Effect of inorganic salt ions on the adsorption of quinoline using coal powder

Quinoline is a typical nitrogenous organic compound commonly seen in the petroleum, coal tar, coal gas and oil shale processing industries. It is widely used as a raw material and solvent in the dye, paint, fungicide and wood-processing industries (Padoley et al. 2008; Rameshraja & Srivastava 2012; Wang et al. 2016). However, quinoline is a risk to the environment and humans due to being highly toxic. It has a good solubility and low biodegradability which makes it penetrate easily into the soil and groundwater, causing environmental problems and illness.

Physico-chemical and biological methods have been used to treat wastewater containing quinoline. In addition, multiple adsorbents such as granular activated carbon and bagasse fly ash (Rameshraja & Srivastava 2012), kaolinite and montmorillonite (Chen 2002), mesoporous molecular sieve Ti-HMS (Thomas et al. 2010), Pt (Santarossa et al. 2008), fibrous silicates and Patagonian saponite (Vico & Acebal 2006), combusted rundle spent shale (Zhu et al. 1995) have been used in the removal of quinoline. However, various problems including high cost, desorption difficulty after adsorption, transfer of organic pollutants after desorption, poor adsorption effect after multiple adsorption and desorption make it difficult for these adsorbents to be scaled up for industrial application. So an adsorbent with the lowest possible price that does not require regeneration is very desirable.

Coal is an important energy resource widely used with other fossil fuels such as oil and natural gas. It has relatively large pores and specific surface area, and has been tested to adsorb methane, carbon monoxide and other gases well at the laboratory scale (Hao et al. 2013; Jian et al. 2015). In addition, Wang et al. described a method of wastewater treatment using coal as a potential adsorption material and this technology has been successfully used in industrial production (Wang et al. 2014). It should be noted that the requirement for a sustainable industrial wastewater treatment technology should include the following key elements: a sorbent with good sorption properties along with a low price that could go back into the coking system after adsorption instead of entering a regeneration process (Gao et al. 2016a, 2016b). The study found that the chemical oxygen demand (COD) was reduced from 178.99 mg/L to 43.56 mg/L, and the removal rate reached 75.66%, using coking coal powder to treat coking wastewater (Gao et al. 2015). It was interesting to find that the coking coal had a higher adsorption capacity in an acidic solution (Gao et al. 2016a, 2016b). This may be attributed to a greater specific surface area produced by minerals dissolving to form inorganic salt ions under the addition of acid, which enhanced the adsorption effect. In addition, the dissolved inorganic salt ions may have had an impact on the adsorption of pollutants using coking coal.

As can be seen, the use of coal as an adsorbent to treat industrial wastewater, especially the adsorption process under the influence of inorganic ions is still poorly understood. This paper explores the effect of inorganic salt ions on the adsorption of quinoline using coal powders, taking Na⁺ and Ca²⁺ as examples. The data were fitted to Langmuir and Freundlich isothermal adsorption models, and various thermodynamic parameters were also calculated. Sorption kinetics was studied using pseudo-first-order and pseudo-second-order kinetic models. The ζ-potentials (zeta potentials) of coal particles in the solution with different inorganic salt ion concentrations were measured to explain the effect of the inorganic ions on adsorption.

The low-rank coal was obtained from Shenmu, Shaanxi, China. It was crushed to 3 mm, and then ground so that 85% of the coal sample was below 200 mesh before using as the adsorbent for this work. The physical and chemical properties of the coal sample were analyzed using X-ray fluorescence (XRF, Model S8 TIGER, Bruker AXS GmbH, element detection range: Be (4) ∼U (92)), X-ray diffraction (XRD, Model D8 Advance, Bruker AXS GmbH, X-ray tube voltage 40 kV, current 30 mA, anode target material Cu target, radiation Ka), scanning electron microscope (SEM, Model FEI Quanta TM 250, Thermo Fisher Scientific Inc., scanning acceleration voltage 30 kV, resolution less than 3 nm, coating treatment sprayed on before test) and BET automatic nitrogen adsorption instrument (BELSORP-max, BEL JAPAN Inc.).

A 1 g/L quinoline stock solution was prepared by dissolving 1 g quinoline in deionized water in a 1 L volumetric flask and stirring well to ensure the quinoline dissolved completely. The quinoline was supplied by Kemiou Chemical Reagent Co., Ltd. NaCl and CaCl₂, supplied by Sinopharm Chemical Reagent Co., Ltd, were added to prepare solutions of different inorganic salt ion concentrations, respectively. All chemicals used in this study were of analytical reagent (AR) standard.

For each adsorption test, a certain amount of quinoline stock solution of 1 g/L was pipetted to a 250 ml conical flask. Deionized water was added to the flask to make a quinoline solution. The concentration of this diluted solution in the flask was defined as the initial concentration. Coal powder was then added to the solution to adsorb quinoline. The conical flask was sealed with a plastic wrap and shaken in a thermostatic water bath shaker at a set temperature for a set time. After shaking, the solution was filtered using a Buchner funnel and vacuum pump. The filtrate was analyzed using a UV-vis spectrophotometer (UV-4802S, Shangkuo Instrument Technology Co., Ltd) to determine the residual concentration of quinoline in the solution. The maximum absorbance wavelength of quinoline was found to be 277 nm. To explore the effect of inorganic salt ions on the adsorption process, different amounts of NaCl or CaCl₂ were added to the quinoline solution before adsorption. Also, deionized water was used instead of simulated quinoline wastewater for a blank test, to eliminate the possibility that the dissolved organic matter on the surface of coal could interfere with the absorbance of the UV-vis spectrophotometer.

Coal powder adsorbent suspensions were prepared for ζ-potential measurement by ζ-potential analyzer (Model ZetaPALS, Brookhaven Instruments). Each suspension had a volume of 50 ml and the concentration of coal was 10 g/L. NaCl or CaCl₂ was added to make adsorbent suspensions containing different inorganic salt ions. Each suspension was stirred at 1,600 rpm for 5 min using a magnetic thermostatic mixer. After one night, 5 ml supernatant was transferred to the electrophoresis tank of the ζ-potential analyzer to measure the ζ-potential of the coal particle surfaces without adjusting the pH.

Through the XRF and XRD analysis shown in Table 1 and Figure 1, the content of inorganic components in the coal was relatively low. Quartz and morganite were the two main inorganic substances found in the coal sample. The lower content of clay minerals was good for the subsequent solid-liquid separation process (Zhang et al. 2003).

As shown in Figure 2, the coal surface was rough and uneven. This porous structure on the coal surface undoubtedly increased the possibility of pollutant adsorption. The mean pore size was found to be 8.176 nm, which was determined by a BET automatic nitrogen adsorption instrument. The specific surface area, total pore volume and mesopore volume of the pulverized coal were 13.131 m²/g, 0.01253 cm³/g and 0.012105 cm³/g. The coal powder had a smaller specific surface area and total pore volume, but it had a more mesoporous structure, as shown in Figure 3, which is better for adsorbing large molecule pollutants from effluents.

NaCl and CaCl₂ were added to the solution to investigate the effect of inorganic salt ions on the adsorption of quinoline using coal powder, with concentrations of Na⁺ and Ca²⁺ from 0 to 100 mmol/L, C₀ = 30 mg/L, m = 20 g/L, t = 60 min, T = 293 K, without adjusting the pH, and the results are shown in Figure 4. When the concentration of Na⁺ or Ca²⁺ increased from 0 to 8 mmol/L, the removal rate and adsorbed amount increased rapidly with an increase in the concentration of Na⁺ or Ca²⁺. The removal rate and adsorbed amount of quinoline reached 94.47%, 1.42 mg/g or 90.21%, 1.35 mg/g when the concentration of Ca²⁺ or Na⁺ was 8 mmol/L, higher than the results obtained without the use of inorganic salts, which were 83.87%, 1.26 mg/g.

Interestingly, the removal rate and adsorption capacity showed a slow and steady increase when the concentration exceeded 8 mmol/L. This might be due to the fact that the addition of inorganic salts reduces the surface charge of the coal particles and compresses the electric double layer, thereby facilitating the motion of quinoline molecules onto the coal surface and enhancing adsorption (Bjelopavlic et al. 1999). In addition, the electric field around Na⁺ and Ca²⁺ might cause a strong electrostatic effect between the inorganic salt ions and water molecules in the quinoline solution, making water molecules gather around ions and enhancing the binding force between water molecules in the quinoline solution; the reduction of free water relative to quinoline molecules favors the adsorption process (Arafat et al. 1999).

Figure 5 shows the changes in ζ-potential of the coal particle surface in different Na⁺ and Ca²⁺ solutions. The absolute value of ζ-potential tended to decrease gradually with the increase in the Na⁺ or Ca²⁺ concentration. According to the Stern double layer model, the electrical double layer is divided into an adsorption layer and a diffusion layer. The inorganic salt ions compressed the diffusion layer, and reduced the repulsion potential energy. Na⁺ and Ca²⁺ could exclude counter ions in the diffusion layer with the same symbol charge as the adsorption layer, thereby reducing the amount of charge on the surface of the coal particles, which was expressed the as ζ-potential absolute value decrease. In addition, Ca²⁺ brought more positive charge than Na⁺ at the same concentration, causing a lower ζ-potential. The result indicated that Ca²⁺ had a beneficial effect on the adsorption of quinoline onto coal.

The isothermal adsorption experiments were carried out with inorganic salt ions at three temperatures: 293 K, 303 K and 313 K. Each temperature was applied to the blank (without adding inorganic salt ions), adding Na⁺ and Ca²⁺, with concentration of Na⁺ or Ca²⁺ 8 mmol/l, C₀ from 5 to 50 mg/L, m = 20 g/L, t = 60 min, and without adjusting the pH. The results are shown in Figure 6.

The results showed that the adsorption of quinoline by coal decreased with the increase of temperature, therefore the adsorption of quinoline by coal is an exothermic process. The adsorption isotherm described the relationship between the adsorbed amount of adsorbate molecules on the surface of the adsorbent and the concentration of the adsorbate molecules in the solution when the adsorption of the adsorbate molecules on the adsorbent surface reaches equilibrium under constant temperature conditions (Rameshraja & Srivastava 2012). Langmuir and Freundlich isotherms were used to fit the isotherm data, and the fitting results of constants and correlation coefficients in the adsorption models are shown in Table 2.

Comparing the values of the correlation coefficient R², the Freundlich isothermal adsorption model had a better correlation with the adsorption of quinoline by coal, and the isothermal adsorption curve was fitted as shown in Figure 6. The surface of the coal powder was dominated by aromatic rings, and the quinoline molecules could have coordination or complexation adsorption. The adsorption state was mainly physical adsorption. Also, as there was surface and pore adsorption, the Freundlich isothermal adsorption model was more suitable. The empirical parameter 1/n of the Freundlich isothermal adsorption model was less than 1, indicating that the adsorption of quinoline by coal is favorable. The value of K_F, which characterized the capacity for adsorption, was higher at the lowest temperature. Furthermore, the addition of NaCl and CaCl₂ produced a higher K_F than the blank, the K_F had the highest value with the addition of CaCl₂ at 293 K. This indicated that the inorganic salt ions could promote the adsorption of quinoline by coal powder. The addition of inorganic salt ions could reduce the electrostatic repulsion between coal particles and quinoline molecules, and the electric field around the salt ions caused the water molecules in the quinoline solution to accumulate around the ions, so the free water was reduced, resulting in the enhancement of adsorption and K_F value.

The values of lnK at different temperatures could be obtained by plotting ln(Q_eq/C_eq) versus Q_eq and extending the line to the intercept. Afterwards, the values of ΔH° and ΔS° were calculated from the slope and intercept of plots of lnK versus 1/T. Further, ΔG° was obtained using Equation (9). The values of the thermodynamic parameters are shown in Table 3.

The negative value of ΔG° showed that the adsorption of quinoline by coal was spontaneous, and the absolute values of ΔG° under the addition of inorganic salt ions were higher than that of the blank condition, which indicated that the inorganic salt ions intensified the adsorption of quinoline. The negative value of ΔH° showed clearly that the adsorption of quinoline by coal was exothermic, and the lower temperature favored the adsorption. The degree of freedom of quinoline molecules on the coal powder surface reduced after adsorption, indicated by the negative value of ΔS°.

Figure 7 shows the effect of adsorption time on the adsorption of quinoline by coal in the presence of inorganic salt ions, at temperatures of 293 K, 303 K and 313 K. For this test, the adsorption time was 10 to 120 min, with concentrations of 8 mmol/L Na⁺ or Ca²⁺, C₀ = 30 mg/L, m = 20 g/L, and without adjusting the pH.

The amount of quinoline adsorbed increased with adsorption time in the blank, Na⁺ and Ca²⁺, and increased rapidly in the first 30 min, and then plateaued after 60 min. In addition, it could be seen that the adsorption capacity was larger under the lower temperature with the same adsorption time.

The pseudo-first-order model and pseudo-second-order model were used to study the adsorption kinetics of quinoline, and the fitting results are shown in Table 4.

The pseudo-first-order model had a lower correlation coefficient R², and the calculated equilibrium adsorption amount Q_e(cal) was much lower than the actual equilibrium adsorption Q_e(exp), indicating that pseudo-first-order cannot fit the adsorption of quinoline. The pseudo-second-order model has a good value of R², and the equilibrium adsorption amount was close to the actual equilibrium adsorption amount. So the pseudo-second-order model can be used to describe the adsorption process of quinoline by coal; the curve is shown in Figure 7.

The adsorption rate of quinoline decreased gradually in the blank, Na⁺ and Ca²⁺ conditions, shown by the changing trend of k₂, which was the rate constant of pseudo-second-order model. This was due to the electric field around the inorganic salt ions causing the water molecules in the solution to collect around the ions, leading to a decrease in free water and a reduction in quinoline solubility. The probability of collision between the quinoline molecules increased, resulting in increased diffusion resistance.

The activation energy could be obtained by the slope of the plot of lnk₂ versus 10³/T as shown in Figure 8. The activation energies of the adsorption reaction were 7.38 kJ/mol, 6.08 kJ/mol and 5.31 kJ/mol respectively in the blank, Na⁺ and Ca²⁺ conditions. The activation energy required for physical adsorption is very small, usually in the range of 5–40 kJ/mol. But for chemical adsorption, it is generally greater than 83.72 kJ/mol (Sun et al. 2014). The result indicated that the adsorption of quinoline by coal was physical adsorption, and a small activation energy corresponds to a easy adsorption process. The values of activation energy with Na⁺ and Ca²⁺ were smaller than that of the blank, which suggested that inorganic salt ions were favorable to the adsorption of quinoline by coal powder.

It was feasible to adsorb quinoline using coal powders as the adsorbent, as indicated by the experimental results. The addition of an appropriate amount of inorganic salt ions (Na⁺ or Ca²⁺) to the solution significantly improved the adsorption effect, as an increase in both the removal rate and adsorption capacity of quinoline was found. ΔG° was negative, indicating that the adsorption of quinoline by coal was spontaneous. The higher absolute value of ΔG° and the smaller activation energy E_a of the adsorption reaction identified when Na⁺ or Ca²⁺ was added suggest that the inorganic salt ions could enhance the adsorption of quinoline. Negative ΔH° suggested the adsorption was an exothermic process and negative ΔS° indicated more quinoline molecules adsorbed on the surface of the coal powder.

The addition of inorganic salt ions could reduce the ζ-potential of coal particles and the electrostatic repulsion between coal particles and quinoline molecules, which was beneficial for quinoline molecules moving to the surface of coal powder to enhance adsorption. In addition, the electric field around Na⁺ or Ca²⁺ could make a strong electrostatic interaction with the water molecules in the quinoline solution. The water molecules gathered around the ions and enhanced the binding force between water molecules in quinoline solution. The free water for the quinoline molecules decreased accordingly, which was also beneficial for the adsorption process.

This research was supported by the National Natural Science Foundation of China (No. 51504254, No. 51274198) and the National Key Technology R&D Program for the 12th Five-Year Plan of China with Project (No. 2014BAB01B04).