An example of a wastewater treatment project for high concentrations of benzene, xylene, acetophenone, and DOPO generated in the production of phosphorus flame retardant

A phosphorus-based flame retardant company in Guizhou Province, China, focuses on the production of phosphorus flame retardants for high-temperature nylon and reactive phosphorus flame retardants. The wastewater mainly comes from the washing water of the reaction exhaust gas and the hydrolysis exhaust gas, with a small amount of other wastewater that contains benzene, xylene, acetophenone, 9, 10-dihydro-9-oxa-10-phosphoxanthine-10-oxide (DOPO), and so on.

As new emerging contaminants, benzene, xylene, acetophenone, and DOPO have drawn much attention over the recent years. A wide range of removal techniques for these pollutants in wastewater, such as adsorption, oxidation, reduction, and biological technology, have been investigated.

A lot of adsorbents comprising carbon materials (Wang et al. 2018a; Gu et al. 2019), covalent organic frameworks (Wang et al. 2018c), metal–organic frameworks (Su et al. 2020), zeolites, and other fabricated adsorbent materials (Wang et al. 2018b; Hao et al. 2020) have been explored to be applied for the adsorptive removal of various phosphorus pollutants from wastewater. Among them, biochar (Du et al. 2020), activated carbon, and carbon nanotubes were the mainly reported adsorbents for phosphorus pollutants adsorption from the solution (Wang et al. 2018a, 2018c; Gu et al. 2019).

The adsorption method is valid for hydrophobic phosphorus pollutants but exhibits much lower removal capacities for hydrophilic counterparts. The design of required adsorbents for hydrophilic phosphorus pollutants is necessary. Adsorbents with high selectiveness and recyclability should be also fabricated based on the adsorption activities and mechanisms of phosphorus pollutants.

In addition, traditional treatment methods and some effective advanced oxidation techniques have been investigated to remove phosphorus pollutants. Since benzene, xylene, acetophenone, and DOPO are more difficult to biodegrade and higher toxic than alkyl organic substances, advanced oxidation processes mainly specialized in the degradation of acrylated organic materials are needed.

Photocatalysis applies TiO₂ as a photocatalyst to degrade pollutants and has shown a great potential in water treatment, owing to its strengths including high stability, easy performance, and sustainable use (Ye et al. 2017; Yu et al. 2019b; Lin et al. 2020), which can rapidly and effectively remove phosphorus organic compounds.

Sulfate radical-advanced oxidation processes have been extensively explored to remove emerging pollutants because they can produce sulfate radicals, which have higher selectivity, standardized redox potentials, and broader pH suitability compared to hydroxyl radicals (Ou et al. 2017; Xu et al. 2017; Hu et al. 2019; Lian et al. 2019; Yu et al. 2019a). It is widely recognized that sulfate radicals can react with a range of emerging organic pollutants (Ou et al. 2017; Xu et al. 2017; Rodríguez-Chueca et al. 2018; Hu et al. 2019; Lian et al. 2019; Yu et al. 2019a).

Hydrogen peroxide-based advanced oxidation is also an extensively used approach to degrade many organic microcontaminants by hydroxyl radical, consisting of UV/H₂O₂, Fenton, and Fenton-like techniques. In terms of phosphorus pollutants removal, the reported researches were mainly concerned with chlorinated organic pollutants (Yuan et al. 2015; Cristale et al. 2016; He et al. 2019; Ji et al. 2020; Son et al. 2020; Yu et al. 2020). Generally, compared with sulfate radical-advanced oxidation processes and photocatalysis by TiO₂, the removal efficiency and corresponding apparent rate constant values of UV/H₂O₂ and Fenton-like process for phosphorus pollutants are lower (Ruan et al. 2013; Ye et al. 2017; Du et al. 2018; Liu et al. 2018; Abdullah & O'shea 2019; He et al. 2019; Yu et al. 2019a; Ji et al. 2020; Yu et al. 2020). Nevertheless, oxidation approaches by H₂O₂ activation, in particular, Fenton and Fenton-like procedures, are being more broadly used for wastewater treatment now.

Ozone oxidation has been explored to remove phosphorus pollutants from municipal wastewater (Lee et al. 2021; Song et al. 2021; Moradi et al. 2023), but the removal efficiency is significantly lower than other kinds of advanced oxidation processes.

Among various advanced oxidation processes, the technologies using UV/TiO₂ and sulfate radical-advanced oxidation processes show better performance on phosphorus pollutant removal. Nevertheless, almost all of the published advanced oxidation processes are negatively affected by the coexisting matters, and their oxidative selectivities for phosphorus pollutants should be enhanced in future studies.

Compared with oxidation technology, a few studies center on the reduction and hydrolysis approaches for phosphorus pollutants degradation (Fang et al. 2018; Li et al. 2020). Previous reduction researches took the electrochemical or reducing agents such as ferrous sulfide, nano-zero-valent iron, and sulfate species to realize the reduction and dechlorination of phosphorus pollutants. Electrochemical reduction as an environmentally friendly and efficient technology has been extensively applied to remove several halogenated organics (Fang et al. 2018; Du et al. 2019; Li et al. 2020, 2021; Yang et al. 2021).

Chemical reduction, particularly electrochemical reduction, is efficient and selective for phosphorus pollutants. Compared to advanced oxidation processes, research studies on reduction methods are much fewer and restricted.

Current studies of biodegradation for phosphorus contaminants mainly concentrate on the use of activated sludge, artificial wetlands, and pure bacteria (Qin et al. 2020; Wang et al. 2021; Pantelaki & Voutsa 2022; Chen et al. 2024). A part of phosphorus pollutants can be removed by adsorption onto the activated sludge (Pantelaki & Voutsa 2022). During the process of treatment and disposal of sewage sludge, it was revealed that the aerobic and anaerobic composting of sludge could result in the biodegradation of phosphorus contaminants (Pang et al. 2023). The hydrolysis, hydroxylation, conjugation, methylation, dechlorination, and other procedures help in biodegradation of phosphorus contaminants.

In comparison with chemical approaches, most biodegradation methods are relatively lower in efficiency. According to the conclusion of phosphorus pollutant biodegradation mechanisms, biohydrolysis is prominent during the degradation procedure. Improving the biohydrolysis reaction may be additionally investigated as the future biotreatment choice.

Although different techniques displayed good performance for removal of the aforementioned contaminants, most of the studies were at the stage of applying pure water solution and the efficiencies of those technologies would be significantly discounted when used in real water environments. Since halogenated phosphorus pollutants and alkyl and aryl phosphorus pollutants commonly co-exist in water environments, the combination of several treatment methods, which may make the most of their strengths, is also a promising approach to remove phosphorus pollutants in real applications. For the combination of different techniques, the adsorption–oxidation–biological technologies or adsorption–electrochemical reduction–biological technologies may be the promising combination technologies for removal of phosphorus pollutants.

With reference to the aforementioned treatment methods, the wastewater generated in this study contains more benzene ring organics, which is more toxic to the organisms; therefore, the first combination process of lime emulsion–flocculation and precipitation–ozone oxidation reaction was adopted to degrade the benzene organics into small molecules with short chains that are easy to be biochemically decomposed, and then the second combination process of upflow anaerobic sludge bed (UASB) anaerobic digestion–air flotation–membrane bioreactor (MBR) biological contact oxidation was used to further reduce the concentration of pollutants such as chemical oxygen demand (COD) and phosphorus, which finally could meet the emission limit values.

To make the entire article easier to understand for everyone, Table 1 presents all acronyms used in this article.

A phosphorus flame retardant company has a production plant with an annual output of 300 tons of phosphorus flame retardant for high-temperature nylon and 800 tons of reactive phosphorus flame retardant, which produces alkaline washing wastewater of crude products and washing wastewater of fine products. Moreover, its tail gas treatment section generates falling film absorption wastewater and alkaline washing wastewater from the alkaline washing tower. These wastewaters contain benzene, xylene, acetophenone, and DOPO. The concentrations of pollutants and the volumes of the influents and effluents are presented in Table 2.

The following are reagents used in this article: silver sulfate (Ag₂SO₄), A.R.; mercuric sulfate (HgSO₄), A.R.; potassium dichromate (K₂Cr₂O₇), A.R.; ammonium iron (Ⅱ) sulfate hexahydrate ((NH₄)₂Fe(SO₄)₂ 6H₂O), A.R.; iron(II) sulfate heptahydrate (FeSO₄ 7H₂O), A.R.; potassium hydrogen phthalate (KC₈H₅O₄), A.R.; magnesium oxide (MgO), A.R.; potassium iodide (KI), A.R.; potassium hydroxide (KOH), A.R.; sodium thiosulfate (Na₂S₂O₃), A.R.; zinc sulfate heptahydrate (ZnSO₄ 7H₂O), A.R.; sodium hydroxide (NaOH), A.R.; boric acid (H₃BO₃), A.R.; sodium carbonate anhydrous (Na₂CO₃), A.R.; ammonium chloride (NH₄Cl), A.R.; amylum ((C₆H₁₀O₅)n), A.R.; potassium persulfate (K₂S₂O₈), A.R.; ascorbic acid (C₆H₈O₆), A.R.; ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄ 4H₂O), A.R.; potassium phosphate monobasic (KH₂PO₄), A.R.; sodium fluoride (NaF), A.R.; sodium acetate trihydrate (CH₃COONa 3H₂O), A.R.; lanthanum nitrate (La(NO₃)₃ 6H₂O), A.R.; phenolphthalein (C₂₀H₁₄O₄), A.R.; perchloric acid (HClO₄), A.R.; potassium antimony tartrate monohydrate (KSbC₄H₄O₇ H₂O), A.R.; acetone (CH₃COCH₃), A.R.; acetic acid (CH₃COOH), A.R.; sulfuric acid (H₂SO₄), G.R.; hydrochloric acid (HCl), A.R.; mercury chloride (HgCl₂), A.R.; potassium sodium tartrate (KNaC₄H₆O₆ 4H₂O), A.R.; nitric acid (HNO₃), A.R.; 3-methylamine-alizarin-diacetic acid (C₁₄H₇O₄ CH₂N(CH₂COOH)₂), A.R.; bromthymol blue (C₂₇H₂₈O₅SBr₂), A.R.; and 1,10-phenanathroline monohydrate (C₁₂H₈N₂ H₂O), A.R.

Multiparameter water quality analyzer (LH-3B), Agilent 1260 high-performance liquid chromatography (HPLC) with a photodiode array detector, and Agilent 8890 GC/MS with fast heating/fast cooling modules were used to measure water samples.

pH was measured by the electrometric method (Patnaik 2017c). COD was determined by the potassium dichromate method (Patnaik 2017d). BOD₅ was measured for the amount of oxygen utilized during a specific incubation period of 5 days generally for the biochemical oxidation of organic materials and oxidizable inorganic ions (Patnaik 2017b). NH₃-N was analyzed by the colorimetric nesslerization method (Patnaik 2017e). Total phosphorus (TP) was determined by colorimetric analysis (Patnaik 2017f). Benzene was analyzed by Agilent 8890 GC/MS (Patnaik 2017g). Xylene was analyzed by Agilent 8890 GC/MS (Patnaik 2017h). Acetonephenone was analyzed by Agilent 8890 GC/MS (Patnaik 2017i). DOPO in influent and effluent was detected by Agilent 1260 HPLC (Patnaik 2017a).

Production wastewater of phosphorus-based flame retardant is higher in organic matters (benzene, xylene, acetophenone, and DOPO) and phosphorus. In this article, three different treatment processes, adsorption, ozone oxidation, and coagulation and precipitation, were selected for the treatment of production wastewater of phosphorus-based flame retardants. In Table 3, the treatment effects, advantages and disadvantages, and operating costs of the three different treatment processes were comprehensively evaluated and comparatively analyzed.

• (1) The alkaline washing wastewater of crude products and washing wastewater of fine products from the production workshop of phosphorus flame retardant for high-temperature nylon and reactive phosphorus flame retardant, the falling film absorption wastewater, and alkaline washing wastewater from alkaline washing tower in the tail gas treatment section converge and then flow into the regulating pool of wastewater.

• (2) The wastewater of the regulating pool is lifted by a pump to the reaction pool for phosphorus removal, where the milk of lime is added to adjust its pH to 10–11, and phosphate ions and calcium ions generate calcium phosphate precipitation.

• (3) The addition of polyacrylamide to the wastewater after the neutralization and precipitation treatment results in the precipitation to continue to gather and increase in size, thus further removing suspended matters with smaller particles from the water. The sludge generated by the settlement is discharged into the sludge thickener, and the effluent overflows into the ozone oxidation reactor. The sludge is sent to the filter press through a piping system.

• (4) Selection of oxidation treatment methods.

In Table 4, the treatment effects, advantages and disadvantages, and operating costs of the three treatment processes such as ozone oxidation, Fenton oxidation, and photocatalytic oxidation were compared and analyzed. Finally, ozone oxidation was selected for the treatment of phosphorus-containing organics.

• (5) The wastewater from the ozone oxidation reactor is homogenized and conditioned in a homogenization basin and then enters the UASB anaerobic treatment unit. Under anaerobic conditions, organic matter in the sewage is utilized by anaerobic bacteria to decompose and generate methane. Phosphate is reduced to soluble NH₄H₂PO₄, which is precipitated along with other ions to form sludge. The soluble inorganic and residual organic matter in the wastewater flows out with the water. The hydraulic retention time of the anaerobic section is 60 h. The COD removal efficiency is more than 80%.

• (6) Air flotation is mainly used to separate and remove fine suspended particles with a density close to that of water from the production wastewater. The air flotation tank is fabricated using a steel structure with a hydraulic retention time of 0.5 h.

• (7) In a biological contact oxidation reactor, natural biota undergoes a contact oxidation reaction with the wastewater to degrade the organic matter. The contact oxidation reaction mainly refers to the elimination and transformation of organic matter in wastewater by exposing it to a higher concentration of biological sludge in the vicinity of the microbial interface, which leads to the rapid decomposition of nutrients at the microbial interface. Through microfiltration–ultrafiltration membrane technology, suspended matters, colloidal matters, and bacteria in wastewater are separated from the water. Ultrafiltration membrane effluent can be used as process water, green water, reclaimed water, etc., or can be discharged directly. The ultrafiltration section has a COD removal efficiency of 90%, a TP removal rate of 50%, and an SS removal efficiency of 95%.

• (8) The sludge from the thickening tank is transported to the plate filter press with a sludge pump, and the dewatered sludge after squeezing is disposed of by a specialized company.

• (9) The filtrate generated from the plate filter press is returned to the wastewater-regulating tank for re-treatment.

Configurations and their relative design parameters of the wastewater treatment section are presented in Table 5.

In general, benzene removal in ozone oxidation reactors shows a trend with less significant changes with time. This is due to the fact that the rate of depletion of ozone is progressively similar to that of the removal of benzene, thus leading to its removal effect varying little. As the reaction time progresses, the ozone in the reactor continues to be depleted while the organic material is being removed.

Benzene removal in an ozone oxidation reactor is affected by other factors such as pH and temperature of water and dissolved oxygen in the water. The best operating conditions for an ozone oxidation reactor include ozone concentration controlled between 1 and 3 mg/L, temperature maintained between 15 and 30 °C, and water pH regulated between 7.0 and 9.0, under which the effect of benzene removal would be better.

In general, the ozone oxidation reactor is suitable for pollutant concentrations of 500 μg/L or less, so if the xylene concentration is within this range, the ozone oxidation reactor can be used for the treatment. Treatment conditions for ozone oxidation reactors include ozone concentration, water pH, and reaction time. The optimal removal of pollutants can only be achieved if the appropriate ozone concentration, water pH, and reaction time are selected. Specifically, the ozone concentration needs to be controlled between 1 and 3 mg/L, and the pH can be adjusted to between 7.0 and 9.0.

Degradation of acetophenone in ozone oxidation reactors might follow a similar approach to benzene. Acetophenone removal in ozone oxidation reactors might be affected by other factors such as ozone concentration, water temperature, and pH. Specifically, the ozone concentration is maintained at 1–3 mg/L, the temperature is controlled between 15 and 30 °C, and the pH of water is adjusted between 7.0 and 9.0, which could be more effective in removing acetophenone. Moreover, improvement of the structural design of the reactor could enhance the removal of acetophenone in the reactor.

As shown in Figure 5(a), DOPO in the ozone oxidation reactor is broken down into nontoxic products through a series of complex oxidation reactions. At the same time, the presence of phosphorus in DOPO would result in some negative influences on DOPO degradation in the ozone oxidation reactor, which might react chemically with other elements in the reactor, thus affecting the DOPO removal effect.

In addition, DOPO removal in ozone oxidation reactors might be influenced by other factors such as ozone concentration, temperature, and pH of water. DOPO could be processed more effectively by maintaining appropriate ozone concentration, temperature and pH of water, a well-designed reactor structure, and combination of other treatment technologies.

Effects of wastewater treatment in each unit are displayed in Table 6, which shows that all indicators of the effluent meet the discharge limits.

Direct and indirect costs are displayed in detail in Table 7.

Production wastewater of phosphorus flame retardant contains benzene, xylene, acetophenone, DOPO, and other organic compounds with larger benzene rings that are more difficult to be directly biodegraded. If the aforementioned wastewater containing these organic compounds was treated directly by microbiological methods, it could result in microbial poisoning.

In this study, most of the phosphorus in the production wastewater was removed by lime emulsion reaction and flocculation and precipitation, and then ozone oxidation was used to degrade the benzene ring organics into short-chain small molecules that are easier to utilize by microorganisms. Then the microbial biochemical treatment was carried out with the combined process of UASB anaerobics digestion–air flotation–MBR biological contact oxidation for the production wastewater, thus ultimately leading the pollutants in the wastewater to the permissible concentration. The wastewater can be discharged outside or reused as supplementary water for other production units.

In addition, mechanisms of benzene, xylene, acetophenone, and DOPO degradation in an ozonation reactor were explained graphically in this article. The effects of benzene, xylene, acetophenone, and DOPO removal in the reactor were analyzed.

However, the shortcoming of this study is the higher equipment cost and operating cost of the ozone oxidation reactor. It is hoped that more economical oxidation methods can be found in the future to reduce equipment and operating expenses.

The authors would like to thank their laboratory colleagues for their help in the experimental work.

Xiaobin Lu was in charge of sample analysis, data validation, writing, reviewing, and editing.

This work was supported by the Guizhou Provincial Basic Research Program (Natural Science) (Qiankehejichu[2020]1Y047).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.