Relationship between the molecular structure of different dithiocarbamates and their oil removal performance

With the development of oilfields, oily wastewater treatment has become a significant problem. Oily wastewater contains dispersed oil, suspended particles, and dissolved solutes (Connor et al. 2017). Therefore, the direct discharge of oily wastewater will cause environmental pollution to the ecosystem. Various treatment methods for oily wastewater can be divided into physical, chemical, and biological processes. The physical methods, gravity separation and dissolved gas flotation, are suitable for initial treatment but not for the break of emulsified oil (Zhang et al. 2014; Adetunji & Olaniran 2021; Oliveira et al. 2021). The biological methods use microbial cells or the active group of biological macromolecules to demulsify oily wastewater, but the treatment performance is not stable due to a long microbial culture cycle, which is challenging to use widely (Le et al. 2013). The chemical methods include flocculation and demulsification using chemicals and have the advantages of simple operation, low cost, excellent performance, and wide use (Grenoble & Trabelsi 2018). These chemicals are named water clarifiers. The treatment mechanism of oily wastewater by water clarifiers includes demulsification, aggregation, and flocculation (Debord 2018).

According to the chemical structure, water clarifiers can be divided into inorganic and organic flocculants. Inorganic flocculants include aluminum sulfate, polymeric ferric sulfate, and polymeric aluminum chloride (PAC). The cost of inorganic flocculants is low, but the disposal of pollutants is slow (Le et al. 2012; Zhai et al. 2017). On the other hand, organic flocculants can be divided into anionic, cationic, amphoteric, and non-ionic types (Abuhasel et al. 2021). Because oil droplets in the wastewater are negatively charged, cationic flocculants have attracted more attention (Matusiak & Grzadka 2020; Tang et al. 2021). Current water clarifiers mainly include polyamine salts, quaternary ammonium salts, and polyacrylamide.

Dithiocarbamate (DTC) is a negatively charged compound synthesized by primary/secondary amines and carbon disulfide and is commonly used as flocculants, metal traps, and bactericides. The original DTC synthesis was reported as a patent in 1952 (Flenner & Del 1952). The researchers found that some DTC can effectively clarify O/W emulsions containing heavy metal cations (Thorn & Ludwig 1962). However, since the 1990s, conventional water clarifiers have been unable to satisfy the 1987U.S. Environmental Protection Agency standard for oily wastewater treatment. Therefore, scientists optimize DTC by adjusting the ratio of amine to carbon disulfide to improve the oil removal performance and control the properties of flocs used to treat oily wastewater in offshore fields (Durham et al. 1989). After that, scientists have gradually developed a variety of DTC to meet the needs of complex oily wastewater (Durham et al. 1991; Rivers 1993; Gao et al. 2011; Debord 2018).

For example, adding amines, alcohols, and ethers inhibits floc adhesion and enhances performance (Durham et al. 1991). In China, the fourth generation of DTC has regulated flocculating substances by compound amines, alcohols, and halogenated lipids (Rivers 1993). Due to the short processing time, narrow space, and short residence time, introducing the DTC is the best choice for treating oily wastewater in offshore fields (Zhai et al. 2017). DTC ligands (–CSS⁻) contain sulfur atoms and can coordinate with most heavy metal cations, such as Fe²⁺, Zn²⁺, Pb²⁺, and Cd²⁺, to generate flocs (Liu et al. 2017; Ayalew et al. 2020). These flocs swept up the dispersed oil droplets in wastewater and quickly raised or sank to remove oil. Various initiators have been used to synthesize different DTC, such as ethylenediamine, diethylenetriamine, and triethylenetetramine (Ayalew et al. 2020). Although different DTC has their advantages, the effects of chemical structures of initiators on the performance of DTC and the properties of flocs have not been researched clearly. Therefore, the relationship between the molecular structure and oil removal performance of DTC is discussed in this paper. The results instructively selected the treatment agents for oily wastewater in the offshore field.

Tetraethylenepentamine (90.0%), bishexamethylenetriamine (95.0%), polyethylene polyamine (AR), carbon disulfide (99.0%), formaldehyde solution (37%–40%) and polyoxyethylene octylphenol ether (OP-10) were purchased from Macklin reagent (Shanghai, China). Jeffamine^® T-403 and Jeffamine^® D-230 polyether amine were purchased from Tianjin SINS Biochemical Technology Co., Ltd (China). Iron (II) chloride tetrahydrate (A.R.), Bisphenol A (BPA, A.R.), and other commonly used chemicals were purchased from Sinopharm Chemical Reagents Co., Ltd (China). Crude oil was obtained from an oil field of China National Offshore Oil Corporation. Composition analysis of crude oil: saturated hydrocarbon 48.0%, aromatic hydrocarbon 17.4%, wax 5.8%, resin 25.2%, and asphaltene 2.8%. All chemicals were used as received without any further purification.

Among the initiators, bisphenol A phenol amine resin was prepared by ourselves according to the literature (Ge et al. 2002; Akintola et al. 2019). Precisely, 0.054 g of KOH, 5.0 g of BPA, and 10.5 mL of deionized water were placed in a flask, then stirred at 50 °C for 15 min. After that, 7.1 mL of formaldehyde solution was added to the flask dropwise. The reaction temperature was adjusted to 90 °C after dropping. After two hours of reaction, excess formaldehyde and water were removed by rotary evaporation. The product was a golden yellow viscous liquid, and the yield of bisphenol A phenol amine resin was 91.2%.

First, 2.8 g of KOH and 10.5 mL of deionized water were placed into a flask. Then, after the KOH was utterly dissolved, 1.93 g of tetraethylenepentamine was added to this flask. Next, 3.2 mL of carbon disulfide slowly dropped into the flask through the constant pressure drop funnel for 0.5 h. The reaction continued for four hours at 25 °C (Durham et al. 1989, 1991; Li et al. 1998). Then nitrogen was introduced to remove the unreacted carbon disulfide, and the product was orange-red. The product was named A-DTC.

When tetraethylenepentamine was replaced by bishexamethylenetriamine, polyethylene polyamine, polyether amino Jeffamine^® T-403, polyether amino Jeffamine^® D-230, or BPA phenolamine resin, different DTCs were synthesized by using the same synthetic procedure. These DTCs were named B-DTC, C-DTC, D-DTC, E-DTC, and F-DTC. All DTCs are presented in Table 1, the specific amounts of raw materials are listed in Table S1, and the initiators of these DTCs are presented in Table S2.

Table 1

Chemical structure of DTC and its corresponding initiator

DTC .	Initiator .	Molecular structure of DTC .
A-DTC	Tetraethylenepentamine
B-DTC	Bishexamethylenetriamine
C-DTC	Polyethylene polyamine
D-DTC	Jeffamine® D-230
E-DTC	Jeffamine® T-403
F-DTC	BPA phenolamine resin

DTC .	Initiator .	Molecular structure of DTC .
A-DTC	Tetraethylenepentamine
B-DTC	Bishexamethylenetriamine
C-DTC	Polyethylene polyamine
D-DTC	Jeffamine® D-230
E-DTC	Jeffamine® T-403
F-DTC	BPA phenolamine resin

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5.1 g of sodium chloride and 33.3 g of calcium chloride were weighed and dissolved in 1,000 mL of distilled water to prepare mineralized water (salinity was 38.4 g/L). Then, 0.025 g of crude oil, 2.5 mL of OP-10 solution (20 g/L), and 400 mL of mineralized water were added into a beaker and finally stirred at 10,000 r/min for 10 min to prepare simulated oily wastewater. The indicators of oily wastewater remained stable for 12 hours.

The flask test of six kinds of DTC water clarifiers was carried out in the beaker, the size and thickness of flocs formed were measured, and the data were compared and evaluated. An optical microscope observed the flocs at the microscopic level to investigate the difference between flocs formed by various DTCs. The corroded N80 steel sheets were used to simulate the pipe surface, and the solution of the floc layer and the N80 steel sheet were placed in an oven for preheating for 30.0 min at 60 °C, respectively. Then the solution of the flocs layer was dumped along the upper part of the metal plate at an angle of 30°, and the steel sheets were continuously scoured with 60.0 °C clear water (flow rate: 30 mL/min). The fluidity of floc with clear water and adhesion to the metal plate were observed.

As shown in Figure 3, the rising time of flocs increases until the ratio A-DTC/Fe²⁺ of 1:3, and then decreases with the Fe²⁺ concentration, while the oil removal efficiency of A-DTC increases until a ratio of 1:4, and then remains stable. Therefore, increasing the ferrous ion concentration is conducive to floc formation and oil removal. When the molar ratio of A-DTC to ferrous ions equals 1:5, the oil removal efficiency of DTC reaches the maximal value. According to the molecular structure of A-DTC, the initiator has two primary amino groups (-NH₂) and three secondary amino groups (-NH-). Therefore, the optimal molar ratio of A-DTC to ferrous ions equals the number of amino groups of the initiator. Meanwhile, the A-DTC can remove 93.14% of oil from the aqueous phase at the optimal molar ratio in 4 min. Therefore, A-DTC is suitable for oil-water separation at the offshore platform because of its high efficiency and short treatment time. To further demonstrate the relationship between the optimal molar ratio and the number of amino groups of the initiator, the proportion of six kinds of DTC to ferrous ion is shown in Table 2.

As shown in Table 2, tetraethylenepentamine contains five amino groups per molecule, and the optimal ratio of A-DTC to ferrous ion is 1:5. Bishexamethylenetriamine has two primary amino groups (-NH₂) and one secondary amino group (-NH-) per molecule. Polyethylene polyamine is a mixture with an average molecular weight of about 275. We can determine from their regular structure that there are 6–7 amino groups per molecule of polyethylene polyamine. Jeffamine^® D-230 contains two amino groups per molecule; Jeffamine^® T-403 contains three amino groups per molecule (Thompson & Asperger 1987). Bisphenol A phenol amino resin has 20 amino groups per molecule (Akintola et al. 2019). As a result, the optimal DTC/Fe²⁺ molar ratios agree with the molecular structure of amino groups per molecule of initiators.

Concentration is one factor determining the oil removal efficiency of DTC. Therefore, when the pH of oily wastewater was 7.0, the experimental temperature was 60 °C and the A-DTC/Fe²⁺ molar ratio was 1:5, the effects of A-DTC concentration on the oil removal efficiency were investigated.

The oil removal mechanism is that DTC chelates with metal ions to form flocs, which sweep and capture the oil droplets. Therefore, the chelating metal ions may influence the oil removal efficiency of DTC. According to the analysis report of oily wastewater in an offshore platform of China National Offshore Oil Corporation, the oily wastewater contains Ba²⁺, Sr²⁺, Mg²⁺, Mn²⁺, Ca²⁺, Fe²⁺, Fe³⁺, Na⁺, and K⁺. In addition, Al³⁺ is a widely used efficient flocculant. Therefore, eight divalent and trivalent metal ions commonly existed in oily wastewater were selected for experiments to explore the chelating effects of various metal ions. The pH of oily wastewater was 7.0, the DTC concentration was 50 mg/L, the molar ratio of A-DTC to metal ions was 1:5, and the treatment temperature was 60 °C.

As shown in Table 3, only Al³⁺, Fe³⁺, Fe²⁺, and Mn²⁺ can react with DTC to generate flocs. Among these four metal ions, Fe²⁺ has the best oil removal performance. According to hard and soft acid-base theory, sulfur atoms belong to soft bases with large atomic radii and abundant electrons. Therefore, DTCs can interact with most transition metal ions other than alkali and alkaline earth metals to form stable four-membered rings as a two-dentate ligand. At the same time, multiple -CSS^- coordination groups can be used as bridging groups, forming supramolecular coordination structures with the central metal ions and eventually forming precipitation (Jia et al. 2009; Liu et al. 2013; Ayalew et al. 2020).

The polyether structure was introduced into D-DTC and E-DTC, and the chain end of polyether is the primary amino group (Thompson & Asperger 1987; Satou 1992). This kind of polyether amino contains both polyether structure and terminal amino groups. Polyether structure is an amphiphilic group, so introducing this structure helps complete the separation of oil and water of the floc and remove oil. And this structure is beneficial to improving the thermal stability of the DTC floc structure, which can stabilize the utility of DTC in a high-temperature environment. In this way, the synthetic DTC is adaptable to high temperatures and easier to follow up, which is convenient to realize the later oil-water separation treatment and effectively improves the oil removal efficiency. However, as shown in Figure 7, the oil removal performance of these two DTC is quite different, mainly because the initiator of D-DTC is a linear structure. The linear structure makes against the generation of flocculation network structure. Therefore, the oil removal ability of D-DTC is poor (Ince et al. 2016; Erturk et al. 2018).

Finally, the initiator of F-DTC has a benzene ring and multi-branching structure. Benzene ring increases the interaction between DTC and oil, while the multi-branching design helps the network structure of flocs. However, Figure 2 shows that F-DTC has the lowest conversion rate of amino groups in DTC synthesis, which may attribute to the hindering effect of molecular structures. Given the lowest conversion rate of amino groups, F-DTC exhibits a high oil removal efficiency and a short rising time of flocs (Akintola et al. 2019).

The rising rate of DTC flocs depends on various factors, including treatment temperature, oil content in oily wastewater, and suspended matter concentration in oily wastewater (Conradi et al. 2019). After unifying the external factors, the influence of DTC structure on the rising rate of flocs can be discussed.

The flocs produced by D-DTC are relatively small and few, so the rising phenomenon is different. Small flocs are directly formed and float up instead of forming large flocs at the bottom of the container, which is related to the linear structure of D-DTC. Each molecule of E-DTC contains three amino groups. E-DTC forms small flocs and constructs complex network structures simultaneously. The flocs can efficiently capture oil droplets with small densities to float up quickly.

B-DTC has a low oil removal efficiency; thus, the rising rate of flocs is small. Therefore, the captured oil decreases the density of flocs and favors the rising of flocs. However, the rising process of the other flocs is similar, and the rising time is also similar, so it is not easy to distinguish. In conclusion, DTC with a high oil removal efficiency will capture more oil droplets; thus, the overall density of flocs is closer to oil, and the rising process is fast.

Although treatment performance and floc properties are affected by DTC molecular structure, there is no relationship between oil removal efficiency and floc properties. Take B-DTC and E-DTC as an example, D-DTC produces small flocs with a poor oil removal efficiency, whereas E-DTC produces small flocs with a high oil removal efficiency. However, dense flocs reduce the rising time of flocs and save the treatment time of oily wastewater (Oliveira et al. 2020). Meanwhile, loose flocs easily block the pipelines, which disturbs the production and enhances the treatment costs.

After the rising process, some flocs will remain on the container wall. In the actual production process, these residual flocs lead to the blockage of pipelines, so the adhesion of flocs on the metal surface should be evaluated (Mascarenhas et al. 2004; Zhang et al. 2020). Because of the process of production and transportation, oilfield oily wastewater pipes mostly use N80 steel, the adhesion test used corroded N80 steel sheet to simulate the real pipeline environment.

Six kinds of DTC with different initiators were synthesized and characterized by infrared spectra. The effects of DTC concentration, the DTC/Fe²⁺ molar ratios, and treatment temperature on the oil removal efficiency have been investigated. Results show the optimal oil removal conditions for six kinds of DTC: the dosage of DTC is 50 mg/L; the treatment temperature is 60 °C; the addition ratio of DTC and ferrous ion equals the amino group number of DTC initiator. At the optimal oil removal condition, the oil removal efficiency of A-DTC, B-DTC, C-DTC, D-DTC, E-DTC, and F-DTC were 93.14%, 87.33%, 95.14%, 78.58%, 93.81%, and 93.26%, respectively. Except for D-DTC, the rising time of A-DTC, B-DTC, C-DTC, E-DTC, and F-DTC flocs are 215 s, 252 s, 190 s, 173 s, and 197 s, respectively. Meanwhile, ferrous ion has been replaced by other transition metal ions, but current studies show that ferrous ion is the most suitable chelating metal ion for DTC.

Increasing amino groups in the initiators could improve the oil removal efficiency. The linear structural DTC exhibits a low oil removal efficiency due to a lack of network structural flocs. The introduction of polyether structure helps reduce the floc volume and make it compact, but it also increases the adhesion of the floc on the metal surface. The introduction of bisphenol A phenol amino resin structure is helpful to the development of the flocs and improves the oil removal efficiency. In the actual production, the above findings will reduce the workload of reagent screening and achieve the goal faster. Novel green clarifiers with a high oil removal efficiency and rate are essential to future research.

This work is supported by the State Key Laboratory of Separation Membranes and Membrane Processes (Tiangong University, No. M202109) and the National Natural Science Foundation of China (grant no. 22073025, 21873075).

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

The authors declare there is no conflict.