Preparation of a novel flocculant and its performance for treating acidic oily wastewater

Abstract

In this study, a series of copolymers of methyl methacrylate (MMA), methacrylic acid (MAA) and chitosan (CS), named P(MMA–MAA–CS), were prepared via emulsion copolymerization (CS content in monomer mass was less than 3 wt%). The copolymer was characterized by infrared spectroscopy, X-ray powder diffraction, thermogravimetry and differential scanning calorimetry. The surface tensions and flocculation performances of different copolymers for treating acidic oily wastewater were also investigated. Interestingly, it was found that P(MMA–MAA–CS) has pH-sensitive surface activity and excellent flocculation performance. The effects of dosage, temperature, stirring speed and stirring time on the treatment of P(MMA–MAA–CS) were studied and the highest oil removal reached was 96%. This copolymer may have potential applications in treating acidic oily wastewater produced from oil well acidizing.

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Introduction

Acidization of oil and gas wells is one of the most frequently used stimulation techniques for increasing well productivity.^1–3 It commonly involves pumping acid into the near-wellbore region to dissolve formation damage and create new pathways for production. Usually, there are several problems during the stimulation of oil-producing wells. One of the most significant problems created by the acidization treatment is the subsequent formation of acidic oily wastewater in the produced fluids from the well.³ The acidic oily wastewater is usually produced in the form of extremely stable oil in water (O/W) emulsions and chemical agents are needed to separate the oil from water.

Usually, the oil droplet in common oily wastewater produced from oilfield is negatively charged, and the chemical agents are cationic flocculants, such as poly aluminum chloride (PAC), polydiallyldimethylammonium chloride, cationic polyacrylamide, polyethylene and so on.^4–6 Among these flocculants, PAC was the most widely used because of its excellent flocculation capacity, inexpensive cost and nontoxic property.^7–10 Electrostatic neutralization is supposed to be the main flocculation mechanism of PAC and the best performance could be achieved at pH of 9 to 11.^11,12 However, PAC was not effective for treating acidic oily wastewater of which pH was usually lower than 5 and the oil droplet is cationic. Therefore, the acidic oily wastewater could be adjusted to be basic (pH > 9) using NaOH or CaO before treated by PAC in oil field.¹³ In addition, although the oil droplet in acidic oil wastewater is positively charged, anionic polyacrylamide did not show good flocculation performance alone because its bridging effect for small oil droplet is limited.^12,14 Therefore, it is usually used combined with PAC.¹⁴

Acrylate emulsion is commonly used as a type of demulsifier in oil field for O/W emulsion separation. It can destroy the interfacial film between water and oil at some conditions.^15–17 In addition to excellent demulsification performance, acrylate emulsion also presents outstanding injectability and pumpability. It has been used in an offshore oilfield in South China Sea for several years.¹⁵ But it has never been used for treating acidic oily wastewater. In this context, terpolymer of methyl methacrylate (MMA), methacrylic acid (MAA) and chitosan (CS) was prepared via emulsion copolymerization. The addition of CS was expected to enhance the molecular weight of copolymer. Infrared spectroscopy (IR), X-ray powder diffraction (XRD), thermogravimetry (TGA) and differential scanning calorimetric (DSC) were used for the characterization of the prepared polymer. The intrinsic viscosity, surface tension and pH-sensitivity of the terpolymer were also studied. Interestingly, we found that the terpolymer can be used as flocculant for treating acidic oily wastewater. To our knowledge, until now, study regarding the synthesis of terpolymer of MMA, MAA and CS by emulsion polymerization and its performance for treating acidic oily wastewater has not been reported in the literature. This copolymer may find potential application in treating oily wastewater produced from oil well acidizing.

Experimental section

Materials

Chitosan (>95% of deacetylation degree, viscosity average molecular weight of 300 kDa), cetyl trimethyl ammonium bromide (CTAB), MMA, MAA and ammonium persulphate (APS) were purchased from KeLong Reagent Company, Chengdu, China, in analytical grade. MMA was passed through an alumina column before used and the others were used without further purification. Acidic oily wastewater and PAC were obtained from one oilfield in china with oil content in water (OiW) of 6400 mg L⁻¹ and pH of 4.2.

Preparation of copolymer

Firstly, chitosan and CTAB (used as emulsifier) solution were prepared with 1.0% acetic acid in a three necked flask with constant stirring and bubbling of a slow stream of nitrogen at ambient temperatures. Then, the flask was placed in a preset water bath (70 °C), and a given amount of APS (used as initiator) was added into the solution as initiator. After that, the mixture of MMA and MAA was added dropwise for 1 h. Later, the reaction was continuously stirred at 350 rpm for 2 h. At the end of reaction, the copolymer emulsions were demulsified by freeze-thaw method. Thereafter, copolymer samples were washed firstly by 1% HAc aqueous solution, then washed by water for several times until there was no surface tension decrease for the water, which indicates that no residual CTAB existed in the copolymer. At last, the copolymer samples were dried in a vacuum oven at 50 °C until a constant weight was obtained.

The schematic representation of synthetic route is shown in Fig. 1 and four copolymer samples were prepared under the condition shown in Table 1.

Fig. 1 Schematic representation of copolymerization.

Table 1 Copolymerization under different conditions

Entry	Mass of 1% HAc aqueous solution (g)	Mass of CTAB (g)	Mass of MMA (g)	Mass of MAA (g)	CS (g)	Abbreviation of copolymer
1	135	0.8	6.3	10	0	P(MMA–MAA)
2	135	0.8	6.3	10	0.1	P(MMA–MAA–CS)-1
3	135	0.8	6.3	10	0.3	P(MMA–MAA–CS)-2
4	135	0.8	6.3	10	0.5	P(MMA–MAA–CS)-3

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Characterization

Particle size distribution of emulsion was measured by dynamic light scattering (DLS), which was performed on a Zeta Plus apparatus (Zeta Plus, Brookhaven Instruments, US).

IR spectra were recorded on a Thermo Fisher Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific Corp., USA) and the samples were prepared as KBr pellets.

The TGA was performed on a STA449F3 thermogravimetric analyzer (NETZSCH instruments). Samples between 10 and 15 mg were heated from 40 to 800 °C at heating rate of 10 °C min⁻¹ in a He atmosphere.

DSC (in the range of 40–400 °C) of the polymers were carried out in a nitrogen atmosphere at heating rate of 10 °C min⁻¹, using TA Q20 (TA instruments).

XRD patterns were recorded using Pert Pro X-ray diffractometer (PANaly tical Company, Netherlands) with a copper target at 40 kV and 40 mA.

Intrinsic viscosities ([η]) of polymers were measured with the “five-spot” dilution method using an Ubbelohde viscometer at 30 °C according to ref. 18, where the solvent was 0.1 mol L⁻¹ NaOH.

Surface tensions were measured by drop shape analyzer (DSA30, KRÜSS GmbH Co., Hamburg, Germany) at 20 °C.

Flocculation test

Firstly, 25 mL of acidic oily wastewater was added into a beaker at the desired temperature for 10 min. Then, a certain volume of flocculant solution was added under stirring (the flocculant solution was prepared by adding 1 g copolymer emulsion into 99 g 0.05 mol L⁻¹ NaOH solution). The stirring was continued for several minutes followed by a standing of 5 min. At last, the water quality was observed and the OiW was measured. The OiW was measured by OilTech121A, Environ Lab&Tech Inc., USA.

Results and discussions

Characterization

Characterization of copolymer emulsion. DLS of different copolymer emulsions are shown in Fig. 2. Obviously, the particle size of emulsion increases with increasing CS content and it changes from nanometer order to micron order. The median particle size of P(MMA–MAA–CS)-3 emulsion is 1.165 μm. When there is CS in the reaction, several P(MMA–MAA) molecules may link onto one CS molecule. CS is in favor of formation of larger molecule. Therefore, the particle size increases with increasing CS content.

Fig. 2 DLS data of the different copolymer emulsions.

After the polymerization, P(MMA–MAA–CS)-3 emulsion is milk-white. When NaOH solution is added to the emulsion, it changes to be transparent and homogeneous. Under the acidic condition, disassociation of MAA is weak and P(MMA–MAA–CS) is not water-soluble. When NaOH solution is added, MAA changes to be sodium methacrylate (SMA). As SMA is a type of strong electrolyte and the copolymer is water-soluble, the emulsion turns to be aqueous solution eventually. When H₂SO₄ solution is added, SMA changes to be MAA again and the solution appears to be cloudy because P(MMA–MAA–CS) was not water-soluble. In addition, this phenomenon confirms indirectly that MAA has been copolymerized with MMA.

Characterization of copolymer. The [η]s of different copolymers were measured. It was found that the [η] increases with increasing CS content and P(MMA–MAA–CS)-3 has the highest [η] of 3.42 dL g⁻¹. This result shows that CS can enhance the molecular weight of P(MMA–MAA–CS) and confirms indirectly that CS has been participated in the copolymerization.

The infrared spectra of different polymers are shown in Fig. 3. The band at 1597 cm⁻¹, the characteristic peak of primary amino N–H vibration in chitosan, disappears in the profile of P(MMA–MAA–CS), indicating the deformation of the primary amine in the copolymer. This implies that the copolymerization has occurred at –NH₂ groups. The new peaks appearing at 1061 cm⁻¹, should be assigned to the C–O stretching of the primary alcohol. Its strength increases with CS content. The peak at 1741 cm⁻¹ is related to the stretching vibration of C=O bands for the P(MMA–MAA) parts of the copolymer. These spectra demonstrate that the terpolymer has been successfully prepared.

Fig. 3 The IR spectra of different polymers.

The XRD profiles of the prepared terpolymer present distinct crystalline peaks, compared with that of chitosan (Fig. 4). The profiles of chitosan show one peak around 2θ = 20.1°, which corresponds to the orthorhombic crystal structure of chitosan (JCPDS no. 39-1894).^19,20 For the terpolymer, this peak disappeared, which might be attributed to the fact that copolymerization disrupt the crystalline structure of CS, especially by the loss of hydrogen bonding.²¹

Fig. 4 XRD patterns of chitosan and P(MMA–MAA–CS)-2.

The TGAs of CS and copolymers were measured. CS exhibits two stages of distinct weight loss and copolymers have three stages. For all the polymers, the first stage ranges between 0 and 200 °C is caused by the adsorbed and bound water weight loss. During this stage, the water loss of terpolymer increases with CS, which may be due to the increased hydroxyl content brought by CS. During the second stage, the weight loss of copolymers is much smaller than that of CS and the onset temperature of this transition is also lower than that of CS, indicating that copolymers have much better thermal stability than that of CS. After the second stage, CS has no obvious weight loss and the max weight loss of CS is 60.84%. The main weight loss of copolymer happens in the third stage. It can be noticed that the weight loss in the third stage decreases with CS content. Because CS is difficult to degrade when temperature is more than 400 °C, the terpolymer with higher CS content has lower weight loss in the third stage.

Fig. 5 shows the DSC thermograms of CS and copolymers. They all have two endothermic peaks due to the evaporation of adsorbed and bound water and the flux of the part of polymer, respectively. Corresponding to the two endothermic peaks, there are two flux temperatures (T_f1 and T_f2). The T_f2 of P(MMA–MAA–CS) (185.92 °C) is much higher than that of P(MMA–MAA) (170.19 °C). It is evident that some of the crystalline areas of chitosan have became part of the terpolymer. The result strongly indicates the formation of P(MMA–MAA–CS) terpolymer.

Fig. 5 DSC thermograms of CS and copolymers (exothermic is down).

The surface activity of copolymer

The surface tensions of different copolymers at different concentrations were measured. When 0.05 mol L⁻¹ NaOH is the solvent, we found that four samples have the similar surface activities as their concentrations increasing. All of them can decrease the surface tension of the solvent and have surface tensions of about 30 mN m⁻¹ at concentration of 2000 mg L⁻¹, which represents that they are surfactants in the NaOH solution. In the NaOH solution, the MMA parts are hydrophobic and sodium methacrylate parts are hydrophilic. Therefore, the four samples are amphiphilic macromolecules. In addition, the molecular weight has no great influence on the surface tensions of theses copolymers.

As can be seen from Fig. 6, the pH (from 7 to 4) has a great influence on the surface tensions of these copolymers. As the pH decreasing, their surface tensions increase obviously. However, the effect of pH on P(MMA–MAA) is different from that on P(MMA–MAA–CS). When pH changes, the P(MMA–MAA) and P(MMA–MAA–CS) have a mutation and a gradual change, respectively. For P(MMA–MAA), when pH changes from 5 to 4, the surface tension is suddenly changed from 36.8 mN m⁻¹ to 71.4 mN m⁻¹. In comparison, the surface tension of P(MMA–MAA–CS) changes more slowly, which can be attributed to the existence of CS. When the pH was higher than 5, MAA parts have been changed to be SMA and copolymer is water-soluble. The hydrophilicity of copolymer has no great change at pH higher than 5. Therefore, the surface tension has no great change. When the pH is 4, P(MMA–MAA) is water-insoluble and there is no surfactant in the water. Thereby the surface tension changes to be 71.4 mN m⁻¹. For P(MMA–MAA–CS), when pH is lower than 7, H⁺ can be combined with COO⁻ in SMA and the ether groups and amido groups in CS. The transition rate from COO⁻ to COOH can be slowed down by the ether groups and amido groups in CS. Therefore, the hydrophilicity and surface tension of terpolymer are changed gradually.

Fig. 6 Effect of pH on surface tensions of copolymers (polymer concentration is 500 mg L⁻¹).

Performance of copolymer for treating acidic oily wastewater

At the neutral and basic condition, P(MMA–MAA–CS) is water-soluble and has outstanding surface activity (as shown in Fig. 6). It may absorb onto the surface of several oil droplets. As the pH decreases, the water-soluble ability and surface activity of P(MMA–MAA–CS) decreases as well. Eventually, P(MMA–MAA–CS) would be separated out from the oily wastewater with bridging several oil droplets together, due to the hydrophobic groups decorated on its surface. Therefore, flocculates might be produced when the P(MMA–MAA–CS) is used in acidic oily wastewater treatment.

The treatment results of different flocculants. At 40 °C, the acidic oily wastewater was treated by different flocculants with a dosage of 200 mg L⁻¹, a stirring speed of 150 rpm and a stirring time of 10 min, and the results are shown in Fig. 7. PAC has the lowest oil removal of 9%, which indicates that PAC is not suitable for treating acidic oily wastewater. However, PAC presents good flocculation performance when the pH of acid oily wastewater was adjusted to be higher than 9.^12,14 As for copolymer emulsion, it can be used as flocculant directly without the pH adjustment process. As compared with P(MMA–MAA), P(MMA–MAA–CS) displays better flocculation performance as well as higher molecular weight. Therefore, when separated out from the acidic oily wastewater, the bridging capacity of P(MMA–MAA–CS) is much better. This indicates that molecular weight of copolymer has a significant influence on the flocculation performance. As shown in Fig. 7, both P(MMA–MAA–CS)-2 and P(MMA–MAA–CS)-3 have high molecular weight and excellent performance. As the size of P(MMA–MAA–CS)-2 emulsion droplets is much smaller than that of P(MMA–MAA–CS)-3 emulsion droplets, P(MMA–MAA–CS)-2 emulsion is supposed to be more stable. Therefore, P(MMA–MAA–CS)-2 was employed as the flocculant in the following experiments.

Fig. 7 The treatments of different flocculants (dosage = 200 mg L⁻¹, temperature = 40 °C, stirring speed = 150 rpm, stirring time = 10 min).

The effect of concentration. The effect of P(MMA–MAA–CS)-2 concentration on the flocculation performance was studied with a temperature of 40 °C, a stirring speed of 150 rpm and a stirring time of 10 min and the result is shown in Fig. 8. It can be found that the dosage of P(MMA–MAA–CS)-2 has great influence on the treatment performance. When the dosage is lower than 400 mg L⁻¹, the flocs can float onto the surface, the oil removal increases with the increase of dosage. The oil removal has little change when the dosage is higher than 200 mg L⁻¹ (the oil removal is 91% at the dosage of 200 mg L⁻¹). Besides, when the dosage is higher than 400 mg L⁻¹ (including 400 mg L⁻¹), the flocs scarcely float onto the surface.

Fig. 8 The effect of dosage on the treatment of P(MMA–MAA–CS)-2 (temperature = 40 °C, stirring speed = 150 rpm, stirring time = 10 min).

The effect of temperature. When dosage was 200 mg L⁻¹, stirring speed was 150 rpm and stirring time was 10 min, Fig. 9 shows the effect of temperature on the treatment performance of P(MMA–MAA–CS)-2. Obviously, the oil removal increases with increasing the temperature and remain unchanged when the temperature is above 60 °C. Besides, the stability of oil droplets decreases with increasing the temperature and the flocculants have more outstretched conformation.

Fig. 9 The effect of temperature on the treatment of P(MMA–MAA–CS)-2 (dosage = 200 mg L⁻¹, stirring speed = 150 rpm, stirring time = 10 min).

The effect of stirring speed and stirring time. Fig. 10 shows the effect of stirring speed and stirring time on the treatment performance of P(MMA–MAA–CS)-2. It could be found that the oil removal increases with increase of stirring speed and stirring time. The oil removal reaches a steady state when the stirring speed and stirring time are above 150 rpm and 7 min, respectively.

Fig. 10 The effect of stirring speed and stirring time on the treatment of P(MMA–MAA–CS)-2 of 200 mg L⁻¹ (a) stirring speed was changed, stirring time = 10 min; (b) stirring time was changed, stirring speed = 150 rpm.

Conclusions

In this study, P(MMA–MAA–CS) was designed and synthesized via emulsion polymerization by using CTAB as emulsifier and APS as initiator. Its structure and some properties were studied. When pH is changed, the surface tensions of P(MMA–MAA) and P(MMA–MAA–CS) have a mutation and a gradual change, respectively. For P(MMA–MAA–CS), the transition from COO⁻ to COOH can be slowed down by the ether group and amido group in CS. Therefore, the hydrophilicity and surface tension of P(MMA–SMA–CS) were changed gradually. P(MMA–MAA–CS)-2 has excellent flocculation performance for treating acidic oily wastewater. The effects of dosage, temperature, stirring speed and stirring time on flocculation performance of P(MMA–MAA–CS)-2 were discussed. It's concluded that P(MMA–MAA–CS)-2 displays best flocculation performance with the dosage larger than 200 mg L⁻¹, temperature above 60 °C, stirring speed higher than 150 rpm and the stirring time longer than 10 min.

Acknowledgements

This work is funded by the National Natural Science Foundation of China (51504201) and the Key Technologies R&D Program of China (NO. 2016ZX05025003-003).

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