Aerobic biodegradation of polydiallyldimethylammonium chloride-acrylic-acrylamide-hydroxyethyl acrylate/ZnO nanocomposite in an activated sludge system

Abstract

Biodegradation studies of polydiallyldimethylammonium chloride-acrylic-acrylamide-hydroxyethyl acrylate/ZnO (P(DMDAAC-AA-AM-HEA)/ZnO) nanocomposite were performed in a simulated aerobic activated sludge system. Batch experiments were conducted with different initial substrate concentrations between 100 and 1000 mg L⁻¹ at pH 7 and 25 °C. The biodegradability tests of the samples by activated sludge exhibited significant degradation after 30 days of inoculation. The removal ratios were 71.8%, 67.1% and 63.6% at initial P(DMDAAC-AA-AM-HEA)/ZnO concentrations of 100, 500, and 1000 mg L⁻¹, respectively, indicating that the biodegradation efficiency decreased with increasing initial substrate concentration. Kinetic studies showed that the Monod model could accurately describe the biodegradation process. The estimated values of the maximum specific rate of substrate degradation (ν_max) and saturation rate constant (k_s) were 2.56 h⁻¹ and 336 mg L⁻¹, respectively. The biodegradability of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite was further confirmed through dynamic light scattering (DLS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR) and gel permeation chromatography (GPC) analysis. The action of microorganisms in the activated sludge caused the partial disruption of nano ZnO-matrix bonding, and was followed by random chain scission in the P(DMDAAC-AA-AM-HEA) polymer chains. These changes were accompanied by significant losses in the concentration and molecular weight of the samples.

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1. Introduction

Dimethyl diallyl ammonium chloride (DMDAAC), the most important commercial polymer of the allyl monomers, is a water-soluble cationic polyelectrolyte that has been extensively used in oilfields, papermaking, water-treatment, leather finishing, textile printing and dyeing, medicine, cosmetics, etc.^1–6 ZnO nanoparticles are considered to be an ideal filler material for high performance polymer/inorganic nanocomposites, because of their chemical and physical properties, high stability, excellent photocatalytic ability and electrical conductivity.^7–9 Recently, polymer/inorganic nanocomposites have attracted considerable attention due to their improved thermal stability, mechanical strength and toughness.^10,11 In our previous research,¹² DMDAAC was grafted with acrylic acid, 2-hydroxyethyl acrylate and acrylamide acrylate to produce P(DMDAAC-AA-AM-HEA) vinyl polymer, and the resultant product contained –OH, –COOH, and –CONH₂ groups and was endowed with enhanced overall properties. Furthermore, P (DMDAAC-AA-AM-HEA)/ZnO nanocomposite was prepared using the polymer diallyl dimethyl ammonium chloride-chloride-acrylic acid-2-hydroxyethyl acrylate/acrylamide P(DMDAAC-AGE-MAA) and nano-ZnO, in an attempt to further combine the properties of the individual constituents and broaden the application range of the nanocomposite.¹³ The carboxyl functional group of organic components is a good modifier to improve the adhesion strength between inorganic nanoparticles and organic components.^14,15 However, the nanoparticles released from different nanomaterials used in industrial commodities find their way through waste disposal routes into wastewater treatment facilities, and end up in wastewater sludge.¹⁶ Nano-ZnO has been found to be toxic to microorganisms at various levels.^17,18 The cationic quaternary ammonium group of PDMDAAC inhibits the activities of microorganisms in activated sludge processes.¹⁹ At the same time, P(DMDAAC-AA-AM-HEA)/ZnO leads to a considerable load of chemical oxygen demand (COD) in wastewater.

Because environmental protection has now become a global issue, a cleaner and greener technology is warranted for the abatement of industrial pollution.²⁰ As we know, to meet discharging specifications, concentrations of nutrients, suspended solids, metals, synthetic organic chemicals, and pathogens in wastewater are mainly treated by activated sludge processes.²¹ The biodegradability of a chemical under the conditions of biological treatment in a wastewater treating plant is significant in the evaluation of its environmental fate and friendliness. In consideration of the potential environmental impact of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite, it is necessary to evaluate the entry of the nanocomposite into wastewater treatment plants, and methods are needed for industry to assess its potential removal during wastewater treatment. Therefore, it is quite necessary to evaluate the biodegradability of the nanocomposite in wastewater activated sludge.

Recently, many studies have focused on the biodegradation behaviour of nanocomposites, such as montmorillonite,^22–25 clay,^26,27 silica,²⁸ and cellulose nanocomposites.²⁹ Nano-ZnO has been found to be toxic to microorganisms, fish, and plants at various levels.^21,30,31 A literature³² report has described the thermal degradation behaviour of polyacrylate/ZnO nanocomposites. The study indicated that ZnO nanoparticles exhibited a greater effect on polyacrylate/ZnO thermal degradation. However, detailed studies on the biodegradability of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite in wastewater treatment plants have not been reported.

The aim of this work was to investigate the biodegradation potential of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite in the presence of an aerobic activated sludge system at the laboratory scale. Batch experiments were conducted to investigate the degradation ability. With this as a basis, the kinetics of the biodegradation process were studied in accord with the widely applied Monod model, by taking into consideration both substrate inhibition and endogenous metabolism. In addition, this research provides an intensive evaluation of the weight loss, size distribution, chemical degradation and morphological properties of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite before and after biomass inoculation.

2. Materials and methods

2.1 Materials

P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite was provided by the Key Laboratory of Chemistry and Technology for Light Chemical Industry of Ministry of Education according to literature,¹³ but it was prepared via in situ polymerization. Activated sludge was taken from an aeration basin used in cyclic activated sludge system (CASS) processes in a full scale municipal wastewater treatment plant (MWTP) in Xi'an, China. The sludge was acclimatized in the laboratory by feeding it with glucose, NH₄Cl, and K₂HPO₄ (COD/N/P ratio 200 : 5 : 1). During the steady state, the concentration of mixed liquid suspended solids (MLSS) was 6.5 g L⁻¹. This activated sludge solution was used for the biodegradation studies, and it was continuously aerated until use. Other chemicals were purchased from Xi'an Chemical Co., China. All other chemicals were of analytical grade and used without further purification.

2.2 Biodegradation experiments

Batch biodegradation experiments were carried out by adding P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite to three 10.0 L aeration tanks; the COD concentrations of P(DMDAAC-AA-AM-HEA)/ZnO in the batch culture experiments were 100, 500, and 1000 mg L⁻¹, respectively. Activated sludge was directly added to each tank, and the initial mixed liquor volatile suspended solid (MLVSS) concentration was maintained at 1500 mg L⁻¹. The mixture was adjusted to pH 7.0 ± 0.3 using 0.1 M HCl or 0.1 M NaOH. Over the course of the experiment, the aerobic tank was aerated by an air pump, and the dissolved oxygen (DO) concentration was maintained at about 2.8 to 3.0 mg L⁻¹. The glass tank was placed in an incubator to maintain the temperature at 25 °C. The tests were run simultaneously for 30 days. Biodegradation was monitored by plotting the biological respiration curve, COD and TOC (total organic carbon) removal ratios. A blank test, which contained only the activated sludge and distilled water, was conducted. Every test was conducted in triplicate.

The concentration of P(DMDAAC-AA-AM-HEA)/ZnO was expressed as COD. The COD concentration was determined using a standard method, and the absorbance was analyzed with a UV spectrophotometer (UV7200).³³ The data are the averages of three separate measurements. The COD removal ratio was defined as eqn (1):

COD removal ratio (%) = 100 × (COD₀ − COD_t)/COD₀ (1)

where COD₀ is the original chemical oxygen demand of the test sample solution (mg L⁻¹), and COD_t is the chemical oxygen demand of the sample after biodegradation at time t (day).

The total organic carbon (TOC) was measured using a TOC analyzer (elemental Y-Liqui TOCII 35091009). The data are the averages of three separate measurements. The TOC removal ratio was measured using eqn (2):

TOC removal ratio (%) = 100 × (TOC₀ − TOC_t)/TOC₀ (2)

where TOC₀ is the initial and TOC_t is the final total organic carbon of the test sample solution (mg L⁻¹) after biodegradation in t days.

The biomass concentration was determined by the measurement of the OD (Optical Density) of the sample at 600 nm using a UV spectrophotometer (UV7200).³⁴ The observed OD600 values were converted to MLVSS concentrations using the calibration curve, which was used to relate the absorbance of the culture to the biomass concentration. MLVSS and MLSS analysis were carried out according to standard methods.³⁵ The oxygen consumption of degradation was detected by a BOD analyzer (HaCH BODtrackII). DO and pH measurements were carried out using the DO and pH meter probes.

2.3 Characterization

2.3.1 Morphological and elemental analysis. Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800 field emission scanning electron microscope, which was equipped with an energy dispersive X-ray spectroscope (EDS) for morphological and elemental analysis. The activated sludge sample was dried in a vacuum freezing drying oven, and the sample surface was coated with gold. The sample surface morphology was observed using a 1.0 kV acceleration voltage. EDS was performed on an EDAX 32 system simultaneously.

2.3.2 Fourier transform infrared spectroscopy (FT-IR) analysis. FT-IR analysis of the activated sludge and the original P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite were carried out using a Fourier transform infrared spectrometer (Vector-22, Brucker). For FT-IR analysis, the samples were prepared by drying under atmospheric conditions for 24 h and mixed with spectrographic KBr by tableting. The scan wavenumber was in the range of 400–4500 cm⁻¹.

2.3.3 Dynamic light scattering (DLS) measurements. The particle size and size distributions of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite before and after biodegradation were determined using a Zeta PALS dynamic light scattering detector (Nano-ZS, Malvern Instruments Ltd.). The nanocomposite sample for DLS characterization was diluted with deionized water and ultrasonically treated for 10 min.

2.3.4 Gel permeation chromatography (GPC). The average molecular weight (M_w) and polydispersity index (PDI) of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite before and after biodegradation were determined using a Agilent 1100LC gel permeation chromatograph. The eluent was nitric acid at a flow rate of 1.0 mL min⁻¹ and 30 °C.

3. Results and discussion

3.1 P(DMDAAC-AA-AM-HEA)/ZnO biodegradation

3.1.1 Effect of substrate concentration on P(DMDAAC-AA-AM-HEA)/ZnO biodegradation. Three different initial COD concentrations (100, 500, and 1000 mg L⁻¹) of P(DMDAAC-AA-AM-HEA)/ZnO were studied for their effects on the biodegradation capacity of activated sludge. The experiments were designed to evaluate the long-term exposure of the activated sludge. The long-term effects were evaluated after 30 days of continuous incubation. In all the experiments, the pH was maintained at pH = 7 ± 0.3. The dissolved oxygen (DO) concentration ranged from 2.8–3.0 mg L⁻¹. Fig. 1 shows that the removal efficiency gradually increased with increasing reaction time. It is notable that in the curves, there was a relatively sharp drop of nanocomposite concentration at the beginning, followed by a relatively gentle decline. The overall average percent removals of the nanocomposite with initial concentrations of 100 mg L⁻¹, 500 mg L⁻¹, and 1000 mg L⁻¹ were found to be 30.9%, 24.8% and 19.3% in the first day, respectively. The result confirmed that the nanocomposite biosorption before biodegradation was primarily dependent on the physicochemical interactions between the substrate, and the biomass active sites on the cell surfaces for substrate binding were quickly occupied during the initial rapid phase. Subsequently, the biodegradation efficiency continued to increase; the removal ratios at the 10th day reached 60.6%, 55.6%, and 50.9% for the treatment groups with 100 mg L⁻¹, 500 mg L⁻¹, and 1000 mg L⁻¹ of the nanocomposite, respectively. This trend was likely caused by bioaccumulation and biodegradation. However, the increase of the nanocomposite removal efficiency for the three treatment groups tended to slow after 10 days of incubation. The removal ratio was 71.8% for the initial substrate concentration of 100 mg L⁻¹ at the end of 30 days of incubation. The extent of the removal ratio decreased to 67.1% for the initial substrate concentration of 500 mg L⁻¹, and decreased further to 63.6% for the initial substrate concentration of 1000 mg L⁻¹ at the end of the incubation period.

Fig. 1 P(DMDAAC-AA-AM-HEA)/ZnO biodegradation at different initial substrate concentrations.

With increasing substrate concentration, the removal efficiency of P(DMDAAC-AA-AM-HEA)/ZnO decreased. This is because more nanocomposites would have little chance to bind to the constant concentration of microorganisms.³⁶ Furthermore, fragments such as ZnO have special bio-toxicity, leading to microorganism inactivation^17,18 and thereby retarding the biodegradation. The toxic effects of the nanocomposite on the microorganisms could increase with increasing initial substrate concentration.

3.1.2 Kinetic studies. Based on the results obtained from the batch experiments, the biodegradation kinetics were investigated with the Monod model. The Monod equation is one of the best-known kinetic models describing microbial growth, which shows a functional relationship between the specific growth rate and an essential substrate concentration.^37,38 Monod's model was found to describe the biodegradation of P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite when present as a single substrate better than other bio-kinetic models. The specific growth rate μ (h⁻¹) for Monod's model is expressed by the following eqn (3):where μ_max is the maximal specific microbial growth rate (h⁻¹), s is the substrate concentration (mg L⁻¹), and k_s is the saturation constant (mg L⁻¹). The model assumes that some constant fraction of the consumed substrate is transformed into microbial biomass:where ν is the specific rate of substrate degradation (h⁻¹), and ν_max is the maximal specific rate of substrate degradation (h⁻¹).

Application of the experimental data presented in Fig. 1 to the Monod equation gave an excellent fit to the experimental data, as R² was observed to be 0.99. The parameters of the Monod equation for P(DMDAAC-AA-AM-HEA)/ZnO were obtained from the experimental data, and are as follows:

ν_max = 2.56 h⁻¹

k_s = 336 mg L⁻¹

Therefore, the Monod kinetic equation for the biodegradation of P(DMDAAC-AA-AM-HEA)/ZnO by municipal activated sludge can be represented with the following eqn (5):

Fig. 2 shows the experimental and modeled P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite concentration profiles during the biodegradation of 500 mg L⁻¹ P(DMDAAC-AA-AM-HEA)/ZnO in the activated sludge. It can be seen that there was little change in the nanocomposite concentration in the modeled profile after 1 day. However, a sharp drop in the total nanocomposite concentration can be seen after 1 day of aerobic system operation. This is in good accordance with the amount of nanocomposite that was adsorbed in the activated sludge on the first day. The model corroborates the experimental data very well from 5 to 20 days. A comparison between the model predictions and the experimental values shows that the Monod model is suitable to describe the biodegradation of P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite.

Fig. 2 Experimental data and Monod kinetic model fitting for the aerobic biodegradation of P(DMDAAC-AA-AM-HEA)/ZnO at an initial COD concentration of 500 mg L⁻¹.

3.1.3 TOC removal. Biodegradation was further confirmed by TOC removal ratio analysis.³⁹ When the soluble carbon of the biodegradable nanocomposite in the wastewater is utilized as the source of carbon and energy for the growth of microorganisms in activated sludge, it is gradually consumed by the microorganisms, resulting in a decrease of the TOC value after treatment with activated sludge. Fig. 3 shows the removal percentages of P(DMDAAC-AA-AM-HEA)/ZnO with an initial COD concentration 500 mg L⁻¹; it could be easily seen that the TOC in solution decreased rapidly in 5 days of aerobic digestion. The percent TOC removals of P(DMDAAC-AA-AM-HEA)/ZnO within 5 days reached 29.5%. Subsequently, the biodegradation efficiency continued to increase, and the TOC removal ratio on the 10th day reached 33.7%. From this period onward, the biodegradability of the nanocomposite slowly increased with time. The TOC removal ratio was 41.8% at the end of 30 days of treatment, which is in agreement with the tendency of the COD removal ratios. The decrease of TOC can be attributed to the mineralization of P(DMDAAC-AA-AM-HEA) by microorganisms.

Fig. 3 TOC removal of P(DMDAAC-AA-AM-HEA)/ZnO under activated sludge process.

3.1.4 Oxygen consumption. Respiration is an essential activity of aerobic microorganisms. A chemical that can be readily biodegraded will be utilized as a source of carbon and energy for the growth of organisms in activated sludge. As a result, the respiration of activated sludge will be enhanced.⁴⁰ Biodegradation tests with P(DMDAAC-AA-AM-HEA)/ZnO were performed with an initial COD concentration of 500 mg L⁻¹. As shown in Fig. 4, little oxygen consumption was observed during the first day, implying that the biodegradation process was retarded. Afterwards, the microorganisms present in the activated sludge adapted to the nanocomposite environment, and the oxygen consumption increased rapidly from 1 to 10 days. From this period onward, the oxygen consumption slowly increased. The tendency of the respiration curve was similar to that of the degradation curve (Fig. 1), which implied that there was a significant correlation between oxygen consumption and substrate degradation. The respiration curve of activated sludge in the presence of the nanocomposite is above the endogenous respiration curve. This fact indicates that the nanocomposite has the potential to biodegrade, and could be readily biologically treated in an effluent treating plant.

Fig. 4 Oxygen consumption curves of blank (A) and activated sludge in the presence of P(DMDAAC-AA-AM-HEA)/ZnO (B).

3.2 SEM-EDS analysis

To further understand the biodegradability of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite, SEM was employed to observe the morphology changes of the activated sludge before and after degradation. A SEM image of the nanocomposite is shown in Fig. 5A, where the black substances are P(DMDAAC-AA-AM-HEA) particles. Some small ZnO particles, which are relatively bright and sheet-like, appear to be agglomerated in the P(DMDAAC-AA-AM-HEA) latex. The agglomeration of inorganic nanoparticles is due to the high specific surface area.¹⁰ This result demonstrated that ZnO nanoparticles are well embedded in the P(DMDAAC-AA-AM-HEA) matrix. Fig. 5B shows a SEM image of the bare activated sludge, which is characterized by loose volume and a visible surface morphology and pores. It can be observed that the number of pores or holes on the surface of the activated sludge clearly decreased after one day of treatment. The slight accumulation that can be seen in Fig. 5C is believed to be caused by the adsorption of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite onto the sludge. However, after 30 days of incubation, the activated sludge appears as amorphous clumps of matter and lacks distinct features (Fig. 5D). In addition, some white aggregates can be seen in the 30 days sample, which clearly indicates the distribution of ZnO nanoparticles on the surface of the activated sludge. The evident microstructural changes of the activated sludge may be due to the biodeterioration of the nanocomposite chains, causing further fragmentation. The changes in the morphologies of the activated sludge before and after degradation observed in the SEM images can be corroborated with the concentration loss of the nanocomposite upon degradation, as shown in Fig. 1. Consequently, it could be concluded that P(DMDAAC-AA-AM-HEA)/ZnO had preferable biodegradability.

Fig. 5 SEM image of P(DMDAAC-AA-AM-HEA)/ZnO (A); SEM images of the activated sludge in the aerobic reactor during different biodegradation periods: (B) 0 day (bare activated sludge), (C) 1 day, (D) 30 days; EDS spectra of the activated sludge in the aerobic reactor during different biodegradation periods: (E) 0 day (bare activated sludge), (F) 1 day, (G) 30 days.

EDS analysis was conducted to semi-quantify the surface and near surface amounts of each element present. As shown in Fig. 5E, it is clear that the structural elements of activated sludge are mainly composed of C, O, Si, Al, Ca, Na, P, K, and Fe elements. EDS spectra of the activated sludge after biodegradation are also provided in Fig. 5F and G. Clearly, Si content accounted for the majority of elemental content in Fig. 5E and G, whereas the elements C and O are higher in content than Si in Fig. 5F. The appearance of additional elemental peaks for Zn and Cl was also noted in Fig. 5F. This demonstrates the amount of P(DMDAAC-AA-AM-HEA)/ZnO adsorbed on the activated sludge surface after 1 day treatment. Through surface binding, the P(DMDAAC-AA-AM-HEA)/ZnO particles accumulated and adsorbed to the surface of the activated sludge. When comparing Fig. 5G to Fig. 5F, the peak for Zn significantly increased after 30 days of biodegradation. Due to the collapse and disruption of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite, there would be more Zn ions released. However, no Cl elemental peak could be observed in the 30 days sample, indicating that depolymerization of the adsorbed P(DMDAAC-AA-AM-HEA) occurred. Moreover, it is obvious that the peak of elemental C in Fig. 5E is weaker in comparison with Fig. 5F, but stronger than in Fig. 5G. The increased percent content of elemental C in the matrix on the first day could be the reason for the presence of large amounts of P(DMDAAC-AA-AM-HEA)/ZnO adsorbed on the activated sludge surface after one day of adsorption. However, the decrease in C content after 30 days might be due to the fragmentation of the long chains of P(DMDAAC-AA-AM-HEA) into smaller chains and CO₂.

3.3 Particle size distribution analysis

The particle size distribution of P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite before and after biodegradation for 30 days was investigated by DLS analyses, and the results are displayed in Fig. 6. The original particle size distribution of the nanocomposite is bimodal, mainly containing two particle classes, primary particles (295–950 nm) and nanoparticles (50–90 nm). This is because the P(DMDAAC-AA-AM-HEA)/ZnO latex particles include both ZnO and P(DMDAAC-AA-AM-HEA); in addition, there may be aggregation of nano-ZnO. After 30 days of aerobic digestion, the nanocomposite particles were transformed to a monomodal particle size and had a low intensity of distribution, ranging from 140 nm to 915 nm. Obviously, the P(DMDAAC-AA-AM-HEA)/ZnO particles decreased in size after degradation, which further demonstrated that the polymer chains in the molecules of P(DMDAAC-AA-AM-HEA)/ZnO ruptured under biodeterioration by microorganisms.

Fig. 6 Particle size distribution of P(DMDAAC-AA-AM-HEA)/ZnO before (A) and after (B) biodegradation for 30 days.

3.4 FT-IR spectrum analysis

The chemical structures of the original P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite and the activated sludge before and after biodegradation for 30 days were detected by FT-IR, and the results are presented in Fig. 7. In the FT-IR spectra of P(DMDAAC-AA-AM-HEA)/ZnO, the bands at 1398 and 2925 cm⁻¹ are ascribed to the CH₃ and CH₂ stretching vibrations, respectively. The peak at 1735 cm⁻¹ represents the stretching vibration absorption of the C=O⁴¹ of the carboxyl group. The peak at 1617 cm⁻¹ is assigned to the C=O stretching vibration, which is a characteristic absorption of –CONH₂ vibrations. The absorption peak at 1313 cm⁻¹ is related to the –OH vibration. The absorption peak for C–O at 1037 cm⁻¹ is related to the ester bond. In addition, absorption peaks for C–N and –NH₂ can be clearly found at 1419 and 615 cm⁻¹, respectively. The peak that appears at 435 cm⁻¹ was due to the Zn–O stretching vibration.

Fig. 7 FT-IR spectra of P(DMDAAC-AA-AM-HEA)/ZnO (A); FT-IR spectra of the activated sludge before (B) and after (C) biodegradation for 30 days.

On comparing the FT-IR spectra of activated sludge before and after biodegradation for 30 days, it has been observed that the spectrum of activated sludge after biodegradation shows variations in intensity, and a shifting of the peak from 1637 to 1617 cm⁻¹ appears due to –CONH₂ vibrations. The new bands appearing at 435 and 1246 cm⁻¹ are due to the Zn–O and C–N⁺˙⁴² stretching vibrations, respectively. The characteristic absorption band of the ester bond at 1037 cm⁻¹ is obviously strengthened after biodegradation for 30 days. The presence of vinyl groups (CH₂=CH–) was also detected at 883 cm⁻¹, indicating the scission of the main polymer chain. Higher quantities of ester bonds, vinyl groups and Zn–O were produced after biodegradation. The changes in the FT-IR spectra indicate that the P(DMDAAC-AA-AM-HEA)/ZnO molecules are not completely mineralized but undergo depolymerization to a certain degree.

3.5 GPC analysis

GPC analysis of the P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite was conducted to investigate the effect of biodegradation on the molecular weight. As shown in Fig. 8, the molecular weight of the nanocomposite decreased significantly after 30 days of biodegradation; the M_w of the nanocomposite changed from 1.443 × 10⁵ g mol⁻¹ to 2.121 × 10⁴ g mol⁻¹. The decrease in the M_w of the nanocomposite might be due to the fragmentation of long chains into smaller ones by microbial attack and consumption of the smaller fragments, thereby leading to M_w loss.⁴³ Meanwhile, the PDI increased from 15.8 to 33.3, indicating that random chain fragmentation in the P(DMDAAC-AA-AM-HEA)/ZnO backbone caused a greater variation in the polymer chain length, subsequently resulting in the broadening of the molecular weight distribution.⁴⁴ Therefore, it can be concluded that as the biodegradation process occurs in the activated sludge, microorganisms first rupture the P(DMDAAC-AA-AM-HEA)/ZnO chains into smaller sized fragments, which is followed by the mineralization of the polymer by microbial metabolism.²⁴

Fig. 8 GPC results of P(DMDAAC-AA-AM-HEA)/ZnO before (A) and after (B) biodegradation for 30 days.

4. Conclusions

Biological treatment of P(DMDAAC-AA-AM-HEA)/ZnO nanocomposite by activated sludge over a period of 30 days was investigated, and the aerobic biodegradation kinetics were studied. The biodegradation efficiency decreased with increasing initial substrate concentration. About 71.8%, 67.1% and 63.6% of the total P(DMDAAC-AA-AM-HEA)/ZnO was removed for the treatment groups with 100 mg L⁻¹, 500 mg L⁻¹, and 1000 mg L⁻¹ concentrations of the nanocomposite, respectively. The Monod model successfully predicted the kinetic data obtained from batch experiments. The biodegradability tests were supplemented with SEM, EDS, FT-IR, DLS, and GPC analysis. The results showed that the biodegradation process occurs in the activated sludge mainly through biodeterioration and depolymerisation by microorganisms.

Acknowledgements

This work was supported by 973 Program (no. 2011CB612309), Scientific Research Plan of Shaanxi Province (no. 2012K08-03).

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