of the total eight heavy metals in coking waste sludge comparing to influent wastewater of A-O1-H-O2 system was found to be 6232, suggesting that coking sludge had strong enrichment ability for heavy metals. In the process of coking wastewater treatment, heavy metals might enter into the sludge phase through precipitation, complexation, microbial adsorption, and accumulation (Pagnanelli et al., 2009), which might be one reason for the high enrichment ability of coking sludge. Another reason might be due to the decomposition of organic and inorganic substance into CH4, CO2, N2, H2S and other gases in the process of A-O1-H-O2 biological system for coking wastewater treatment, leading to the observed content of heavy metal in coking sludge increased due to the fact the calculation about heavy metal content in coking sludge was based on dry weight (Chipasa, 2003). The obtained Ks/w value was the average of A, O1, H and O2 tank, so the enrichment ratio of A and H tank would be much higher than this value based on the results of Fig. 3b. According to previous study (Karvelas et al., 2003), the heavy metals in sludge phase were closely related to the organic matter. There are large amount of dissolved inorganic and organic matter in coking wastewater including ammonia nitrogen, phenol, cyanide, sulfide and other hundreds of organics which might regulate the distribution of heavy metals. If the regulation law really exist, it might be a reason why coking sludge has high enrichment capacity for heavy metals. Herein, we determined the concentration of ammonia nitrogen, thiocyanide,
Fig. 3. The total concentration of heavy metals in aqueous phase (a) and sludge phase (b) of A-O1-H-O2 system. (The green line represents the change trend of various substances.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 237
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Fig. 4. The simplified A-O1-H-O2 system that used for the calculation of enrichment ratio.
in the range of 5.0–8.5, Cu and Cr ions will be precipitated. While for the precipitation of Zn, Cd, and Ni ions, stronger alkaline condition is needed (Alwan, 2008). The chemical precipitation mechanism is presented in Eqs. (4) and (5). The CN−, SCN−, NH3 and OH− have a stronger ability in chelating with heavy metals (Dash et al., 2009; Kim et al., 2002; Schroder et al., 2001), and the corresponding reaction equation as Eqs. (6)–(9). Combined with the chemical reaction occurring in A-O1-H-O2 system (Table 1), it is meaningful to discuss the reaction related to heavy metals which may explain the reason why coking sludge has a strong enrichment ability for the heavy metals.
M 2+ + 2OH
M 2+
+
S2
M 2+ + nCN M 2+
+ nSCN
M 2+ + nNH3
M 2+
Fig. 5. The total concentration of typical compounds in aqueous phase of A-O1H-O2 system. (The green line represents the change trend of various substances.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
+ nOH
(4)
M (OH )2
(5)
MS [M (CN )n]2 [M (SCN )n
n (n
=1
]2 n (n
=1
[M (NH3)n]2+ (n = 1
[M (OH )n
]2 n (n
=1
(6)
4)
(7)
4)
(8)
6)
(9)
4) −
A tank: There are a high concentration of CN and NH3 in A tank (Table S3). Because of complexation, a certain amount of [M(CN)n]2−n Table 1 The main reaction occurred in A-O1-H-O2 system.
cyanide and sulfide in the aqueous phase and the results were shown in Fig. 5. As presented in Fig. 5, the concentration of ammonia nitrogen, thiocyanide, cyanide and sulfide in the aqueous phase of A-O1-H-O2 system followed the sequence of A > O1 > H > O2, which was in accordance with the changes trend of total heavy metals in aqueous phase. The pH values of A, O1, H and O2 tank were 8.2, 7.4, 8.5 and 7.2, respectively. The pH values reflected the concentration of hydroxide in coking wastewater. These phenomena indicated the interaction between heavy metals and typical compounds in coking wastewater were really existed.
Unit
Main Reaction
A
M2++S2−→MS↓ M2++OH−→M(OH)2↓ M2++SCN−→[M(SCN)n]2-n M2++CN−→[M(CN)n]2-n M2++NH3→[M(NH3)n]2+ SCN-+H2O+O2→HCO3−+NH4++SO42− +H+ NaCN+H2O+O2→NaHCO3+NH3 C6H5OH+O2→CO2+H2O S2− +O2→SO42M2++NH3→[M(NH3)n]2+ M2++SO42−→MSO4↓ M2++OH−→M(OH)2↓ HCN+H2O→HCOOH+NH3 HSCN+H2O→S2− +HCNO+SH− HCNO+H2O→CO2+4NH3 C6H6ONa+H2O→C6H5OH+NaOH M2++OH−→M(OH)2↓ M2++S2−→MS↓ SCN-+H2O+O2→HCO3−+NH4++SO42− +H+ C6H5OH+O2→CO2+H2O S2− +O2→SO42NH4+→NO2− NO2−→NO3− Organic compounds+O2→CO2+H2O M2++OH−→M(OH)2↓ M2++SO42−→MSO4↓ M2++CO32−→MCO3↓
O1
3.2. Regulation mechanism of typical compounds on the distribution of heavy metals in A-O1-H-O2 coking wastewater treatment system
H
Based on the discussion in 3.1, we infer that the typical compounds in coking wastewater will influence the distribution and fate of heavy metals. Herein, we use CN−, SCN−, S2−, NH3 to represent cyanide, thiocyanide, sulfide and ammonia nitrogen, respectively, in order to facilitate the discussion of chemical reactions occurred in coking wastewater related to these typical substances. OH− and S2− will form chemical precipitation with heavy metals (Fu and Wang, 2011). The concentration of OH−, which is marked as pH of the solution, is key for control chemical precipitation and complexation reaction of heavy metals. For different heavy metals, pH value required for precipitation is slightly different from each other. For example, when the pH value is
O2
238
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Fig. 6. The main chemical reaction about heavy metals in A-O1-H-O2.
and [M(SCN)n]2−n would be formed. The Ksp value of MS (Cd2+, Hg2+, Cu2+, Zn2+, Pb2+ and Ni2+) was lower than the M(OH)2, suggesting that S2− had a stronger ability to bind free M2+. Therefore, the chemical precipitation species in coking sludge were mainly in the form of MS and this might be also the reason why the content of heavy metals in A sludge was relatively high. The simplified model about the main chemical reaction in A-O1-H-O2 concerning about heavy metals can also be seen in Fig. 6. O1 tank: The complexes of [M(CN)n]2−n and [M(SCN)n]2−n in A tank would then transfer to O1 tank through self-flow in the wastewater treatment process. In O1 tank, organic substances and most reducing inorganic substances such as CN− and SCN− were oxidized to HCO3−, NH4+ and SO42−, so heavy metals would be released from the [M (CN)n]2−n and [M(SCN)n]2−n complexes and then existed in free form. With ammoniation of nitrogen-containing organic compounds in O1 tank, some free heavy metals will complex with NH3 and then enter H reactor. The other free M2+ would form M(OH)2 with OH− because of the alkaline aqueous solution in O1 tank (pH = 7.4). H tank: The main reactions occurred in H reactor was denitrification and the concentration of ammonia nitrogen would be decreased along with the release of N2 into the atmosphere. As results, some free M2+ will be released from [M(NH3)n]2+complex. Along with the denitrification reaction, the pH value of H tank increased slightly (pH = 8.5), and a large amount of free heavy metal ions will precipitate in the form of metal hydroxides. Besides, H reactor is almost anoxic and partial SO42− will be reduced to S2− because of almost anaerobic environment of H tank, which can be realized by regulating DO (dissolved oxygen), so some free heavy metal ions will form MS precipitation with S2−. Altogether, the total content of heavy metals in the sludge phase of H tank was higher than that of A, O1 and O2 tank. O2 tank: The main reactions happened in O2 reactor was nitrification and oxidation. The reductive ligands in residual M2+-reductive ligand complexes such as MS and [M(SCN)n]2−n self-flowing from H tank would be oxidized in O2 tank. At the same time, free M2+ would be released from M2+-reductive ligand complexes then be precipitated with -OH- (the pH of O2 tank was about 7.2 which was alkaline), SO42− and CO32−, further be enriched in sludge phase of O2 tank.
the form of anions in the solution and repulsive effect might be existed among Cr, As and the typical compounds (OH−, S2−, CN−, SCN− and NH3) in the coking wastewater, so Cr and As were not taken into consideration in the DFT calculation. The most stable structure of M2+-OH, M2+-S2-, M2+-NH3, M2+-CN- and M2+-SCN- were shown in Table 2. M2+ referred to the divalent heavy metal ions. As presented in Table 2, the configurations of M2+-OH-, M2+-S2-, M2+-NH3, M2+-CN- and M2+-SCN- were V type, linear type, trigonometric cone type, linear type and V type, respectively. △E value of heavy metals and typical compound complexes in coking wastewater were following the sequence (Table 3): OH: Cu2+ > Pb2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; S2−: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; CN−: Zn2+ > Cu2+ > Cd2+ > Hg2+ > Pb2+ > Ni2+; SCN−: Zn2+ > Cd2+ > Pb2+ > Hg2+ > Cu2+ > Ni2+; NH3: Cu2+ > Zn2+ > Cd2+ > Pb2+ > Hg2+ > Ni2+. On the whole, based on the complexing ability between heavy metals and anions, we can determine which metal ions are preferentially combined with the typical compounds in coking wastewater so as to provide a theoretical basis for the prevention and treatment of heavy metals in coking wastewater. It would also provide guidance for analysis of specific existence forms of heavy metal components in coking wastewater. For example, the ΔE (au) of Cu2+-NH3 was higher than that of the other M2+-NH3, indicating that Cu2+ would be priority to form M2+-NH3. 4. Conclusions The regulation mechanism of typical compounds including OH−, CN−, SCN−, S2−, and NH3 on the distribution of 8 heavy metals including Cd, Hg, Pb, Ni, Zn Cr, As and Cu in coking wastewater treatment system were systematically discussed for the first time on the basis of fully considering the biological and chemical reactions in each unit of the A-O1-H-O2 coking wastewater treatment system. The changes in the concentrations of NH3, SCN−, CN− and S2− in the system was in accordance with these of total heavy metals in aqueous phase (with an order of A > O1 > H > O2). The interaction between typical compounds and heavy metals were further confirmed by DFT calculation, showing consistency with the engineering data observed in the A-O1-HO2 system, suggesting that the regulation mechanism of typical compounds on heavy metals was effective. Altogether, heavy metals in coking sludge could achieve enrichment and separation based on hydrolysis, complexation, precipitation, and oxidation and reduction effect by using A-O1-H-O2 biological coking wastewater treatment system under no sludge reflux operation mode. The no sludge reflux mode used in the A-O1-H-O2 system allowed the risk management of heavy metals to be achieved during the coking wastewater treatment, which is a
3.3. DFT calculation The components in coking wastewater including NH3, CN−, SCN−, OH and S2− might regulated the distribution of heavy metals according to the discussion in section 3.2. Herein, DFT calculation was used to verify the regulation mechanism of the typical compounds on heavy metals in coking wastewater. Since Cr and As mainly occurred in −
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Table 2 Optimized structure of OH−, S2−, NH3, CN− and SCN− with common heavy metal ions. Heavy metals
OH−
S2-
CN−
NH3
SCN−
Cd2+
Hg2+
Cu2+
Zn2+
Pb2+
Ni2+
revolutionary improvement compared to the traditional technology in which the risk of heavy metal was not well controlled owing to the reflux of sludge.
Table 3 △E value of heavy metals and typical compound complexes in coking wastewater. Typical compounds OH
−
S2-
CN−
SCN−
NH3
Heavy metals 2+
Cd Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+
EHomo(au)
ELumo(au)
ΔE(au)
−0.25010 −0.25477 −0.34584 −0.27125 −0.34138 −0.30172 −0.19198 −0.19437 −0.22968 −0.20233 −0.25362 −0.21614 −0.32659 −0.31978 −0.38503 −0.34031 −0.35795 −0.35903 −0.25340 −0.22808 −0.32356 −0.26582 −0.30015 −0.30742 −0.36992 −0.29306 −0.50872 −0.40865 −0.40942 −0.37815
−0.05186 −0.06390 −0.08782 −0.05267 −0.10348 −0.18853 −0.04033 −0.04902 −0.05484 −0.04142 −0.07391 −0.13410 −0.05051 −0.06773 −0.09356 −0.04822 −0.13758 −0.21077 −0.06954 −0.09745 −0.22956 −0.07206 −0.13062 −0.21379 −0.06603 −0.09793 −0.11907 −0.06638 −0.14460 −0.22536
0.19824 0.19087 0.25802 0.21858 0.23790 0.11319 0.15165 0.14535 0.17484 0.16091 0.17971 0.08204 0.27608 0.25205 0.29147 0.29209 0.22037 0.14826 0.18386 0.13063 0.09400 0.19376 0.16953 0.09363 0.30389 0.19513 0.38965 0.34227 0.26482 0.15279
Acknowledgements The financial support of the National Nature Science Fund of China (No. 5187080143, 51878290, 51778238, 51808297), the State Key Program of National Natural Science of China (No. 21037001), the Development Foundation of Applied Science and Technology of Guangdong Province, China (No. 2018A050506009, 2015B020235005, 2017A02016001) is greatly appreciated. This work is also funded by the short-term overseas visiting project of doctoral students from South China University of Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2019.06.061. References Alwan, G.M., 2008. pH-control problems of wastewater treatment plants. Al-Khwarizmi Eng. J. 4 (2), 37–45. Alwan, G.M., Mehdi, F.A., Arazak, A.A., 2010. Operation and Ph control of a wastewater treatment unit using labview. Eng. Technol. J. 28 (17), 5524–5546. Chiu, J.M.Y., Degger, N., Leung, J.Y.S., Po, B.H.K., Zheng, G.J., Richardson, B.J., Lau, T.H., Wu, R.S., 2016. A novel approach for estimating the removal efficiencies of endocrine disrupting chemicals and heavy metals in wastewater treatment processes. Mar. Pollut. Bull. 112, 53–57. Chipasa, K.B., 2003. Accumulation and fate of selected heavy metals in a biological wastewater treatment system. Waste Manag. 23 (2), 135–143. Clarke, L.B., 1993. The fate of trace elements during coal combustion and gasification: An overview. Fuel 6, 731–736 1993.
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Q. Kong, et al. Dash, R.R., Gaur, A., Balomajumder, C., 2009. Cyanide in industrial wastewaters and its removal: A review on biotreatment. J. Hazard Mater. 163, 1–11. Diao, C., Wei, C., 2017. Fraction of heavy metals in sludges from typical coking wastewater treatment plants by modified bcr sequential extraction procedure. Environ. Protect. Eng. 43, 191–207. Fu, F., Wang, Q., 2011. Removal of heavy metal ions from wastewaters: A review. J. Environ. Manag. 92, 407–418. Jiao, B., Xu, G., Li, D., Luo, J., Yang, K., 2012. Hazards of heavy metals in coal. Disaster Advances 5, 1812–1818. Karvelas, M., Katsoyiannis, A., Samara, C., 2003. Occurrence and fate of heavy metals in the wastewater treatment process. Chemosphere 53, 1201–1210. Kim, S.J., Lim, K.H., Joo, K.H., Lee, M.J., Kil, S.G., Cho, S.Y., 2002. Removal of heavy metal-cyanide complexes by ion exchange. Korean J. Chem. Eng. 19, 1078–1084. Kong, Q., Wu, H., Liu, L., Zhang, F., Preis, S., Zhu, S., Wei, C., 2018. Solubilization of polycyclic aromatic hydrocarbons (PAHs) with phenol in coking wastewater treatment system: Interaction and engineering significance. Sci. Total Environ. 628–629, 467–473. Moscatello, N., Swayambhu, G., Jones, C.H., Xu, J., Dai, N., Pfeifer, B.A., 2018. Continuous removal of copper, magnesium, and nickel from industrial wastewater utilizing the natural product yersiniabactin immobilized within a packed-bed column. Chem. Eng. J. 343, 173–179. Mu, L., Peng, L., Liu, X., Bai, H., Song, C., Wang, Y., Li, Z., 2012. Emission characteristics of heavy metals and their behavior during coking processes. Environ. Sci. Technol. 46, 6425–6430.
Pagnanelli, F., Mainelli, S., Bornoroni, L., Dionisi, D., Toro, L., 2009. Mechanisms of heavy-metal removal by activated sludge. Chemosphere 75, 1028–1034. Pan, J., Wei, C., Fu, B., Ma, J., Preis, S., Wu, H., Zhu, S., 2018. Simultaneous nitrite and ammonium production in an autotrophic partial denitrification and ammonification of wastewaters containing thiocyanate. Bioresour. Technol. 252, 20–27. Schroder, D., Schwarz, H., Wu, J., Wesdemiotis, C., 2001. Long-lived dications of Cu(H2O) (2+) and Cu(NH3)(2+) do exist!. Chem. Phys. Lett. 343, 258–264. Sun, Y., Wang, X., Ai, Y., Yu, Z., Huang, W., Chen, C., Hayat, T., Alsaedi, A., Wang, X., 2017. Interaction of sulfonated graphene oxide with U(VI) studied by spectroscopic analysis and theoretical calculations. Chem. Eng. J. 310, 292–299. Yu, X., Xu, R., Wei, C., Wu, H., 2016. Removal of cyanide compounds from coking wastewater by ferrous sulfate: Improvement of biodegradability. J. Hazard Mater. 302, 468–474. Zhang, F., Wei, C., Hu, Y., Wu, H., 2015. Zinc ferrite catalysts for ozonation of aqueous organic contaminants: Phenol and bio-treated coking wastewater. Separ. Purif. Technol. 156, 625–635. Zhang, W., Wei, C., Chai, X., He, J., Cai, Y., Ren, M., Yan, B., Peng, P., Fu, J., 2012. The behaviors and fate of polycyclic aromatic hydrocarbons (PAHs) in a coking wastewater treatment plant. Chemosphere 88, 174–182. Zhang, Z., Baroutian, S., Munir, M.T., Young, B.R., 2017. Variation in metals during wet oxidation of sewage sludge. Bioresour. Technol. 245, 234–241. Zhuang, Y., Biswas, P., 2001. Submicrometer particle formation and control in a benchscale pulverized coal combustor. Energy Fuels 15, 510–516.
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Research article
Investigation of the fate of heavy metals based on process regulationchemical reaction-phase distribution in an A-O1-H-O2 biological coking wastewater treatment system
T
Qiaoping Konga, Zemin Lia, Yasi Zhaoa, Cong Weia, Guanglei Qiua,b, Chaohai Weia,b,∗ a
School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, South China University of Technology, Guangzhou, 510006, PR China
b
ARTICLE INFO
ABSTRACT
Keywords: Coking wastewater Heavy metals Regulation Distribution Waste sludge
Regulation mechanism of typical substances including OH−, CN−, SCN−, S2−, NH3 on the distribution of heavy metals was investigated in coking wastewater treatment plant with our self-designed Anaerobic-Oxic-Hydrolytic-Oxic (A-O1-HO2) system through engineering data exposure and computational density functional theory (DFT) verification. The results showed that coking sludge had superior enrichment ability for heavy metals, especially for the sludge from the A and H tanks. The enrichment ratio of the 8 heavy metals including Cd, Pb, Ni, Zn, Cu, Hg, Cr and As in coking waste sludge was found to be 6232 (comparing to these in the influent wastewater of A-O1-H-O2 system). The distribution of 8 heavy metals was closely related to their chemical (precipitation and/or complexation) and biochemical reaction potential with OH−, CN−, SCN−, S2−, NH3 in the A-O1-H-O2 system. The regulation mechanism of these precipitation and/or complexation agents on heavy metals was confirmed by DFT calculation. The stable energy of complexes formed between typical compounds and common heavy metal ions follow the order: OH: Cu2+ > Pb2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; S2−: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; CN−: Zn2+ > Cu2+ > Cd2+ > Hg2+ > Pb2+ > Ni2+; SCN−: Zn2+ > Cd2+ > Pb2+ > Hg2+ > Cu2+ > Ni2+; NH3: Cu2+ > Zn2+ > Cd2+ > Pb2+ > Hg2+ > Ni2+, providing reference for the judgement of which metal ions were preferentially combined with the typical compounds in coking wastewater. The results of this paper indicated that the enrichment of heavy metal ions in coking wastewater can be achieved by process design combined with the control of operating conditions (dissolved oxygen, hydraulic retention time, sludge retention time and pH), basing on the nature of heavy metal ions. Finally, the separation and differential management of heavy metals can be achieved.
1. Introduction Heavy metals including Cd, Cr, Cu, Pb, Hg, Ni and Zn are commonly founded in industrial wastewater resulting from mining, smelting, electroplating, battery manufacture and so on (Chiu et al., 2016; Moscatello et al., 2018). For example, in the process of coking and coking wastewater treatment, heavy metals in coal will enter into surrounding water in the form of evaporative gasification-nucleationcondensation (Zhuang and Biswas, 2001) or ash particles, posing serious threats to ecosystem. In general, the biological process as one of the traditional coking wastewater treatment technologies mainly targets on removing organic substance by the degradation of microorganisms in activated sludge, neglecting the removal of heavy metals due to the weak economic benefits. Because of the relatively low concentration of heavy metals in the raw coking wastewater (Diao and Wei, ∗
2017) and the effluent has been stable up to the standard, people have ignored their influence for a long time. According to our long-term monitoring of the full-scale biological treatment system with stable operation for more than 10 years, we found that the concentration of heavy metals in the sludge phase of coking wastewater was relatively high (Diao and Wei, 2017). The coking sludge containing heavy metals have a certain but not prominent environmental risk (Zhang et al., 2017). Up to now, rarely studies have been conducted about the characteristic of heavy metals in coke production process, and most of them are focused on the solid-air interface, such as dust, fly ash, PM2.5 (Clarke, 1993), but not wastewater treatment system. Mu et al. (2012) have studied the distribution of heavy metals (Cu, Zn, As, Pb, Cr, Ni, Co, Cd, Fe and V) during coking production process, but the reason for the distribution law was unclear. According to our previous publications (Yu et al., 2016; Zhang et al.,
Corresponding author. School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China. E-mail address: [email protected] (C. Wei).
https://doi.org/10.1016/j.jenvman.2019.06.061 Received 9 April 2019; Received in revised form 29 May 2019; Accepted 13 June 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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2015), some typical substances including OH−, CN−, SCN−, S2− and NH3 existed in coking wastewater. These substances may also influence the distribution of heavy metals in the coking wastewater treatment process. For example, pH, a representative of OH− concentration, plays an important role in determining the efficiency of wastewater treatment and it will also influence the physical and chemical performance of heavy metals (Alwan et al., 2010). As we all know, the concentration of typical compounds in coking wastewater treatment system including OH−, CN−, SCN−, S2− and NH3 varies greatly in different unit, so it will be better to analyze the distribution of heavy metals in each separate reaction unit so as to investigate the interaction between typical compounds and heavy metals. But sludge recirculation is generally used in wastewater treatment process (AO (Anaerobic-Oxic), AAO (Anaerobic-Anoxic-Oxic) and AOO (Anoxic-Oxic-Oxic)), among which the components in each unit of coking wastewater will be uniformity mixed and heavy metals will be equally distributed in each unit of coking wastewater treatment system. Because of this, it would be difficult for the investigation of regulation mechanism of typical compounds on heavy metals in each unit of coking wastewater treatment system. In order to solve the equal distribution of heavy metals caused by sludge recirculation, we use our self-designed Anaerobic-Oxic-Hydrolytic-Oxic (A-O1-H-O2) system to operate without sludge reflux during our test period (06/2018-12/2018). A is an anaerobic biofilm reactor, O1 is a high-load aerobic reactor, H is a hydrolytic denitrification reactor, and O2 is a complete nitrification/mineralization reactor. This A-O1-H-O2 system has been running steadily for more than 10 years and this A-O1H-O2 has been mentioned many times in our group's previous publications (Kong et al., 2018; Zhang et al., 2012). If the regulation mechanism of typical compound on the distribution of heavy metals in coking wastewater really exist, the heavy metals can be effectively controlled in the wastewater treatment process, the environmental risks of sludge can be well identified and the directional enrichment and resource utilization of heavy metals in wastewater can be also realized. To our best knowledge, no publication to date has focused on the regulation mechanism of typical compound on the distribution of heavy metals in coking wastewater. In China, there are currently more than 800 coking wastewater treatment projects. For example, Guangdong Shaogang iron and steel group company has three coking plant supporting wastewater treatment projects and the amount of coking wastewater produced is approximately 3600–3800 m3/d. China has an annual coke production of 480 million tons. Given that the discharge of wastewater per ton of coke is around 0.6 m3, the coking wastewater generation and treatment capacity in China is about 7.9 × 105 m3/d. Meanwhile, the waste sludge (with 75% water content) generated from these coking wastewater treatment plants (i.e. coking sludge) of China is estimated to be 3200 t/ d. The contents of Cd, Cr, Hg, Cu, Zn, Pb, Ni and As in coking sludge vary from 3.34 to 788 mg/kg (Diao and Wei, 2017). The heavy metals resources in coking sludge are very rich considering the large amount of coking sludge generated around the world, especially in China. If we can use the regulation mechanism of typical compound on the distribution of heavy metals in coking wastewater to realize the directional enrichment of heavy metals in specific units of wastewater treatment, it will reduce the risk of heavy metals disposal, and facilitate the recovery and reuse of heavy metals in coking sludge. The same concept may be applied to the treatment and resource recovery of other heavy metalcontaining waste sludges. The objectives of this study include: (1) the investigation of the distribution of 8 kinds of heavy metals including Cd, Pb, Ni, Zn, Cu, Hg, Cr and As in each unit of A-O1-H-O2 coking wastewater treatment system; (2) the investigation about the interaction between OH−, CN−, SCN−, S2−, NH3 on the distribution of heavy metals through engineering data analysis. For explicit understand the regulation mechanism of OH−, CN−, SCN−, S2−, NH3 on the distribution of heavy metals, DFT calculation was also used to calculate the binding energies of M2+ (divalent heavy metal ions)-OH-, M2+-CN-, M2+-SCN-, M2+-S2-,
M2+-NH3 in coking wastewater. The results of this study aim to clarify the distribution law of heavy metals in coking wastewater from the aspects of process control, chemical reaction and phase distribution, so as to obtain the reasons for the accumulation of heavy metals in coking sludge, and to provide new ideas for the regulation, separation and resource utilization of heavy metals in coking wastewater. 2. Materials and methods 2.1. Chemicals and materials Hydrochloric acid, nitric acid, perchloric acid, acetic acid, potassium permanganate and hydroxylamine hydrochloride were all purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). H2O2 (30%), sodium borohydride, and ethylene were purchased from Chemical Reagent Co., Ltd. (Laiyang, China). Stock solutions of Cd, Pb, Ni, Zn, Cu, Hg, Cr and As (100 mg/L) were purchased from Chinese National Standard Sample Center. Calibration standard solutions were prepared by further diluting heavy metal stock solution with 2% HNO3 for the determination of total heavy metals. All the glassware used in this study was soaked with 20% HNO3 and then washed by deionized water in order to eliminate interference from foreign impurities. 2.2. Sampling of coking wastewater The performance of coking wastewater treatment plant, the subsidiary of Shaogang iron and steel group company which has been running stably for more than 10 years, located in Shaoguan city, Guangdong Province, China, applying high efficiency biological fluidized bed technology with A-O1-H-O2 wastewater treatment systems, was studied (Fig. S1). The hydraulic retention time (HRT) of A, O1, H and O2 were 28–32.5 h, 12–14 h, 20–23.3 h and 16–18.6 h, respectively. The sludge retention time (SRT) of A, O1, H and O2 were 180–200 d, 10–15 d, 150–180 d and 50–60 d, respectively. No sludge reflux model was adopted during our test period (06/2018-12/2018). The characteristic of raw coking wastewater, operational information and main parameter of A-O1-H-O2 system were shown in Tables S1, S2, S3 and S4, respectively. The coking wastewater samples were collected from the influent, A, O1, H, O2 tank and effluent. The sludge samples were collected from A, O1, H, O2 and sewage sludge. The sampling points of the sludge sample were selected at the sediment discharge port of each unit and the sludge outlet of the sludge pressure filtration locomotives. The detailed sample point distribution was shown in Fig. 1. The sampling time was selected during the continuous and stable operation period of entire coking wastewater treatment system. The wastewater and sludge samples were collected once every 0.5 h for 3 times to ensure that the collected wastewater and sludge samples were representative. All the samples were stored in brown volumetric bottles washed with acetone and Milli-Q water. In pre-process, sludge samples with mixed liquor suspended solids (MLSS) concentration ranging from 4 to 6 g/L were first freeze-dried, then crushed through 20 mesh sieve, and stored at −20 °C before use (Kong et al., 2018). 2.3. Analytical methods The concentration of 8 kinds of heavy metals, which are the common heavy metal components mentioned above in aqueous and sludge phase of coking wastewater, was determined using an AA-6300C flame atomic absorption spectrometer (FAAS, Shimadzu, Japan). As for the determination of heavy metals content in coking wastewater and coking sludge, pretreatment of the aqueous and sludge samples was performed according to Diao et al. (Diao and Wei, 2017). The concentration of heavy metals in aqueous and sludge phase of coking wastewater were recorded as C (μg/L) and Q (mg/kg), respectively. The quality control substance of heavy metals used in the experiment was 235
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Fig. 1. The sampling point location distribution in A-O1-H-O2 system.
GBW08303 (the standard substance for analyzing the soil composition of the polluted farmland). The concentration of NH3, CN−, SCN−, and S2− was determined according to the method described in the previous publication of our group (Pan et al., 2018).
N and S atom. The solvation effects were simulated by using a conductor polarizable continuum model (CPCM). ΔE(au) = ELUMO(au)EHOMO(au). When the value of ΔE is larger which means that the value of EHOMO is smaller and the value of ELUMO is larger, the molecular system will less likely to participate in the chemical reaction and the molecular system will be more stable. All calculations were carried out with the Gaussian 16 software package.
2.4. The enrichment ratio of heavy metals in coking wastewater by coking sludge The enrichment ratio (Ks/w(L/g)) reflects the enrichment ability of coking sludge for heavy metals and it can be calculated by Eq. (1) (Karvelas et al., 2003). Theoretically speaking, the input and output loads of total heavy metals in the A-O1-H-O2 system should be equal because heavy metals can't be degraded but settle out in the sludge or remain in the water stream. Based on mass balance, we can obtain the content of heavy metals in the waste sludge (Q) by using the values of known variables including C1, V1, C2, V2, mA, mO1, mH and mO2 (Eqs. (2) and (3)).
Ks / w =
Q C1
3. Results and discussion 3.1. Distribution of heavy metals in biological reactors with A-O1-H-O2 system The concentration of heavy metals in raw coking wastewater were mainly depending on the content of heavy metals in the coal. Besides, the coking production technology will also influence the distribution of heavy metals. The heavy metals that symbiotic with coal mines includes Pb, Hg, Zn, Cu, Ca, As, Cr, Ni, etc. (Jiao et al., 2012). These heavy metals can be transformed into the air, water and soil in coal combustion and coking production process, endangering humans and the ecological environment. In order to find out the distribution law of heavy metals in coking wastewater, we monitored the total concentration of 8 heavy metals in A, O1, H and O2 reactors of coking wastewater treatment plant with A-O1-H-O2 system. The operating parameters of A-O1-H-O2 system is shown in Table S2. The concentration of heavy metals in the aqueous and sludge phase within different unit of our self-designed A-O1-H-O2 system was investigated and the results was shown in Fig. 2. The total amount of 8 heavy metals in the sludge phase of A, O1, H and O2 tank were in the range of 21.3–774.1 mg/kg, 1.5–170 mg/kg, 4.6–2400 mg/kg and 2.1–130 mg/kg, respectively, which were much higher than that of aqueous phase. Specially, the concentration of Cu, Pb, Ni and Zn in tank A aqueous phase were much higher than others. The content of Ni, Cu and Zn occupied a larger proportion in H tank. The concentration of As, Cd and Hg were quite low in the aqueous phase of A-O1-H-O2 system. The coking sludge of A, O1, H and O2 tanks showed different enrichment ability for the above 8 heavy metals. In sludge phase, the major components in tank A were corresponding to Pb and Cu, while Zn appeared to be the most abundant metal in O1, H and O2 tanks. The phase distribution of the studied heavy metals appeared different from each other, indicating that heavy metals had formed different chemical
(1)
C1 V1 C V = 2 2 + Qm 1000 1000
(2)
m = mA + mO1 + mH + mO2
(3)
where C1 (μg/L) and C2 (μg/L) are the total concentration of the 8 heavy metals in A influent and O2 effluent, respectively; V1 (L) and V2 (L) are the volume of A influent and O2 effluent, respectively; Q, is the total concentration of the 8 heavy metals in the sewage sludge, mg/kg; mA, mO1, mH and mO2 are the mass of A tank sludge, O1 tank sludge, H tank sludge and O2 tank sludge, respectively. m was the total amount of sludge with the mixed of A, O1, H and O2 sludge due to the fact that the coking sludge from different stage of biological reactors was centralized processed in most case. 2.5. DFT calculation To fully understand the interaction between OH−, S2−, NH3, CN−, SCN− and heavy metals, the Perdew-Burke-Erzenrhof exchange-correlation functional (PBE1PBE) of DFT widely applied in geometrical optimization was used (Sun et al., 2017). The basis set by the LANL2DZ were used for heavy metal atom, whereas the 6-31G(d) was used for C, 236
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Fig. 2. The aqueous (a) and sludge (b) concentration of heavy metals in different unit of coking wastewater treatment A-O1-H-O2 system.
forms with different water solubility substance within pH ranging from 7.2 to 8.5 in A-O1-H-O2 coking wastewater treatment system. The analysis of the total amount of heavy metals in the aqueous phase and sludge phase of coking wastewater treatment plant is conducive to the treatment of heavy metals in aqueous phase and the classified disposal of coking sludge containing heavy metals. As shown in Fig. 3a, the total amount of heavy metals in aqueous phase of A-O1H-O2 system followed the sequence of A > O1 > H > O2, indicating that each step of A-O1-H-O2 biological reactors was effective to remove a certain amount of heavy metals. The total amount of heavy metals in sludge phase of A-O1-H-O2 was quite different from these in the aqueous phase. The 8 heavy metals were mainly accumulated in A and H tanks. By comparing the sludge retention time in A, O1, H and O2, it can be found that the accumulation rule of the total amount of heavy metals is consistent with the sludge residence time (SRT) of each pool and the SRT of A, O1, H and O2 in our self-designed A-O1-H-O2 system were 180–200 d, 10–15 d, 150–180 d and 50–60 d, respectively. A-O1-H-O2 seemed to have a special ability in the phase distribution of examined heavy metals. Heavy metals directionally accumulated in A and H tanks would be beneficial for the splitting and disposal of coking sludge containing heavy metals. Besides, it would help control the flow of suspended sludge, reducing the re-release of heavy metals in the sludge. The heavy metals accumulated in sludge phase might be also the reason for the decrease in heavy metals concentrations in the aqueous phase of coking wastewater. Based on the fact that the amount of heavy metals accumulated in the sludge of different unit were different (Fig. 3b), it was recommended to deal with the sludge based on the different process stage so as to better prevent the environmental risk caused by heavy metal pollution. In addition, heavy metal extraction and reuse can be considered, basing on the distribution of heavy metals in sludge at different stages of coking wastewater treatment process.
To better understand the enrichment ratio of coking sludge, we used the following simplified model (Fig. 4) to calculate Ks/w. In the actual production process, the value of m was about 600 t per month. The sludge residence time (SRT) of A, O1, H and O2 in our self-designed AO1-H-O2 system were 180–200 d, 10–15 d, 150–180 d, 50–60 d, respectively. To facilitate the calculation, we set the SRT values of A, O1, H and O2 tank as 200 d, 15 d, 180 d and 60 d, respectively. According to Eq. (1), the value of Ks/w of the total eight heavy metals in coking waste sludge comparing to influent wastewater of A-O1-H-O2 system was found to be 6232, suggesting that coking sludge had strong enrichment ability for heavy metals. In the process of coking wastewater treatment, heavy metals might enter into the sludge phase through precipitation, complexation, microbial adsorption, and accumulation (Pagnanelli et al., 2009), which might be one reason for the high enrichment ability of coking sludge. Another reason might be due to the decomposition of organic and inorganic substance into CH4, CO2, N2, H2S and other gases in the process of A-O1-H-O2 biological system for coking wastewater treatment, leading to the observed content of heavy metal in coking sludge increased due to the fact the calculation about heavy metal content in coking sludge was based on dry weight (Chipasa, 2003). The obtained Ks/w value was the average of A, O1, H and O2 tank, so the enrichment ratio of A and H tank would be much higher than this value based on the results of Fig. 3b. According to previous study (Karvelas et al., 2003), the heavy metals in sludge phase were closely related to the organic matter. There are large amount of dissolved inorganic and organic matter in coking wastewater including ammonia nitrogen, phenol, cyanide, sulfide and other hundreds of organics which might regulate the distribution of heavy metals. If the regulation law really exist, it might be a reason why coking sludge has high enrichment capacity for heavy metals. Herein, we determined the concentration of ammonia nitrogen, thiocyanide,
Fig. 3. The total concentration of heavy metals in aqueous phase (a) and sludge phase (b) of A-O1-H-O2 system. (The green line represents the change trend of various substances.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 237
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Fig. 4. The simplified A-O1-H-O2 system that used for the calculation of enrichment ratio.
in the range of 5.0–8.5, Cu and Cr ions will be precipitated. While for the precipitation of Zn, Cd, and Ni ions, stronger alkaline condition is needed (Alwan, 2008). The chemical precipitation mechanism is presented in Eqs. (4) and (5). The CN−, SCN−, NH3 and OH− have a stronger ability in chelating with heavy metals (Dash et al., 2009; Kim et al., 2002; Schroder et al., 2001), and the corresponding reaction equation as Eqs. (6)–(9). Combined with the chemical reaction occurring in A-O1-H-O2 system (Table 1), it is meaningful to discuss the reaction related to heavy metals which may explain the reason why coking sludge has a strong enrichment ability for the heavy metals.
M 2+ + 2OH
M 2+
+
S2
M 2+ + nCN M 2+
+ nSCN
M 2+ + nNH3
M 2+
Fig. 5. The total concentration of typical compounds in aqueous phase of A-O1H-O2 system. (The green line represents the change trend of various substances.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
+ nOH
(4)
M (OH )2
(5)
MS [M (CN )n]2 [M (SCN )n
n (n
=1
]2 n (n
=1
[M (NH3)n]2+ (n = 1
[M (OH )n
]2 n (n
=1
(6)
4)
(7)
4)
(8)
6)
(9)
4) −
A tank: There are a high concentration of CN and NH3 in A tank (Table S3). Because of complexation, a certain amount of [M(CN)n]2−n Table 1 The main reaction occurred in A-O1-H-O2 system.
cyanide and sulfide in the aqueous phase and the results were shown in Fig. 5. As presented in Fig. 5, the concentration of ammonia nitrogen, thiocyanide, cyanide and sulfide in the aqueous phase of A-O1-H-O2 system followed the sequence of A > O1 > H > O2, which was in accordance with the changes trend of total heavy metals in aqueous phase. The pH values of A, O1, H and O2 tank were 8.2, 7.4, 8.5 and 7.2, respectively. The pH values reflected the concentration of hydroxide in coking wastewater. These phenomena indicated the interaction between heavy metals and typical compounds in coking wastewater were really existed.
Unit
Main Reaction
A
M2++S2−→MS↓ M2++OH−→M(OH)2↓ M2++SCN−→[M(SCN)n]2-n M2++CN−→[M(CN)n]2-n M2++NH3→[M(NH3)n]2+ SCN-+H2O+O2→HCO3−+NH4++SO42− +H+ NaCN+H2O+O2→NaHCO3+NH3 C6H5OH+O2→CO2+H2O S2− +O2→SO42M2++NH3→[M(NH3)n]2+ M2++SO42−→MSO4↓ M2++OH−→M(OH)2↓ HCN+H2O→HCOOH+NH3 HSCN+H2O→S2− +HCNO+SH− HCNO+H2O→CO2+4NH3 C6H6ONa+H2O→C6H5OH+NaOH M2++OH−→M(OH)2↓ M2++S2−→MS↓ SCN-+H2O+O2→HCO3−+NH4++SO42− +H+ C6H5OH+O2→CO2+H2O S2− +O2→SO42NH4+→NO2− NO2−→NO3− Organic compounds+O2→CO2+H2O M2++OH−→M(OH)2↓ M2++SO42−→MSO4↓ M2++CO32−→MCO3↓
O1
3.2. Regulation mechanism of typical compounds on the distribution of heavy metals in A-O1-H-O2 coking wastewater treatment system
H
Based on the discussion in 3.1, we infer that the typical compounds in coking wastewater will influence the distribution and fate of heavy metals. Herein, we use CN−, SCN−, S2−, NH3 to represent cyanide, thiocyanide, sulfide and ammonia nitrogen, respectively, in order to facilitate the discussion of chemical reactions occurred in coking wastewater related to these typical substances. OH− and S2− will form chemical precipitation with heavy metals (Fu and Wang, 2011). The concentration of OH−, which is marked as pH of the solution, is key for control chemical precipitation and complexation reaction of heavy metals. For different heavy metals, pH value required for precipitation is slightly different from each other. For example, when the pH value is
O2
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Fig. 6. The main chemical reaction about heavy metals in A-O1-H-O2.
and [M(SCN)n]2−n would be formed. The Ksp value of MS (Cd2+, Hg2+, Cu2+, Zn2+, Pb2+ and Ni2+) was lower than the M(OH)2, suggesting that S2− had a stronger ability to bind free M2+. Therefore, the chemical precipitation species in coking sludge were mainly in the form of MS and this might be also the reason why the content of heavy metals in A sludge was relatively high. The simplified model about the main chemical reaction in A-O1-H-O2 concerning about heavy metals can also be seen in Fig. 6. O1 tank: The complexes of [M(CN)n]2−n and [M(SCN)n]2−n in A tank would then transfer to O1 tank through self-flow in the wastewater treatment process. In O1 tank, organic substances and most reducing inorganic substances such as CN− and SCN− were oxidized to HCO3−, NH4+ and SO42−, so heavy metals would be released from the [M (CN)n]2−n and [M(SCN)n]2−n complexes and then existed in free form. With ammoniation of nitrogen-containing organic compounds in O1 tank, some free heavy metals will complex with NH3 and then enter H reactor. The other free M2+ would form M(OH)2 with OH− because of the alkaline aqueous solution in O1 tank (pH = 7.4). H tank: The main reactions occurred in H reactor was denitrification and the concentration of ammonia nitrogen would be decreased along with the release of N2 into the atmosphere. As results, some free M2+ will be released from [M(NH3)n]2+complex. Along with the denitrification reaction, the pH value of H tank increased slightly (pH = 8.5), and a large amount of free heavy metal ions will precipitate in the form of metal hydroxides. Besides, H reactor is almost anoxic and partial SO42− will be reduced to S2− because of almost anaerobic environment of H tank, which can be realized by regulating DO (dissolved oxygen), so some free heavy metal ions will form MS precipitation with S2−. Altogether, the total content of heavy metals in the sludge phase of H tank was higher than that of A, O1 and O2 tank. O2 tank: The main reactions happened in O2 reactor was nitrification and oxidation. The reductive ligands in residual M2+-reductive ligand complexes such as MS and [M(SCN)n]2−n self-flowing from H tank would be oxidized in O2 tank. At the same time, free M2+ would be released from M2+-reductive ligand complexes then be precipitated with -OH- (the pH of O2 tank was about 7.2 which was alkaline), SO42− and CO32−, further be enriched in sludge phase of O2 tank.
the form of anions in the solution and repulsive effect might be existed among Cr, As and the typical compounds (OH−, S2−, CN−, SCN− and NH3) in the coking wastewater, so Cr and As were not taken into consideration in the DFT calculation. The most stable structure of M2+-OH, M2+-S2-, M2+-NH3, M2+-CN- and M2+-SCN- were shown in Table 2. M2+ referred to the divalent heavy metal ions. As presented in Table 2, the configurations of M2+-OH-, M2+-S2-, M2+-NH3, M2+-CN- and M2+-SCN- were V type, linear type, trigonometric cone type, linear type and V type, respectively. △E value of heavy metals and typical compound complexes in coking wastewater were following the sequence (Table 3): OH: Cu2+ > Pb2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; S2−: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Hg2+ > Ni2+; CN−: Zn2+ > Cu2+ > Cd2+ > Hg2+ > Pb2+ > Ni2+; SCN−: Zn2+ > Cd2+ > Pb2+ > Hg2+ > Cu2+ > Ni2+; NH3: Cu2+ > Zn2+ > Cd2+ > Pb2+ > Hg2+ > Ni2+. On the whole, based on the complexing ability between heavy metals and anions, we can determine which metal ions are preferentially combined with the typical compounds in coking wastewater so as to provide a theoretical basis for the prevention and treatment of heavy metals in coking wastewater. It would also provide guidance for analysis of specific existence forms of heavy metal components in coking wastewater. For example, the ΔE (au) of Cu2+-NH3 was higher than that of the other M2+-NH3, indicating that Cu2+ would be priority to form M2+-NH3. 4. Conclusions The regulation mechanism of typical compounds including OH−, CN−, SCN−, S2−, and NH3 on the distribution of 8 heavy metals including Cd, Hg, Pb, Ni, Zn Cr, As and Cu in coking wastewater treatment system were systematically discussed for the first time on the basis of fully considering the biological and chemical reactions in each unit of the A-O1-H-O2 coking wastewater treatment system. The changes in the concentrations of NH3, SCN−, CN− and S2− in the system was in accordance with these of total heavy metals in aqueous phase (with an order of A > O1 > H > O2). The interaction between typical compounds and heavy metals were further confirmed by DFT calculation, showing consistency with the engineering data observed in the A-O1-HO2 system, suggesting that the regulation mechanism of typical compounds on heavy metals was effective. Altogether, heavy metals in coking sludge could achieve enrichment and separation based on hydrolysis, complexation, precipitation, and oxidation and reduction effect by using A-O1-H-O2 biological coking wastewater treatment system under no sludge reflux operation mode. The no sludge reflux mode used in the A-O1-H-O2 system allowed the risk management of heavy metals to be achieved during the coking wastewater treatment, which is a
3.3. DFT calculation The components in coking wastewater including NH3, CN−, SCN−, OH and S2− might regulated the distribution of heavy metals according to the discussion in section 3.2. Herein, DFT calculation was used to verify the regulation mechanism of the typical compounds on heavy metals in coking wastewater. Since Cr and As mainly occurred in −
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Table 2 Optimized structure of OH−, S2−, NH3, CN− and SCN− with common heavy metal ions. Heavy metals
OH−
S2-
CN−
NH3
SCN−
Cd2+
Hg2+
Cu2+
Zn2+
Pb2+
Ni2+
revolutionary improvement compared to the traditional technology in which the risk of heavy metal was not well controlled owing to the reflux of sludge.
Table 3 △E value of heavy metals and typical compound complexes in coking wastewater. Typical compounds OH
−
S2-
CN−
SCN−
NH3
Heavy metals 2+
Cd Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+ Cd2+ Hg2+ Cu2+ Zn2+ Pb2+ Ni2+
EHomo(au)
ELumo(au)
ΔE(au)
−0.25010 −0.25477 −0.34584 −0.27125 −0.34138 −0.30172 −0.19198 −0.19437 −0.22968 −0.20233 −0.25362 −0.21614 −0.32659 −0.31978 −0.38503 −0.34031 −0.35795 −0.35903 −0.25340 −0.22808 −0.32356 −0.26582 −0.30015 −0.30742 −0.36992 −0.29306 −0.50872 −0.40865 −0.40942 −0.37815
−0.05186 −0.06390 −0.08782 −0.05267 −0.10348 −0.18853 −0.04033 −0.04902 −0.05484 −0.04142 −0.07391 −0.13410 −0.05051 −0.06773 −0.09356 −0.04822 −0.13758 −0.21077 −0.06954 −0.09745 −0.22956 −0.07206 −0.13062 −0.21379 −0.06603 −0.09793 −0.11907 −0.06638 −0.14460 −0.22536
0.19824 0.19087 0.25802 0.21858 0.23790 0.11319 0.15165 0.14535 0.17484 0.16091 0.17971 0.08204 0.27608 0.25205 0.29147 0.29209 0.22037 0.14826 0.18386 0.13063 0.09400 0.19376 0.16953 0.09363 0.30389 0.19513 0.38965 0.34227 0.26482 0.15279
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