Toxicity reduction of reverse osmosis concentrates from petrochemical

Prévia do material em texto

Chemosphere 286 (2022) 131582
Available online 16 July 2021
0045-6535/© 2021 Elsevier Ltd. All rights reserved.
Toxicity reduction of reverse osmosis concentrates from petrochemical 
wastewater by electrocoagulation and Fered-Fenton treatments 
Chenhao Gong *, Xiaojing Ren, Junxing Han, Yue Wu, Yaling Gou, Zhongguo Zhang **, Peiran He 
Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, No.1 Gao Li Zhang Road, Beijing, 100095, China 
A R T I C L E I N F O 
Handling Editor: E. Brillas 
Keywords: 
Toxic organic matters 
Electrocoagulation 
Size fractionation 
Excitation-emission matrix 
Heavy metals 
A B S T R A C T 
In this work, both Electrocoagulation (EC) and Fered-Fenton (FF) technologies were used to treat reverse osmosis 
concentrates (ROC) from petrochemical production. The toxicity reduction capacity and mechanism were 
comparatively assessed during these two treatments. The results showed that FF exhibited higher capacity to 
reduce toxicity than EC in the 30 min treatment, which could be attributed to the removal of organic pollutants 
and heavy metals. The results showed that the ROC contained organics with molecular weight of 1200 g mol− 1 
and 220 g mol− 1, which mainly consisted of the soluble microbial by-product-like and humic acid-like sub-
stances. The removal of these organics directly led to the noticeable toxicity reduction. Alkanes, haloalkanes, 
ketones, PAHs, and other four organic pollutants were the dominant species in the ROC, and the removal of small 
molecular weight organic pollutants played an essential role in reducing toxicity. FF exhibited stronger capacity 
to remove PAHs, BTEXS and haloalkanes, and the removal efficiencies for the PAHs were in the following order: 
5-ring > 4-ring > 3-ring > 2-ring. The promotion of heavy metals removal appeared to be favorable for 
decreasing toxicity in ROC. This study illustrated the mechanism of the toxicity reduction and the characteristics 
of pollutants removal during FF and EC treatments, and provided valuable guidance for petrochemical 
manufacturing to the toxicity reduction and operation of wastewater treatment facilities. 
1. Introduction 
Nowadays, the petrochemical industry wastewater is one of the most 
critical environmental issues worldwide. The petrochemical industry 
produces substantial quantities of hazardous materials, such as petro-
leum hydrocarbon, aniline, nitrobenzene, polycyclic aromatic hydro-
carbons (PAHs), phenols and their derivatives (Fu et al., 2016), which 
are potentially responsible for generating hazardous effluents to be 
discharged in the environment (Rocha et al., 2012). Thus, the removal of 
these persistent organic contaminants has become a significant problem 
in environmental research, especially in view of water reclamation 
purposes. Among several different techniques used to solve this prob-
lem, the application of reverse osmosis (RO) holds promise for effi-
ciently removing these organic contaminants (Cartagena et al., 2013). 
However, during the course, the biorefractory organic pollutants is 
accumulated and concentrated, and RO concentrates (ROC) are gener-
ated (Panizza and Cerisola, 2009). 
To minimize hazardous organic pollutants and reduce the toxicity 
posed by such contaminated wastewater, several technologies have been 
investigated and applied for the ROC in the petrochemical industry. For 
example, adsorption treatment by powdered activated carbon (PAC) is 
effective to remove dissolved organics in the ROC, but the high organic 
charge of ROC strongly influences the removal efficiency compared to 
the adsorption capacity of PAC (Zhao et al., 2012). In addition, mem-
brane distillation and evaporation treatments are also conducted in the 
ROC treatment, however, these technologies are also limited by the high 
operation and construction cost (Cui et al., 2018). The wet air oxidation 
(WAO) method is also applied to treat ROC, which could achieve high 
organic pollutants removal efficiency in a short reaction time, however, 
it is reported the interactions between oxygen molecules and organic 
compounds only take place under high temperature and high-pressure 
conditions (e.g., 120–350 ◦C and 5–200 bar) (Oliviero et al., 2003), 
which inevitably could enhance the operational cost and difficulty. 
During the past two decades, the electrocoagulation (EC) treatment 
is considered a cost-effective technique to remove the refractory or-
ganics and eliminate heavy metals and soluble ionic contaminants from 
aqueous conditions, and has been widely applied in the industrial 
wastewater treatment (Kanakaraju et al., 2018; Nasrullah et al., 2019). 
* Corresponding author. 
** Corresponding author. 
E-mail addresses: chenhaogong@163.com (C. Gong), cnzhang@163.com (Z. Zhang). 
Contents lists available at ScienceDirect 
Chemosphere 
journal homepage: www.elsevier.com/locate/chemosphere 
https://doi.org/10.1016/j.chemosphere.2021.131582 
Received 22 April 2021; Received in revised form 29 June 2021; Accepted 15 July 2021 
mailto:chenhaogong@163.com
mailto:cnzhang@163.com
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Recent studies indicate that the main reactions of EC treatment comprise 
of coagulation, redox reaction and flotation. The dynamic generated 
metal hydroxide flocs in the sacrificial anode is the foremost reactions of 
EC, which is effective to reduce the organic pollutants (Mollah et al., 
2014; Brillas and Martinez-Huitle, 2015). Moreover, the oxidation re-
actions in the EC treatment can promote the pollutants removal, but how 
to enhance the biorefractory organics pollutants removal performance is 
one of the significant issues in industrial wastewater treatment (Akansha 
et al., 2020). 
The traditional Fenton process is widely used for the industrial 
wastewater treatment as it could continuously generate hydroxyl radi-
cals (•OH), a non-selective and powerful oxidizing agent (Rosales et al., 
2018). To ensure adequate Fenton reaction, it requires an external 
addition of divalent iron ion and hydrogen peroxide, which increase the 
reagents cost. The Fered-Fenton (FF) treatment is proposed and received 
great attention for wastewaters remediation as FF treatment only needs 
external addition of hydrogen peroxide in the EC treatment. Therefore, it 
could provide several technical superiorities such as high adaptability, 
easy installation, the possibility for automation (Akansha et al., 2020). 
Up to now, there is a lack of research on the toxicity reduction of ROC in 
the comparative treatments of EC and FF; moreover, according to the 
reported work, the toxicity reduction is attributed to the removal of 
refractory organic and inorganic pollutants in the ROC (Sojiadeyinka 
and Rimrukeh, 1999). To the best of our knowledge, the different 
characteristics of pollutants in ROC and the relevant removal perfor-
mance during the EC and FF treatments have not been thoroughly 
described to date, and the effect of the pollutants removal on the toxicity 
reduction during EC and FF treatments has not been reported in the 
literature. Lack of knowledge in this regard makes it hard to minimize 
discharged effluents’ hazardous impact from petrochemical industries. 
This study aims to promote the toxicity reduction of ROC from 
petrochemical industries, and reveal the effect of pollutants removal and 
degradation characterizations on the toxicity reduction during the EC 
and FF treatments. To understand the pollutants removal characteriza-
tions and toxicity reduction mechanism, size exclusion chromatography 
(SEC) with organic carbon and UV detection (SEC-OCD/UV), three- 
dimensionalexcitation emission matrix fluorescence (3DEEM) and 
other analytical instruments are applied. The findings of this work 
would provide valuable insights into the toxicity reduction mechanism 
in the ROC during the EC and FF treatments, and offer guidance for the 
toxicity reduction and pollutants removal method in the petrochemical 
industrial treatment. 
2. Materials and methods 
2.1. Petrochemical ROC 
The ROC was obtained from a petrochemical industry located in the 
northern part of China, and the ultrafiltration of this effluent was treated 
prior to RO in the industrial plant treatment. The obtained ROC was 
stored at 4 ◦C in the laboratory before using. The main characteristics of 
the ROC were provided in Table SM. 1. 
2.2. EC and FF treatments 
The EC and FF treatments were conducted at room temperature and 
ambient pressure in a cylindrical glass container with 200 mL volume 
under constant magnetic stirring at a speed of 150 rpm. The iron elec-
trodes were used in this study, one anode and one cathode electrode (5 
cm long, 4 cm wide, and 1 mm thick) were wired to the power supply 
(Dahua, MC-100/5) separately, the spacing between electrodes was 3 
cm, and the voltage and current could be accurately controlled and read 
by the power supply. For the EC treatment, the treated samples were 
collected at regular time intervals from the cylindrical glass container, 
and then filtered by a flat-sheet acetate fiber membrane (Navigator/ 
13–0.22, Changsheng, China) to eliminate the flocs and prepared for 
further analysis. For the FF treatment, the hydrogen peroxide was added 
to the ROC with sufficiently stirring before connecting to power supply, 
and 5 M NaOH solution was added instantly after the treatment to 
terminate the oxidation reaction, and then treated samples were 
collected and filtered for the further use. Each sample or experiment was 
measured three times to get an average value. 
2.3. Analytical methods 
The ROC was characterized by 3DEEM fluorescence using a Lumina 
spectrometer (Thermo Scientific, USA), the scanning emission spectra 
from 250 to 600 nm, and excitation spectra from 200 to 500 nm with 5 
nm sampling interval was applied in the ROC sample analysis. Based on 
the operationally defined fluorescence boundaries of organic matters, 
five different EEM types of substances, were identified in ROC (Jacquin 
et al., 2017). The calculation of the detected fluorescence in the different 
region was used by Jacquin’s method (Jacquin et al., 2017). (Table SM. 
2). According to the category, the regions 1 and 2 was the aromatic 
protein substances, region 3 was for the fulvic acid-like (FA-like) sub-
stances, region 4 and region 5 was for the soluble microbial 
by-product-like (SMP-like) and humic acid-like (HA-like) substances. 
Size exclusion chromatography with organic carbon and UV detec-
tion (SEC-OCD/UV) was applied to determine the size fractionation of 
organic matters in water samples, which was equipped with a TSK-GEL 
G3000PWxl column (Tosoh Bioscience, Japan). The total organic carbon 
(TOC) of ROC was analyzed by an analyzer (Elementar Liqui TOC II, 
Germany). Conductivity and pH were measured using a multi-function 
meter (Leici-318, China). The identification and quantification of 
heavy metals were analyzed by ICP-MS (Agilent 7700, USA), the 
chemical oxygen demand (COD) was analyzed by a UV–vis spectro-
photometer (Lianhua, China). The biological oxygen demand (BOD5) 
was measured by BOD controller analyzer (WTW, Germany). The total 
phosphorus and nitrogen were measured by a UV spectrophotometer 
(Shimadzu 2550, Japan), the toxicity of samples was analyzed using 
Modulus Luminometer (Modulus 96, USA). The organics in the samples 
were measured using a gas chromatography coupled with mass spec-
trometry (GC/MS) system (Agilent, 7890 GC/5977 A MS), and the 
procedure was provided in Supplementary Information. Each sample 
was measured three times to get an average value. 
3. Results and discussion 
3.1. TOC removal during EC and FF treatments 
The TOC removal performance by EC and FF after 30 min treatment 
under different current densities was presented in Fig. 1a. Averagely, the 
FF showed higher TOC removal efficiencies under 10, 20 and 30 mA 
cm− 2 current density. The TOC removal efficiencies of EC under 10, 20 
and 30 mA cm− 2 were 27%, 32% and 35%; in comparison, the FF ob-
tained 30%, 35% and 40% of TOC removal efficiencies with 10 mM 
H2O2 dosage, respectively. According to the reaction, the addition of 
H2O2 in the EC could accelerate the organic removal, as the hydroxyl 
radicals could be produced by reactions between H2O2 and Fe (II) ions 
(Eq. (1)) (Brillas et al., 1998; Chiou, 2007), and the generated hydroxyl 
radical is regarded as one of the most reactive free radicals and shows 
high activity for the removal and degradation of organic molecules 
(McQuillan et al., 2020). The conclusion that the FF treatment could 
obtain higher TOC removal efficiencies was also supported by the other 
research concerning real wastewater treatment, Atmaca and co-workers 
applied FF to treat landfill leachate wastewater, and it was found that 
the COD removal was increased by 10% by adding external H2O2 dosage 
in the EC treatment (Atmaca, 2009). 
Fe2+ +H2O2 → Fe3+ +OH− +•OH (1) 
The TOC removal performance under different H2O2 dosage in FF 
C. Gong et al. 
Chemosphere 286 (2022) 131582
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treatment was presented in Fig. 1b. The increase of H2O2 dosage in FF 
treatment could enhance the TOC removal. The TOC removal effi-
ciencies under 5, 10, 15 and 30 mM H2O2 dosage was 38.5%, 40%, 43% 
and 45.5%. The increased efficiency on the TOC removal was attributed 
to the increase of hydroxyl radical concentration by adding H2O2 in EC 
treatment (Lopez et al., 2004). The TOC removal trend line at the initial 
10 min showed steeper than the remaining 20 min, and the prolonged 
treatment time and increased dosage just gave milder TOC removal in-
crease. The phenomenon can be explained that the initial concentration 
of H2O2 played an important role in the FF treatment, appropriate 
amount of H2O2 can promote the formation of hydroxyl radicals and 
ultimately achieve the efficient removal of pollutants; but the hydroxyl 
radical scavenging effect of H2O2 (Eq. (2) and (3)) and the recombina-
tion of the hydroxyl radical could not be conducive to the degradation of 
pollutants (Perez et al., 2002). Hence, proper control of H2O2 dosage in 
the FF treatment was critical for the promotion of the organic pollutants’ 
removal efficiency. 
H2O2 +•OH → HO2• + H2O (2) 
HO2•+ •OH→+ H2O + O2 (3) 
3.2. Toxicity analysis 
To analyze the toxicity change of ROC during the EC and FF treat-
ments and reveal the relationship between pollutants removal and 
toxicity, the current density of 30 mA cm− 2 for EC and FF treatments and 
30 mM H2O2 dosage for FF treatment was applied in the toxicity and 
pollutants characterization analysis. The results of toxicity change 
during the EC and FF treatments were presented in Fig. 2. Approxi-
mately 68% luminescence inhibition was observed in the ROC due to the 
existence of trace pollutants. After 30 min treatment, the luminescence 
inhibition for both EC and FF treatments showed a fluctuation trend, and 
luminescence inhibition was reduced by 63.2% (FF) and 52.9% (EC), FF 
could obtain 10.3% lower luminescence inhibition than EC during the 
treatment. The results suggested that the luminescence inhibition was 
greatly decreased at the initial 5 min for both treatments, and then 
showed obvious increase between 5 and 10 min and finallycontinuously 
decreased in the remaining treatment. The increase of luminescence 
inhibition occurred during the treatments can be explained that lumi-
nescence inhibition was sensitive to various factors that cannot be 
controlled and studied separately during the realistic effluent treatment, 
the formed by-products and pollutants concentration change would in-
fluence the luminescence inhibition (Ganiyu et al., 2016). Similar 
findings were also reported by Kateb’s study, the authors addressed that 
the luminescence inhibition showed a fluctuation trend, which was 
directly influenced by the toxic organic pollutants contained in landfill 
leachates and formed by-products in the electrochemical advanced 
oxidation process (Kateb et al., 2019). Therefore, it was necessary to 
draw a possible relationship between pollutants removal and toxicity 
change by evaluating the characterization of pollutants in the treatment. 
3.3. The organic pollutants characterization analysis 
3.3.1. 3DEEM analysis 
The 3DEEM results of ROC, EC-treated ROC, FF-treated ROC and the 
fluorescent intensity evolution were presented in Fig. 3a–d, respectively. 
It was addressed that the ROC was mainly composed of soluble micro-
bial by-product-like (SMP-like) substances (region 4) and humic acid- 
like (HA-like) substances (region 5) as shown in Fig. 3a, and their 
fluorescent intensities accounted for approximately 80% for all EEM 
regions. Besides, ROC also contained some aromatic proteins (region 1 
and 2) and fulvic acid-like (FA-like) substances (region 3). Significant 
changes were observed in fluorescent intensity for all EEM regions after 
30 min EC and FF treatments (Fig. 3b and c), and FF showed higher 
removal efficiency on the fluorescent substances (Fig. 3d). After 30 min 
treatment, FF could completely remove aromatic proteins I, II and FA- 
like fluorescent substances (region 1, 2 and 3). Comparatively, EC 
treatment achieved the 95% of removal efficiency for aromatic protein I 
substances, 90% for aromatic protein II substances and 87% for fulvic 
acid-like substances, respectively. Unlike the aromatic proteins and FA- 
like substances, SMP-like and HA-like fluorescent substances were 
appeared to be recalcitrant for EC and FF treatments, EC and FF could 
remove 45% and 65% of SMP-like substances, and 62% and 81% of HA- 
like fluorescent substances. Based on the EEM results, the FF showed 
higher removal efficiency on the fluorescent substances than EC treat-
ment, which can be explained that recalcitrant organic pollutants or 
Fig. 1. TOC removal results for EC and FF treatments of the ROC: (a) obtained with different applied current densities (initial pH = 6.2, H2O2 dosage = 10 mM for FF, 
temperature = 25 ◦C); (b) TOC removal results obtained with different H2O2 dosage (initial pH = 6.2, current density = 30 mA cm− 2, temperature = 25 ◦C). 
Fig. 2. Evolution of luminescence inhibition under EC and FF treatments. 
(initial pH = 6.2, current density = 30 mA cm− 2, H2O2 dosage = 30 mM for FF, 
temperature = 25 ◦C). 
C. Gong et al. 
Chemosphere 286 (2022) 131582
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partial degradation products in ROC can be further oxidized in FF 
treatment (Pinto et al., 2012), and the formed hydroxyl radicals in the FF 
treatment were capable of removing the recalcitrant substances 
including SMP-like and HA-like fluorescent substances in ROC. In 
addition, it would be indicated that the reduction of main pollutants of 
SMP-like and HA-like substances led to the toxicity reduction (Fig. 2) 
and significant TOC removal (Fig. 1b). 
3.3.2. Size fractionation of organic matters analysis 
The SEC-OCD/UV results provided important information on the 
removal of different organic fractions by EC and FF treatments. As 
shown in Fig. 4, a medium molecular weight (MMW) peak at approxi-
mately 9.1 min, and a low molecular weight (LMW) shoulder peak at 
approximately 10.3 min were observed in the TOC chromatograms. 
Corresponding apparent molecular weights were approximately 1200 g 
mol− 1 and 220 g mol− 1, respectively. Moreover, both FF and EC treat-
ments could not effectively remove medium molecular weight organic 
matters, and their removal efficiency on the 1200 g mol− 1 organic 
fraction reached 38% and 33%, respectively. But the removal efficiency 
of small molecular weight organic pollutants by FF and EC treatments 
was quite different, FF treatment approximately could remove 86% of 
220 g mol− 1 organic pollutants, and EC just reached 67% of removal 
efficiency (Fig. 4). These results demonstrated that EC and FF obviously 
removed the MMW and LMW organic pollutants, FF showed higher 
removal efficiency than EC treatment and the biggest difference be-
tween these two treatments was the removal of small molecular weight 
organic pollutants. Combined the 3DEEM and toxicity results, it was 
found that the organic pollutants with molecular weight of 1200 g mol− 1 
and 220 g mol− 1 mainly consisted of the soluble microbial by-product- 
like and humic acid-like substances in ROC, and their removal directly 
led to the noticeable reduction of toxicity and TOC, especially the 
increase removal of small molecular weight organic pollutants in FF 
treatment had crucial impact on the toxicity reduction. 
3.3.3. Identification of organic matters by GC-MS analysis 
The change of organic pollutants with low and medium molecular 
weight had direct impact on the toxicity during the EC and FF treat-
ments. Therefore, the characterization and removal of the organic pol-
lutants was investigated using GC-MS and the results were displayed in 
Fig. 3. Fluorescence excitation emission matrix spectra: (a) ROC, (b) 30 min EC-treated ROC, (c) 30 min FF-treated ROC and (d) different fluorescent regions change 
under EC and FF treatments. (initial pH = 6.2, current density = 30 mA cm− 2, H2O2 dosage = 30 mM for FF, temperature = 25 ◦C). 
Fig. 4. Comparison of the effects of EC and FF on the size fractionation of the 
ROC TOC. (initial pH = 6.2, current density = 30 mA cm− 2, H2O2 dosage = 30 
mM for FF, treatment time = 30 min, temperature = 25 ◦C). 
C. Gong et al. 
Chemosphere 286 (2022) 131582
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Fig. 5. Alkanes, haloalkanes, ketones, PAHs, olefins, BTEXS, phenols and 
organic acids were detected in ROC, with a percentage of 54%, 9%, 6%, 
13%, 4%, 7%, 4% and 3% of the total organic pollutants, respectively. 
Among these pollutants, alkane, PAHs, haloalkane and BTEXS were the 
dominant species (Fig. 5a). The occurrence of these organic pollutants 
was strongly related to the extraction and refining procedures (Wu et al., 
2017). Moreover, it can be concluded that these organics and persistent 
organic pollutants (POPs) were main organic category in ROC, which 
was potentially toxic and harmful to surrounding environment safety 
and human being’s health (Sponza and Oztekin, 2010). The removal of 
these organic pollutants by EC and FF treatments was analyzed and the 
result was presented in Fig. 5b. In general, the FF treatment had higher 
removal efficiency than EC treatment, it was showed that the removal 
efficiency for the eight organic categories in the ROC was above 60%. 
The removal efficiency for phenols and organic acids was as high as 
89%. Comparatively, EC had lower removal efficiency for these organic 
pollutants, the removal efficiency was mainly between 34% and 60%, 
for example, the removal efficiency of PAHs just reached 34%. 
As shown in Fig. 5, alkanes accounted for 54% of total organic pol-
lutants. The main component of alkanes was octadecane. EC and FF 
showed removalefficiency of 45% and 67%, respectively. PAHs, hal-
oalkane and BTEXS accounted for 28% of the organic pollutants in ROC 
and they were identified as carcinogenic, mutagenic and teratogenic 
(Joshi et al., 2017). Thus, the removal of these toxic organic pollutants 
during EC and FF treatments was compared to reveal the effect of 
organic pollutants removal on the toxicity reduction (Fig. 6). The PAHs 
in the ROC mainly contained 2-ring PAHs (dichloronaphtalene and 
naphthalene), 3-ring PAHs (phenanthrene, fluorene and acenaph-
thylene), 4-ring PAHs of fluoranthene, and 5-ring PAHs of pyrene. Based 
on the results, FF showed higher removal efficiency than EC, greater 
than 87% of pyrene and 67% of fluoranthene were removed by FF. The 
removal efficiency for the 3-ring PAHs was mainly maintained 55%– 
62%, for the 2-ring PAHs of dichloronaphtalene and naphthalene was 
40% and 38%. As evidenced from Fig. 6, the trimethylbenzene and 
2-ethylnitrobenzene were found to be the dominant BTEXS. Bromoform 
and 1-brompentane were the main haloalkanes in ROC. FF had effective 
effect on the removal of BTEXS and haloalkanes, the removal efficiencies 
of BTEXS and haloalkanes reached 52%–64%. In contrast, EC exhibited 
lower removal efficiencies, for instance, it just removed 25% of bro-
moform. The results showed that the PAHs, BTEXS and haloalkanes had 
undergone different removal performances during FF and EC treat-
ments. The removal performances differed in EC and FF treatments, 
which was related to their reaction mechanisms. Besides the adsorption 
reaction through Fe hydroxide flocs in the EC treatment, the formed 
hydroxyl radicals by adding H2O2 in FF treatment had stronger 
Fig. 5. Characterization of toxic organic matters in the ROC (a) identification of ROC, (b) the different toxic organic matters removal under EC and FF treatments. 
(initial pH = 6.2, current density = 30 mA cm− 2, H2O2 dosage = 30 mM for FF, treatment time = 30 min, temperature = 25 ◦C). 
Fig. 6. The PAHs, haloalkane and BTEXS change after EC and FF treatments. 
(initial pH = 6.2, current density = 30 mA cm− 2, H2O2 dosage = 30 mM for FF, 
treatment time = 30 min, temperature = 25 ◦C). 
C. Gong et al. 
Chemosphere 286 (2022) 131582
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oxidation reaction which had effective impacts on the degradation of 
toxic organic pollutants (Zodi et al., 2011; Gong et al., 2017). The 
removal efficiencies for the PAHs were in the following order: 5-ring >
4-ring > 3-ring > 2-ring during the FF treatment, since the recalcitrance 
and biotoxicity of PAHs to microbial degradation increased directly with 
the molecular weight (Ozaki et al., 2015), thus the remarkable removal 
of the heavier PAHs by FF which directly reduced the toxicity. Through 
contrastive analysis, it was found that the FF had higher removal effi-
ciency of BTEXS and haloalkanes than EC. BTEXS and haloalkanes 
showed high toxicity, and thus their removal had positive influence on 
the toxicity reduction, which was also addressed by other authors (Mello 
et al., 2019; Kaur et al., 2021). Moreover, the molecular weight distri-
bution of these pollutants was between 120 and 252 g mol− 1, which 
could support the statement of SEC-OCD/UV analysis that the removal 
of small molecular weight organic pollutants had crucial impact on the 
toxicity reduction. 
3.4. Heavy metals removals 
The heavy metals can be continuously transported, transformed and 
enriched in the environment. Their existence could directly threat the 
surrounding environment’s safety and toxicity (Al Aji et al., 2012). 
Therefore, it was necessary to analyze these heavy metals reduction 
throughout the EC and FF treatments. Fig. 7 showed the effect of EC and 
FF on the removal of heavy metals contained in the ROC. As expected, 
the removal efficiencies of four heavy metals increased, with increasing 
the treatment time. FF had higher removal efficiency than EC, 64% of 
As, 43% of Cr, 57% of Ni and 36% of Pb were removed by FF. In contrast, 
the removal efficiencies of these heavy metals under EC treatment 
reached 35%, 17%, 21% and 28%, respectively. The difference on the 
heavy metals’ removal performances in EC and FF treatments can be 
attributed to several factors, such as initial concentrations, current 
density, pH and organic (Kobya et al., 2010; Aoudj et al., 2015). For 
example, in Moreno-Casillas’ study, more than 97% of Ni can be 
removed in 15 min EC treatment, but the increasing treatment time 
obviously changed the pH condition of reaction which directly influence 
the formation of Fe (III) hydroxides and Fe (II) hydroxides during EC 
treatment, that finally affect the Ni removal (Moreno-Casillas et al., 
2007). In the Fered-Fenton treatment, the electrical reduction and the 
generated sludge in the reaction could simultaneously promote the 
removal of heavy metals (Shih et al., 2013; Moreira et al., 2017). As in 
the FF treatment, the heavy metals ions could be reduced and then 
deposited onto the surface of the cathode. The valence state of heavy 
metals and concentration of organic matters could interfere the elec-
trical reduction, which ultimately influence the removal efficiency. 
Moreover, the generated sludge could also effectively remove the heavy 
metals by chemical precipitation, and the removal efficiency was mainly 
influenced by pH value and the sludge dosage (Zhang et al., 2012; Shih 
et al., 2013). Thus, it was hard to distinguish these heavy metals removal 
mechanisms during the ROC treatments by EC and FF; however, it was 
concluded that FF showed better performance on these heavy metals 
removal, which reduced ROC’s toxicity correspondingly. 
4. Conclusions 
The toxicity reduction mechanism of reverse osmosis concentrates 
from petrochemical production was systematically investigated during 
electrocoagulation and Fered-Fenton in this study. It was found that FF 
showed higher performances for toxicity reduction, and could obtain 
10.3% lower luminescence inhibition than EC after 30 min treatment. 
The results showed that the toxicity reduction was directly influenced by 
the organic pollutants and heavy metals removal in the ROC, and it was 
also suggested that the increase removal of small molecular weight 
organic pollutants had crucial impact on the toxicity reduction. The 
organics with molecular weight of 1200 g mol− 1 and 220 g mol− 1 mainly 
consisted of the SMP-like and HA-like substances in ROC, which were 
observed to be the most refractory organic pollutants, FF could remove 
65% of SMP-like substances, and 81% of HA-like substances which 
directly promoted the toxicity reduction and TOC removal. In compar-
ison, FF treatment approximately could remove 86% of 220 g mol− 1 
Fig. 7. The heavy metal removals under EC and FF treatments (a) As, (b) Cr, (c) Ni, (d) Pb. (initial pH = 6.2, current density = 30 mA cm− 2, H2O2 dosage = 30 mM 
for FF, treatment time = 30 min, temperature = 25 ◦C). 
C. Gong et al. 
Chemosphere 286 (2022) 131582
7
organic pollutants and EC just reached 67% of removal efficiency. And 
FF had higher removal efficiency for the PAHs, BTEXS and haloalkanes, 
and exhibited stronger capacity to remove heavier PAHs, which could 
promote the toxicity reduction. In addition, the removal of relatively 
low initial concentrations of As, Cr, Ni and Pb was also considered as a 
crucial parameter on the toxicity reduction. Overall, the results 
demonstrated that FF treatment was more effective for the toxicity 
reduction of reverse osmosis concentrates from petrochemical produc-
tion. However, thetoxic pollutants and heavy metals cannot be 
completely removed during the treatment, therefore, post-treatment 
was needed to remove residual toxic pollutants from the treated effluent. 
Author contribution 
Chenhao Gong: Conceptualization; Methodology; Investigation; 
Writing - Original Draft; Supervision. Xiaojing Ren: Formal analysis; 
Investigation; Writing - Original Draft. Junxing Han: Investigation; 
Writing - Original Draft. Yue Wu: Conceptualization; Methodology. 
Yaling Gou: Conceptualization; Methodology. Zhongguo Zhang: Re-
sources; Project administration. Peiran He: Project administration. 
Declaration of competing interest 
The authors declared that they have no conflicts of interest to this 
work. We declare that we do not have any commercial or associative 
interest that represents a conflict of interest in connection with the work 
submitted. 
Acknowledgements 
This study was funded by Beijing Natural Science Foundation 
(L182030) and Reform and Development Project of Beijing Academy of 
Science and Technology (2021G-0004; PY2020HJ30; BGS202013). 
Appendix A. Supplementary data 
Supplementary data to this article can be found online at https://doi. 
org/10.1016/j.chemosphere.2021.131582. 
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Shih, Y., Lin, C., Huang, Y., 2013. Application of Fered-Fenton and chemicaland Fered-Fenton tre ...
	1 Introduction
	2 Materials and methods
	2.1 Petrochemical ROC
	2.2 EC and FF treatments
	2.3 Analytical methods
	3 Results and discussion
	3.1 TOC removal during EC and FF treatments
	3.2 Toxicity analysis
	3.3 The organic pollutants characterization analysis
	3.3.1 3DEEM analysis
	3.3.2 Size fractionation of organic matters analysis
	3.3.3 Identification of organic matters by GC-MS analysis
	3.4 Heavy metals removals
	4 Conclusions
	Author contribution
	Declaration of competing interest
	Acknowledgements
	Appendix A Supplementary data
	Referencesand Fered-Fenton tre ...
	1 Introduction
	2 Materials and methods
	2.1 Petrochemical ROC
	2.2 EC and FF treatments
	2.3 Analytical methods
	3 Results and discussion
	3.1 TOC removal during EC and FF treatments
	3.2 Toxicity analysis
	3.3 The organic pollutants characterization analysis
	3.3.1 3DEEM analysis
	3.3.2 Size fractionation of organic matters analysis
	3.3.3 Identification of organic matters by GC-MS analysis
	3.4 Heavy metals removals
	4 Conclusions
	Author contribution
	Declaration of competing interest
	Acknowledgements
	Appendix A Supplementary data
	References