Parametric, equilibrium and kinetic studies on the removal of mercury using ion exchange resin

Rajamohan, N.; Rajasimman, M.

doi:10.2166/wpt.2017.037

This experimental research was an investigation into removal of mercury by using a strong acid cation resin, 001 × 7. Parametric experiments were conducted to determine the optimum pH, resin dosage, agitation speed and the effect of change in concentration in the range of 50–200 mg/L. High resin dosages favoured better removal efficiency but resulted in lower uptakes. Equilibrium experiments were performed and fitted to Langmuir and Freundlich isotherm models. Langmuir model suited well to this study confirming the homogeneity of the resin surface. The Langmuir constants were estimated as q_max = 110.619 mg/g and K_L = 0.070 L/g at 308 K. Kinetic experiments were modeled using Pseudo second order model and higher values of R² (>0.97) were obtained. The Pseudo second order kinetic constants, namely, equilibrium uptake (q_e) and rate constant (k₂), were evaluated as 59.17 mg/g and 40.2 × 10⁻⁴ g mg⁻¹ min⁻¹ at an initial mercury concentration of 100 mg/L and temperature of 308 K.

isotherm, kinetics, mercury, resin

INTRODUCTION

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Heavy metal contamination in terrestrial and aquatic ecosystem is widespread in all the developing countries and poses serious threat to the flora and fauna through their accumulation in food chain. Depollution of heavy metal contamination is more challenging as they are non-biodegradable and have increased tendency to bio accumulate. Mercury is one of most harmful heavy metal contaminants used in mining, caustic-chlorine, crude refining, medical and battery production facilities (Wan Ngah & Hanafiah 2008; Plaza et al. 2011). Mercury finds its use in various forms, including elemental, organic and ionic forms. Mercury contamination through methyl mercury chloride has been reported in aquatic organisms, fishes and birds. Minamata disease is a proven evidence for the harmful effect of mercury contaminated fish and has created a negative impact on human health (Yavuz et al. 2006). Mercury poisoning in humans lead to a disorder called hydragyria which damages the central nervous system, lungs and kidney. This effect of mercury was reported to occur due to the increased affinity of mercury for protein binding. It is inevitable to remove or recover mercury from the contaminated water. Mercury removal can be investigated using a number of physico-chemical conventional methods like coagulation, precipitation, solvent extraction, foam floatation, filtration and evaporation (Yavuz et al. 2006; Pan et al. 2010; Fu & Wang 2011; Lee et al. 2007). Most of these traditional methods suffer from demerits like increased operating costs, excess usage of chemicals and low efficiency. Several studies have focused on utilization of novel biomass based adsorbents using plant material (Al Rmailli et al. 2008; Rajamohan et al. 2014), algae (Plaza et al. 2011) and other natural sources for the removal of metals (Park et al. 2010). Ion exchange technology is proposed as an alternative for the remediation of heavy metal contaminated water. Ion exchange resins involve interchange of ions between two phases through a resin, which is a cross-linked polymer network. The advantage of this method lies with the insolubility of the resin which makes the separation easier by filtration (Alexandratos 2009). Studies on the removal of nickel and zinc from aqueous solutions by ion exchange resins have been conducted (Alyuz & Veli 2009; Franco et al. 2013). Boron removal using strong base anion-exchange resin, Dowex 2 × 8, was carried out in column experiments (Ennil Kose & Ozturk 2008). Gel resin containing sulfonate groups, Dowex 50 W, was successfully applied for the removal of copper, zinc, nickel, cadmium and lead ions (Pehlivan & Altun 2006). Research studies on equilibrium and kinetic modeling on heavy metal exchange using resins have been reported (Shek et al. 2009). In this experimental study, a strong acid cation resin, 001 × 7, has been investigated for its potential to remove mercury from aqueous solutions. The optimal conditions suitable for maximum mercury removal were identified through the parametric studies. Equilibrium experiments were conducted and the isotherm parameters were determined. Kinetic mechanism was elucidated using pseudo-second order model.

MATERIALS AND METHODS

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Chemicals

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Mercury solution was prepared by diluting aliquots of the stock solution which contains 1,000 mg/L of Hg (NO₃)₂.1/2 H₂O in double distilled water. The pH of the working solution was maintained using analytical reagent (AR) grade HCl, NaOH and buffer solutions (Merck, Germany). All other reagents used were of AR grade and obtained from Sigma Aldrich chemicals.

Activation of ion exchange resin

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The resin used in this study is a strong acid cationic resin −001 × 7 which contains sulphonic acid (−SO₃H) as the functional group. This cation exchange resin has moisture content in the range of 45–55% and was used for water softening applications. The particle size of the supplied resin was in the range of 0.42–1.2 mm in diameter. The resin was washed with HCl (1 M) and NaOH (1 M) for the removal of impurities and with deionized water (Millipore Milli-Q) repeatedly for several times and dried at 50 °C in a vacuum oven for 12 h. It was stored in a desiccator for further use.

Parametric studies

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Experimental data on the ion exchange system was obtained to optimize the process parameters in batch conditions. A sample of 100 ml of the metal solution was agitated in an orbital shaker at different experimental conditions for an equilibrium time of 180 min which was fixed using preliminary experiments. In the first set of experiments, the effect of pH was studied in the range of 2.0–9.0 at fixed initial mercury concentration of 100 mg/L. The adjustment of pH of the samples was done by adding either 0.1 N HCl or NaOH as the buffering agent. The influence of resin dosage on mercury removal efficiency and uptake was studied from 0.5 to 5.0 g/L at the optimal pH and an initial metal concentration of 100 mg/L. The effect of initial mercury concentrations on the percentage metal removal was studied in the range of 0–200 mg/L at pre-determined conditions of optimal pH and resin quantity. In order to identify the importance of homogeneity, the effect of agitation speed was studied in the range of 0–400 rpm. Samples were taken at predetermined intervals of time and the contents of the flasks were filtered using 45 μM Millipore membranes. The mercury concentration was estimated with the filtered samples and the filtrate was analysed for residual metal concentrations using atomic absorption spectrophotometer at an analytical wavelength, λ_max of 253.7 nm (Rajamohan et al. 2014). The mercury uptake was calculated by using the following Equation ():

1

where q is the amount of metal adsorbed by resin at any time, t (mg g⁻¹);

and

are the initial and final mercury concentrations (mg L⁻¹), respectively; V is the volume of sample solution (L) and M is the resin dosage (g).

The percentage mercury removal efficiency is defined by Equation ():

2

Equilibrium studies

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Langmuir and Freundlich isotherms (Equations () and () respectively) were employed to fit the experimental data.

3

4

where (mg/g) and (mg/L) are the amount of metal adsorbed on the resin and concentration of the metal at equilibrium, respectively; (mg/g) is the maximum adsorption capacity and (L/mg) is the Langmuir constant related to the free energy of adsorption, (mg/g) is the Freundlich constant which is an estimate of relative adsorption capacity of the resin and is the adsorption intensity.

Kinetic modeling

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Kinetic data are required for the modeling and design of the separation system. The Pseudo-second order model (Ho & Mckay 1999) can be represented in the following form:

5

where,

is the amount of metal adsorbed at equilibrium (mg/g),

is the initial sorption rate (mg/g min) and

(g/mg min) is the pseudo-second-order constant. The model constants are determined experimentally from the slope and intercept of plot

versus t.

RESULTS AND DISCUSSION

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Effect of pH

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The extent of hydrogen ion density in an aqueous solution is represented by the solution pH and the metal speciation dynamics is strongly influenced by pH. Figure 1 represents the effect of pH on the mercury removal efficiency by the resin 001 × 7 at operating conditions of 100 mg/L mercury concentration, resin dosage of 3.0 g/L and operating temperature of 308 K for an equilibrium time of 180 min. The removal efficiency increased from 28% to 78% when the pH increased from 2.0 to 5.0 followed by a flat region in the pH range of 5.0–6.0. When pH was increased above 6.0, the removal efficiency decreased. The optimal pH was identified as 5.0 and used in all experiments conducted later. Higher concentrations of hydrogen ions at low pH values provided a competitive effect for the exchange of metal ions with the resin. This phenomenon was attributed to reduced removal at low pH (Rajamohan et al. 2014). Ghodbane and Hamdaoui (18) have reported the occurrence of electrostatic repulsion at low pH values, 2.0–4.0, which prevented the exchange of Hg²⁺ and Hg (OH)⁺ ions. Precipitation could be reason for the decrease in efficiency at pH values greater than 6.0.

Figure 1

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Effect of pH on percentage mercury removal (t = 180 min, C₀ = 100 mg/L, M = 3.0 g/L).

Effect of resin dosage

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Ion exchange process requires availability of more contact area and resin dosage, a representative of surface area, was varied in the range of 0.5–5.0 g/L. The effect of resin dosage on mercury removal efficiency and uptake was evaluated. From Figure 2, it was inferred that the mercury removal efficiency increased linearly with increase in resin dosage from 0.5 to 3.0 g/L. But, the increase in efficiency was not proportionate in the range of 3.0–5.0 g/L and the slope of the line decreased. This effect was attributed to the availability of more exchange sites at higher resin dosages. Studies depending on surface area dependent methods like ion exchange and adsorption reported similar relationship (Pehlivan & Altun 2006; Bhattacharya et al. 2008; Moussavi & Barikbin 2010). Mercury uptake, representing the mass of metal removed per unit mass of resin, showed an inverse relation with the resin dosage. The increase in resin dosage from 0.5 to 5.0 g/L has led to the decrease in mercury uptake. This observation was related to the unsaturation of exchange sites and expected to occur at higher sorbent dosages, with fixed input metal loading. Moreover at higher sorbent-sorbate ratios, exchange was reported to occur superficially onto the surface sites (Rajamohan et al. 2014).

Figure 2

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Effect of resin dosage on % mercury removal and uptake (t = 180 min, C₀ = 100 mg/L, T = 308 K).

Effect of initial metal concentration

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The mercury concentrations in the industrial effluent vary depending on the process efficiency. In this set of experiments, the effect of initial mercury concentration was studied in the range of 0–100 mg/L at an optimal pH of 5.0, resin dosage of 3.0 g/L and operating temperature of 308 K. As shown in Figure 3, the removal efficiencies attained were higher at low mercury concentrations and a decrease in efficiency was noticed at higher mercury concentrations with a lowest removal efficiency of 50% at 200 mg/L mercury concentration. The rate of removal was observed to be high during the first 75 minutes of the contact time which was related to the easy accessibility of the surface exchange sites for the metal ions. Saturation of exchange sites was bound to occur during the progress of the ion exchange process (Pehlivan & Altun 2006). Due to the saturation effect and coverage of exchange sites, resistance to diffusion was comparatively higher. Studies on removal of nickel using 001 × 7 reported similar results (Rajamohan et al. 2014).

Figure 3

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Effect of initial mercury concentration on % mercury removal (t = 180 min, C₀ = 100 mg/L, T = 308 K).

Effect of agitation speed

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The homogeneity of a solution and the effective contact between metal ion and resin surface are related to the speed of agitation. The agitation speed was varied in the range 0–400 rpm and its effect on mercury uptake was presented in Figure 4. Under static conditions, the mercury uptake attained was very low and thus emphasized the need for shaking experiments. The mercury uptake increased from 23 to 58.4 mg/g for a corresponding increase in speed of 100–300 rpm. This observation was attributed to the maintenance of uniform concentration gradient, which is the driving force for exchange (Rajamohan et al. 2014). Reduction in the boundary layer thickness and enhancement of the external film transfer coefficient were reported to occur at higher agitation speeds (Ghodbane & Hamdaoui 2008). When the agitation speed was increased above 300 rpm, the mercury uptake decreased due to the creation of turbulent excess shearing force.

Figure 4

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Effect of speed of agitation on mercury uptake (t = 180 min, C₀ = 100 mg/L, T = 308 K).

Equilibrium studies

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The relationship between the quantity of metal removed per unit weight of resin and the amount of metal remaining in the solution system at constant temperature was represented through equilibrium isotherm plots. The equilibrium studies give valuable information about the distribution of the metal between solid and liquid phases at various equilibrium concentrations (Rangabhashiyam et al. 2014). Langmuir isotherm which is based on the assumptions of mono layer adsorption and adsorption at specific homogeneous sites was applied to the experimental data. The linearized form of Langmuir isotherm was plotted between versus ⁠. and shown in Figure 5. The Langmuir isotherm constants, and ⁠, were determined from the slope and intercept of the lines representing three different operating temperatures of 303, 308 and 313 K. The Langmuir isotherm fitted well to the data with high values of R² (≥0.98). The isotherm constants were estimated and given in Table 1. The maximum uptake capacity, _, was found to increase with temperature. The essential features of Langmuir isotherm was represented by a parameter, called separation factor and given by Eq.6.

6

Table 1

Isotherm constants for removal of mercury

T	Langmuir constants			Freundlich constants
K	(mg/g)	(L/mg)	R²	n	(L/g)	R²
303	96.154	0.056	0.980	4.000	3.026	0.967
308	110.619	0.070	0.980	3.817	3.115	0.923
313	112.360	0.078	0.981	3.671	3.430	0.962

T	Langmuir constants			Freundlich constants
K	(mg/g)	(L/mg)	R²	n	(L/g)	R²
303	96.154	0.056	0.980	4.000	3.026	0.967
308	110.619	0.070	0.980	3.817	3.115	0.923
313	112.360	0.078	0.981	3.671	3.430	0.962

Figure 5

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Langmuir isotherm plot for removal of mercury.

All the values of

calculated were within the range of 0–1.0, which confirmed the nature of the process as favorable. Freundlich isotherm is applied for multilayer sorption mechanism and assumes heterogeneity in surface of the resin or sorbent. The equilibrium data on mercury removal was verified using Freundlich isotherm plot and shown in Figure 6. This isotherm proved to be inferior to Langmuir isotherm in goodness of fit and the values of regression coefficient were found to be comparatively lesser. The Freundlich isotherm constants were estimated and tabulated in Table 1. Removal studies on copper, nickel, zinc, cadmium and lead ions using Dowex 50 W resin reported better fit to Langmuir isotherm (Pehlivan & Altun 2006).

Figure 6

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Freundlich isotherm plot for removal of mercury.

Kinetic studies

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The suitability of the experimental data to Pseudo second order model was tested by plotting t versus t/q and presented in Figures 7–9. The slope and intercept of the linearized forms were used to determine the kinetic constants of the pseudo second-order model, and ⁠. Pseudo second-order model represented the kinetic data very well with comparatively higher values of R² (>0.97). Thus, the exchange mechanism was proved as chemisorption in nature. The chemisorption process involves the valence forces in the electron exchange process between the resin and metal (Rajamohan et al. 2014). Increase in rate of mass transfer with increase in concentration gradient was identified as the reason for this observation with rate constant (Moussavi & Khosravi 2010). The pseudo second order rate constant values were presented in Table 2 at different metal concentrations. From Table 2, it was observed that both the concentration of metal ions and temperature influenced the rate of removal.

Table 2

Pseudo-second order model kinetic constants for the removal of mercury

T (K)	(mg/L)	(mg/g)	(g mg⁻¹ min⁻¹)	R²
303	50	17.04	9.47	0.995
	100	38.91	2.24	0.980
	150	54.64	1.08	0.979
	200	64.94	0.81	0.972
308	50	49.50	50.5	0.994
	100	59.17	40.2	0.999
	150	62.89	32.3	0.998
	200	71.43	24.8	0.999
313	50	57.14	67.4	0.996
	100	69.93	52.2	0.999
	150	74.07	43.7	0.999
	200	76.92	36.7	0.999

T (K)	(mg/L)	(mg/g)	(g mg⁻¹ min⁻¹)	R²
303	50	17.04	9.47	0.995
	100	38.91	2.24	0.980
	150	54.64	1.08	0.979
	200	64.94	0.81	0.972
308	50	49.50	50.5	0.994
	100	59.17	40.2	0.999
	150	62.89	32.3	0.998
	200	71.43	24.8	0.999
313	50	57.14	67.4	0.996
	100	69.93	52.2	0.999
	150	74.07	43.7	0.999
	200	76.92	36.7	0.999

Figure 7

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Pseudo-second-order kinetic model plot for the sorption of mercury at 303 K.

Figure 8

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Pseudo-second-order kinetic model plot for the sorption of mercury at 308 K.

Figure 9

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Pseudo-second-order kinetic model plot for the sorption of mercury at 313 K.

CONCLUSIONS

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The results obtained in this study demonstrated the potential of strong acid cation resin 001 × 7 for the removal of mercury from its aqueous solution. The optimum pH and resin dosage for mercury removal were found to be 5.0 and 3.0 g/L respectively. The removal efficiency decreased with increase in mercury concentration and sorbent dosage influenced the uptake in a negative pattern. Monolayer attachment of metal ions was proved to be suitable through Langmuir isotherm. The maximum uptake values increased from 96.154 to 112.360 mg/g when the temperature increased from 303 to 313 K. Kinetic experiments were found to be represented by Pseudo second order model and the rate constants increased with temperature for fixed initial metal concentrations. Thus, it can be concluded that the resin 001 × 7 proved to be an efficient choice for the removal of mercury over a wide range of experimental conditions.