Adsorption Characteristics of Dodecamethylcyclohexasiloxane and Dodecamethylpentasiloxane from Landfill Leachate by Municipal Solid Waste under the Landfill Circumstance

Abstract

The release of siloxane from landfill leachate has attracted wide attention. In this study, dodecamethylcyclohexasiloxane (D6) and dodecamethylpentasiloxane (L5) were chosen as the target pollutants to investigate the adsorption characteristics of cyclosiloxane and linear siloxane on municipal solid waste (MSW) under the landfill circumstance. The adsorption behavior could be well described by the Langmuir–Freundlich model, with a maximum adsorption capacity of 0.677 μg g⁻¹ and 15.864 μg g⁻¹ for L5 and D6, respectively. It seems that D6 has a stronger affinity to MSW compared with L5. The maximum adsorption was observed when the pH was 5.0 and 5.5 for D6 and L5. The optical temperature was 30 °C. The organic acid and inorganic ions in the leachate could restrict the adsorption to a low level. The results suggested that the adsorption of siloxane on MSW could be limited at the methanogenic and mature stages, which may promote the distribution of siloxane in the leachate. The results could help to understand the release behavior of siloxane from landfill leachate during the running of a landfill.

1. Introduction

Siloxane is a kind of compound with a Si-O-Si bond as the main chain. According to the main chain structure, it can be divided into linear siloxane (L) and cyclosiloxane (D). The molecular structures of the linear siloxane and cyclosiloxane are shown in Figure 1. They have low surface tension and surface energy, with high thermal stability, chemical stability, strong lubricity and other excellent physical and chemical properties, and are widely used in industrial production [1]. In 2017, the annual global production of siloxanes reached 2.5 million tons, mainly used in textiles, personal care products, waterproofing agents, rubber, coatings, plastics and electronics. It is estimated that the annual growth rate of functional siloxane consumption is about 10.2% [2]. Due to the chemical stability, the consumption of siloxane products can introduce a large amount of siloxane pollutants into the environment. The main destinations are wastewater treatment systems and landfills [3,4,5,6]. In many developing counties, landfills, including open dumpsites and engineered sanitary sites, are the main method for the treatment of municipal solid waste (MSW). In many developed counties, such as the USA and European Union, engineered sanitary landfills are still an integral part of waste treatment systems [7]. In recent years, the release of siloxane through landfill biogas and leachate has attracted wide attention [8,9]. Siloxane has been proven to have a biological amplification effect and biological toxicity, and the output of siloxane to the surrounding environment through landfill gas and leachate may cause environmental pollution and human health risks [10,11,12,13].

The concentration of siloxane in landfill leachate could be affected by several factors, including the degradation process of the MSW related to the release of siloxane, and the distribution coefficient of siloxane in the solid phase (municipal solid waste (MSW)), liquid phase (leachate) and gas phase (landfill biogas). Adsorption of siloxane on MSW allows it to be stored in MSW, which could greatly affect the distribution of siloxane between MSW, leachate and landfill gas, and subsequently affect the release of siloxane from the landfill through leachate. Several research studies have reported that siloxane has a high N-octanol/water partition coefficient (lgKoc) value. For example, Bletsou et al. [14] reported that nearly 90% of methylsiloxane in water was adsorbed by suspended solids and sludge. Silva et al. [15] found that octamethylcyclotetrasiloxane (D4) and decamylcyclopentasiloxane (D5) in solution could be largely adsorbed by activated carbons and porous silica. These results suggested that siloxane is apt to be adsorbed by the solid adsorbents. Although the adsorption characteristics of siloxane have been investigated in wastewater and biogas [15,16], there are few studies concerning the adsorption characteristics of siloxane on MSW. The adsorption of siloxane on MSW allows it to be stored in MSW, which prevents its discharge through landfill leachate. To fully understand the releasing behavior of siloxane through leachate during the running of a landfill, it is important to investigate the adsorption characteristics of siloxane on MSW under different landfill circumstances.

The observed siloxane compounds in the landfill leachate include hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) [17]. Among them, D6 (C₁₂H₃₆O₆Si₆) was categorized as a high production volume chemical by the Organization for Economic Co-operation and Development and the U.S. Environmental Protection Agency [18,19]. Meanwhile, a high concentration of L5 (C₁₂H₃₆O₄Si₅) was observed in landfill leachate [20,21]. In addition, they share a similar molecular structure with other linear siloxanes and cyclosiloxanes, as exhibited in the Table S1. Therefore, L5 and D6 were chosen as the target pollutants, as they share a similar molecular structure with other linear siloxanes and cyclosiloxanes.

In this study, MSW was used as the sorbent to explore its retention capacity for L5 and D6 in the landfill. The kinetics and isotherms of the adsorption of L5 and D6 on MSW were investigated. The effects of the variations in the landfill circumstances on the adsorption, including pH, temperature, anions and organic acid, were also examined to reveal the evolution of the siloxane distribution pattern with the running of the landfill. The results provide insights into the retention ability of MSW for siloxane in different landfill circumstances, which could affect the distribution of siloxane in the landfill leachate.

2. Materials and Methods

2.1. MSW

The MSW used in this study was collected from the Hangzhou Tianziling municipal landfill sites. The original landfill waste components are the following: kitchen waste 52.5%, trees 1.7%, plastics 11.3%, metal and glass 1.8%, paper and fabric 8.7% and ash 24.0%. Excavators were used for sampling, the sampling depth was 15–18 m away from the top of the landfill final overlay, and the sampling volume was about 20 kg. After the MSW was taken out, it was divided into bags immediately and then sealed with fiber bags outside and transported back to the laboratory. To simulate the practical situation of the landfill, the MSW was used without sorting. The MSW was air-dried and ground (cut) with a 4 mm sieve. After screening, wastes with different particle sizes were mixed and sampled by quartering method.

2.2. MSW Characterization

The contents of individual elements of the MSW sample were analyzed after it was digested according to the method of Yamasaki [22]. An amount of 0.5 g air-dried sample was weighed into a Teflon beaker, and 2.5 mL HNO₃ and 2.5 mL HClO₄ were added and heated on the hot plate at 150 °C for 2–3 h. After cooling, 2.5 mL HClO₄ and 5 mL HF were added and heated at 150 °C for 15 min, and then 5 mL HF was added until the residue became almost dry. The residue was dissolved in 5 mL HNO₃ and diluted to 100 mL. The elemental concentrations in the solution were determined by ICP-OES (Thermo Electron Corporation IRIS/AP, USA). The samples were digested and analyzed in triplicate. In addition, the moisture content was determined by heating the MSW sample at 100 °C, according to ASTM D2216. The loss on ignition (LOI) at 600 °C was determined by heating the MSW sample at 600 °C, according to the Chinese standard GB7876-87. The value of (LOI_{600 °C}–moisture content) indirectly reflected the organic matter content in MSW. The corresponding values were presented in Supplementary Materials.

2.3. Batch Adsorption Studies

Batch adsorption experiments were conducted and equilibrated using a model KYC-1102 air-temperature-controlled shaker (Ning-bo Jiangnan Instrument Factory, China) at 150 rpm. Batch experiments were conducted in triplicate to ensure the accuracy of the obtained data.

According to the result of our landfill reactor study, the concentrations of L5 and D6 ranged from 0 to 24.1 ppb (see Figure S1 in Supplementary Materials). The original concentrations of L5 and D6 in this study were set at 20 ppb.

2.3.1. Time-Adsorption Profile Studies

An amount of 10 g MSW sample and 50 mL siloxane solution were added to a 100 mL beaker for the contact times of 1 min, 10 min, 20 min, 30 min, 120 min, 240 min, 480 min, 840 min, 1320 min and 1980, respectively. The slow adsorption process of siloxane has been reported in the previous literature [23]. The suspension was centrifuged at 3000 rpm for 10 min, and the concentration of siloxane in the supernatant was determined.

2.3.2. Isotherm Studies

MSW samples weighing 10 g were accurately weighed and placed into several 100 mL beakers, respectively. Then, 50 mL siloxane with initial concentrations of 10 ppb, 20 ppb, 40 ppb, 60 ppb, 80 ppb, 100 ppb, 120 ppb, 180 ppb and 200 ppb was added, respectively. Beakers were then sealed with a plug and oscillated at a constant temperature of 30 °C for 10 h.

The amount of siloxane absorbed in the MSW and the removal percentage of L5 and D6 (RP) were calculated according to Equations (1) and (2), respectively:

q_t = (C₀ − C_t) × V/m

(1)

RP = (C₀ − C_t) × 100/C₀

(2)

where q_t is adsorption capacity at time t (μg g⁻¹); C₀ and C_t are initial and instantaneous concentrations of siloxane (μg L⁻¹), respectively; V is the volume of the solution (L); and m is the mass of MSW (g).

The adsorption data were fitted by three adsorption models: Langmuir, Freundlich and Langmuir–Freundlich [24,25,26]. The Langmuir (Equation (3)), Freundlich (Equation (4)) and Langmuir–Freundlich (Equation (5)) isotherms are represented in

Table 1. Langmuir, Freundlich and Langmuir–Freundlich isotherm parameters and correlation coefficients for the adsorption of L5 and D6 in MSW at 298 K.

Model	Contaminant	Parameters	R2
Langmuir	L5	qm = 0.732 μg/g b = 0.128 L/g	0.973
	D6	qm = 0.981 μg/g b = 0.035 L/g	0.934
Freundlich	L5	Kf = 0.181 mg1−1/nL1−n/g n = 3.483	0.849
	D6	Kf = 0.108 mg1−1/nL1−n/g n = 2.337	0.959
Langmuir–Freundlich	L5	qm = 0.677 μg/g	0.994
		Klf = 0.161 L1/νmg−1/ν
		ν = 0.648
	D6	qm = 15.864 μg/g	0.959
		Klf = 1.221 × 10−5 L1/νmg−1/ν
		ν = 2.258

.

$$q_e=\frac{q_{m}b{C}_e}{1+b{C}_e}$$

(3)

$$q_e{=K}_f{C}_e{}^{1/n}$$

(4)

$$q_e=\frac{q_m{(K_{lf}{C}_e)}^{1/v}}{1+{(K_{lf}{C}_e)}^v}$$

(5)

where C_e is the equilibrium concentration (μg L⁻¹), q_m is the maximum amount of siloxane per unit weight of MSW (μg g⁻¹), q_e is the adsorption capacity at equilibrium, and b is the binding energy constant (L g⁻¹). For the Freundlich model, n is the heterogeneity factor and K_f is the Freundlich constant (mg^1−1/nL¹⁻ⁿ/g). For the Langmuir–Freundlich model, K_lf is the Langmuir–Freundlich constant (L^1/νmg^−1/ν) and v is the Langmuir–Freundlich heterogeneity constant.

2.3.3. Environmental Factor Studies

During the running of the landfill, the pH, temperature and organic acid and anion concentrations experience great change, which is often a concern for the landfill. In addition, these parameters are also believed to affect the adsorption behavior of siloxane on MSW. Therefore, they were chosen to investigate the variation in landfill circumstance on the adsorption. The effects of pH (3, 4, 5, 5.5, 6.5, 7.5 and 8.5), temperature (20 °C, 25 °C, 30 °C, 35 °C, 40 °C and 50 °C) and organic acid (0, 8000 mg L⁻¹, 16,000 mg L⁻¹, 24,000 mg L⁻¹, 32,000 mg L⁻¹ and 40,000 mg L⁻¹; see Supplementary Materials, 1.0 g acetic acid equal to 1.06 g COD) and anion concentrations (0, 1000 mg L⁻¹, 2000 mg L⁻¹, 3000 mg L⁻¹, 4000 mg L⁻¹ and 5000 mg L⁻¹) were investigated, which were set according to previous research [27,28,29]. The assays were conducted in beakers with 10 g MSW sample and 50 mL siloxane solution.

2.4. Analytical Procedure

The concentrations of L5 and D6 in the solution were determined by GC-MS. An amount of 10 mg of Tetrakis (trimethylsiloxy)silane (M4Q) was placed in a 100 mL volumetric flask and adjusted to the scale with n-hexane, with a mass concentration of about 100,000 μg L⁻¹. It was diluted with acetone in a ratio of 1:50 to 2000 μg L⁻¹, which is solution I. Then, 50 mL water sample in a flue gas washing bottle was added to 0.2 mL solution I under ultrasonication in a 50 °C water bath, which was pumped for 2 h with an air pump. N-hexane was used to elute the solid phase extraction column (SPE). The pre-treated samples were determined by GC-MS. The GC-MS experimental setup consisted of a gas chromatograph GC 7890B (Agilent, USA) hyphenated to mass spectrometry MS 5975C with a quadrupole Mass Analyzer (Agilent Technologies, Inc., CA, USA). All instruments were provided by Agilent Technology (Germany). The GC 7890A was equipped with a 5% phenyl methyl siloxane stationary phase column (DB-5MS,122-5532UI, 30 m × 0.250 mm, 0.25 μm film thickness, Agilent Technologies, Inc., CA, USA). The detailed conditions for the GC-MS are shown in the Supplementary Materials.

3. Result and Discussion

3.1. Adsorption Characteristics of L5 and D6 on MSW

3.1.1. Time-Adsorption Profile

Figure 2 shows the time-adsorption profiles of L5 and D6 at the initial concentration of 20 ppb. Different from other adsorption studies, the removal percentage did not show a consistent increasing trend. Instead, the adsorption exhibited fluctuation within first 240 min, and then slowly reached equilibrium in 480 min. The peak adsorption rates was observed within 20 min for both L5 and D6, which were 90.3% and 84.0%, respectively. After that, the removal percentages sharply decreased to 39.5% and 22.5%, respectively. This phenomenon might be attributed to change in the solution condition. As shown in Figure 3, the pH and conductivity showed a fast increase in the first 20 min. The increases in pH and salinity could restrict the adsorption of L5 and D6 in the MSW, which is further discussed in Section 3.1.3 and Section 3.1.6. Therefore, the removal percentage decreased. After 480 min, the environmental condition was steady, and the adsorption equilibrium was established. A relatively slow adsorption process (hours to reach equilibrium) was also reported in the literature for D4 in the liquid phase [23]. The slow adsorption process might be due to the large molecular diameter [30,31].

3.1.2. Adsorption Isotherms

An adsorption isotherm provides a relationship between the L5 and D6 concentrations in the solution and the amount of L5 and D6 adsorbed on MSW when the two phases are at equilibrium. The adsorption data were fitted by three adsorption models: Langmuir, Freundlich and Langmuir–Freundlich (see Figure 4). The Langmuir (Equation (3)), Freundlich (Equation (4)) and Langmuir–Freundlich (Equation (5)) isotherms are represented in Table 1.

The theoretical parameters of adsorption isotherms along with regression coefficients (R²) are listed in Table 1. The correlation coefficients (R²) of the Freundlich, Langmuir and Langmuir–Freundlich models for L5 were 0.973, 0.849 and 0.994, respectively. For D6, the correlation coefficients (R2) of the Freundlich, Langmuir and Langmuir–Freundlich models were 0.934, 0.959 and 0.959, respectively. This indicated that the Langmuir–Freundlich model was more suitable to describe the adsorption of L5 and D6 on MSW compared to the Freundlich and Langmuir model. The adsorption capacities of L5 and D6 in the MSW were 0.677 and 15.864 μg g⁻¹, respectively, according to the fitted parameter of the Langmuir–Freundlich equation. Compared with L5, a higher adsorption capacity of D6 on MSW was observed. This suggests that the MSW had a stronger affinity with the D6 than L5. A similar result was also observed for the adsorption of siloxane on activated carbon, which suggested that activated carbon has more adsorption capacity for D5 than L2 [16]. This might be attributed to the high molecular weight and boiling point, as shown in the Supplementary Materials.

3.1.3. Effect of pH on the Adsorption

As shown in Figure 5, the adsorption of L5 and D6 was influenced by pH. The peak removal percentage was observed at a pH of 5.0 for D6 and of 5.5 for L5. Raising or lowering the pH is disadvantageous for the adsorption. The decrease in pH could enhance the leaching of minerals from the MSW and increase the concentration of competitive ion of the solution, which is unfavorable for the adsorption process [32]. The increase in the pH may result in the sedimentation of the heavy metals on the surface of the MSW, which could occupy the adsorption sites and hinder the adsorption.

In addition, siloxane is able to hydrolyze in the solution (Equation (6)). The hydrolysis process was enhanced when acid or alkali was added to the system [1]. Siloxane was transferred into the more polar compound, which have greater affinity with solution. Therefore, the adsorption rate was decreased.

$$\mathrm{RSi}\left({\mathrm{OH}}_3{)}_3{ + \mathrm{H}}_2\mathrm{O}\stackrel{}{\to }\mathrm{RSi}\right({\mathrm{OH}}_3{)}_2{\mathrm{OH}+\mathrm{HOCH}}_3$$

(6)

3.1.4. Effect of Temperature on the Adsorption

The adsorption experienced an increase and decrease with the rise in the temperature (Figure 6). When the temperature rose from 20 °C to 30 °C, the removal percentage increased from 72.0% to 86.2% and 57.3% to 74.8% for L5 and D6, respectively. When the temperature further increased to 50 °C, the removal percentage dropped to 54.0% and 22.7%, respectively. Due to the complicated composition of the MSW, it is very likely that both the chemical action and physical interaction proceeded in the adsorption process. For example, the hydrolysis of siloxane allowed the reaction of silanol with carboxyl. When the temperature increased from 20 °C to 30 °C, the chemical adsorption was enhanced, resulting in the increase in the adsorption. The further increase in the temperature limited the physical adsorption, which was an exothermal process [33]. Therefore, the adsorption was decreased.

Compared with L5, the increase in temperature has a more profound effect for D6, which suggests that the physical adsorption played a more important role and was the main process for D6.

3.1.5. Effect of the Organic Acid on the Adsorption

With the running of the landfill, the accumulation of organic acid was often observed due to the degradation of organic matter of the MSW (Figure 7a), which is due to the anaerobic processes of acidogenesis and acetogenesis. Acetic acid was chosen in this study to simulate the organic acid accumulation, as it is the main species of acid resulting from MSW degradation [33,34,35]. Generally, the adsorption showed a restriction with the increase in the acetic acid. Two aspects should be considered for the increase in organic acid concentration. First, the pH of the solution would be lowered with the increase in the acid concentration, which was unbeneficial for the adsorption. In addition, the increase in the organic matter could compete for the active adsorption sites, which could restrict the adsorption of siloxane in the MSW. Compared with D6, the organic acid showed a more profound effect on the adsorption of L5. This result was expected, as the organic acid shared a similar chemical structure with the linear siloxane rather than cyclosiloxane.

3.1.6. Effect of the Inorganic Ions on the Adsorption

Sodium chloride is an important mineral in MSW due to its great relation with human life. It was chosen in this study to simulate the effect of inorganic minerals on adsorption. The result showed that the increase in inorganic ions limited the adsorption of D6 and L5 in the MSW (Figure 7b). With the increase in sodium chloride from 0 to 5000 mg L⁻¹, the adsorption rate decreased from 84.0% to 66.2% for L5 and 82.0% to 41.7% for D6.

3.2. Evolution of Adsorption Behavior at Different Stages of Landfill

With the running of the landfill, the landfill circumstance experienced great change. Generally, the running of a landfill can be divided into four stages, namely the initial stage, acidogenic stage, methanogenic stage and mature stage. After a short period of aerobiotic degradation, the landfill entered into the acidogenic stage. At the acidogenic stage, the organic matter was degraded into organic acid, which decreased the pH, increased the temperature, and increased the organic acid and inorganic ions of the leachate. The lowest pH was usually from 5.0–6.0 at this stage. After that, the landfill entered into methanogenic and mature stages. The pH rose to 7.0–8.0 due to the degradation of organic acid by the methanogenic bacteria. In addition, the temperature started to rise due to the anaerobic degradation. Therefore, the pH and temperature increased. The level of inorganic ion kept steady due to the shortage of degradation paths. According to the results of Section 3.1.3, Section 3.1.4, Section 3.1.5 and Section 3.1.6, the adsorptions of L5 and D6 on the MSW were greatly affected by the variation in landfill circumstances, including pH, temperature, organic acid and inorganic ions. In this respect, the adsorption behavior might be changed during the running of the landfill. Further research is needed to explore the different adsorption behaviors at the different stages of the landfill operation.

4. Conclusions

MSW has considerable adsorption capacity for siloxane. The capacities of L5 and D6 in the MSW were 0.677 μg g⁻¹ and 15.864 μg g⁻¹, respectively. Therefore, the release of L5 and D6 from landfill leachate could be affected by the adsorption. The adsorption could be well described by the Langmuir–Freundlich model. The maximum adsorptions of L5 and D6 were observed at a pH of 5.0 and 5.5, respectively. The optimal temperature was 30 °C for both L5 and D6. The presence of coexisting ions and organic acid could restrict the adsorption process. Future research is needed to explore the different adsorption behaviors at the different stages of landfill operation.