INTRODUCTION:

Mercury, being a very toxic heavy element, affects the environment adversely and due to this reason, it is very important from environmental pollution and management point of view. Mercury is hazardous for human beings and hence the desirability of its removal from effluent water for clean environment^1-3.

During the last few decades a large number of metal oxides have been explored for their use in adsorption and for various separations of analytical and radiochemical importance^4-7. The interest in these materials has grown mainly due to their extra stability towards ionizing radiations and higher temperatures in comparison to their organic counterparts^8-10. Moreover, some metal oxides show high selectivity for certain ions, offering a convenient means for many difficult separations. The adsorption process has been found to be useful and popular due to its low maintenance costs, high efficiency and ease of operation. Adsorption of mercury on soil (Ahmed and Qureshi¹¹; Del Debbio¹²; Borrow and Cox¹³), silica gel (Koejan¹⁴) and activated carbon (Shirakashi et al.¹⁵; Ma et al.¹⁶) has been described under various related parameters. Hydrous ferric oxide (HFO) and hydrous tungsten oxide (HTO) are most promising adsorbents.

The objective of the present study was to investigate the applicability of HFO and HTO for the treatment of aqueous solutions containing Hg (II). The effect of various parameters such as concentration, temperature and irradiation had been examined. The kinetics and adsorption isotherms during the process have also been discussed and thermodynamic parameters deduced to help in understanding the uptake process.

EXPERIMENTAL:

Synthesis of hydrous ferric oxide:

The HFO was synthesized by slow addition of 1.0 M ammonium hydroxide to a solution of ferric chloride at pH 7.5 under constant stirring, as reported by McKenzie¹⁷. The precipitate thus formed was filtered and washed with distilled water. The precipitate was then mixed with hydrochloric acid solution at pH 4.0 and aged at 330K. After 2h of aging, the synthesized product was thoroughly washed with double distilled water till it was free from chloride ions. The solid was dried in a hot air over at 315K and then sieved to obtain particles of 120-170 mesh size, before using it as absorbent.

Synthesis of hydrous tungsten oxide:

The HTO was synthesized by slow drop wise addition of warm saturated solution of sodium tungstate to 2-3 times its volume of boiling concentrated hydrochloric acid followed by additional heating (for 1 h) on steam bath as reported by Brauer George¹⁸. The precipitate was allowed to settle, washed with 5% ammonium nitrate solution until no further Cl^- detected in the filtrate and dried at 393 K and then sieved to obtain particles of 120-170 mesh size, before using as absorbent.

Infrared and X-Ray Diffraction Analysis:

Infrared spectra of synthesized HFO and HTO in the range 200-4000 cm^-1were measured by a KBr disc method using Jasco FT/IR5300 spectrophotometer. The synthesized compound was also characterized by X-ray diffraction (XRD) technique using a Rigaku, X-ray diffractometer; and data resembled as per ASTM File no. 33-664, pp. 481 for HFO and File no. 18-1418 pp.1208 for HTO.

Sorption Measurements:

Mercury (II) as its nitrate salt was used and stock solution (1.0 mol dm^-3) of metal ion was prepared in double distilled water. The solution was further standardized via the standard method of Flaschka¹⁹ and then diluted to obtain desired experimental concentrations (10^-4–10^-8 mol dm^-3). The radioactive mercury (²⁰³Hg, t_1/2 = 47 d) as its respective nitrate in dilute HNO₃ (ca 166.5 x 10⁶ Bq) was obtained from the Board of Radiation and Isotope Technology (BRIT), Mumbai (India). A very small amount of this radionuclide was used to label the Hg (II) adsorptive solution to obtain a measurable radioactivity of minute aliquots of withdrawn samples from bulk.

The sorption experiments were performed by stirring, at regular intervals, and equilibrating 0.1000 g of HFO and HTO with 10.0 cm³ of labeled adsorptive solution [Hg (II)] of desired concentration. The equilibrated solution was centrifuged for phase separation and then supernatant solution was analyzed for its b-activity measurements using an end-window GM-counter (Nucleonix, Hyderabad, India). Radioactivities of some samples were also checked for their g-activity using a Multi Channel Analyzer (Nucleonix, Hyderabad, India). Procedures for estimation of the amount adsorbed and evaluation of other parameters were identical to those given earlier²⁰.

Irradiation of adsorbents:

HFO and HTO were irradiated with neutrons and g-rays from a 11.1 x 10⁹Bq (Ra - Be) neutron source having an integral neutron flux of 3.9 x 10⁶ n cm^-2 s^-1, associated with a nominal g-dose of ca 1.72 Gy h^-1(total dose: from 41.28 Gy to123.84 Gy for 24-72 h of irradiation).Irradiated absorbents were then employed along with unirradiated materials for the uptake of Hg (II) from aqueous solutions.

RESULTS AND DISCUSSION:

Effect of metal ion concentration:

Concentration effect for the adsorption of Hg (II) on HFO and HTO was carried out by varying the adsorptive concentrations (10^-4- 10^-8 mol dm^-3) at 303 K and results of the uptake of Hg (II) ions are shown in Figs.1 and 2. It is observed that initially a fast uptake of metal ions occurred, which became slower with the lapse of time and an apparent equilibria between the two phases achieved within ca 65 min for Hg-HFO and ca 95 min for Hg-HTO of contact time. No further uptake was observed even after 24 h of contact. Moreover, the smooth and continuous curves leading to saturation for the uptake of Hg (II) ions over HFO and HTO infer about the adsorption of metal ions occurring in a single step and not accompanied by any complexity. The amount of Hg (II) adsorbed at equilibrium increases from 0.650 x 10^-9 to 0.415 x 10^-5 mol g^-1 (for Hg-HFO) and from 0.950 x 10^-9 to 0.735 x 10^-5 mol g^-1(for Hg-HTO) with an increase in adsorptive concentration from 10^-8 to 10^-4 mol dm^-3. The relative change in the uptake, i.e., percentage adsorption, increased from 41.5 to 65.0% (for Hg-HFO) and from 73.5 to 95.0% (for Hg-HTO) for Hg (II) with the increase in the bulk dilution from 10^-4to 10^-8 mol dm^-3. This increase in the percentage adsorption is explicable on the basis of the fact that a relatively smaller number of adsorptive species would be available at higher dilution for deposition on an equal number of surface sites of adsorbents²¹.

Figure 1. Variation of the adsorption of Hg (II) on hydrous ferric oxide at various concentration of adsorptive solution (Temperature: 303K; pH: 8.68).

Figure 2. Variation of the adsorption of Hg (II) on hydrous tungsten oxide at various concentration of adsorptive solution (Temperature: 303K; pH: 4.42).

Equilibrium modeling:

The concentration dependence data were further utilized in equilibrium modeling of the removal process by using the Freundlich and Langmuir equations. It has been observed that the concentration data fitted well by Freundlich equation (1) rather than the Langmuir equation.

Here a_e and C_e are the amounts adsorbed (mol g^-1) at equilibrium and equilibrium bulk concentration (mol dm^-3) respectively and 1/n and K are the Freundlich constants which correspond to adsorption intensity and adsorption capacity, respectively. In order to find out these constants a plot has been drawn between log a_evs log C_e (cf Fig. 3) and straight lines were found for both the systems. These linear plots confirm about the monolayer coverage of Hg (II) at the surface of both adsorbents²². The value of 1/n and K were 0.953 and 2.81 x 10^-2mol g^-1 (for Hg-HFO) and 0.842 and 4.46 x 10^-2mol g^-1(for Hg-HTO) system, respectively. The fractional values of 1/n (0< 1/n<1) obtained for both the systems are considered to be due to the heterogeneous surface structure of adsorbents with an exponential distribution of surface active sites²³. The higher numerical values of K for both the systems again confirm the significant affinity of Hg (II) for HFO and HTO.

Figure 3. Freundlich adsorption isotherm of Hg (II) on hydrous ferric oxide and hydrous tungsten oxide at 303 K.

Effect of temperature:

The effect of solution temperature varying from 303 to 333 K in steps of l0 K on adsorption of Hg (II) ions by HFO and HTO has been investigated; the initial concentration of Hg (II) being kept at 1.0 x 10^-5 mol dm^-3 at pH 8.68 (for Hg - HFO) and at pH 4.42 (for Hg - HTO) systems. It was observed that with the increase in temperature the uptake of metal ion increased from 0.465 x 10^-6to 0.620 x 10^-6 mol g^-1 (for Hg-HFO) and from 0.815 x 10^-6 to 0.935 x 10^-6 mol g^-1 (for Hg-HTO) systems at equilibrium (cf Table 2). This increase in adsorption of metal ions may be either due to acceleration of slow adsorption steps or due to creation of some new active sites²³ or to transport against a concentration gradient, and/or diffusion controlled transport across the energy barrier²⁴.

A kinetic study for the uptake of Hg (II) ions over HFO and HTO has also been worked out, which follows the first order rate law obeying Lagergren equation:

log (a_e-a_t) = log a_e- k₁. t/2.303 (2)

Where, a_eanda_tare the amounts adsorbed at equilibrium and at contact time intervals t and k₁ is the adsorption rate constant. The values of adsorption rate constants at different temperatures have been estimated from the slopes of straight lines obtained from log (a_e- a_t) vs time and the values are listed in Table 1. These values for both systems increase with the increase in temperature, which is in conformity to expectations as adsorption increases with the increase in temperature for non-physical type adsorption.

The Arrhenius plots of log k₁ vs 1/T (Fig. 4) gave the activation energies for the adsorption process of Hg (II) on HFO as 9.11±0.04 and on HTO as 10.46 ±0.08 kJmol^-1. These low values of activation energies for both the systems indicates that the process of uptake can occur even under normal conditions of temperature and pressure and also indicate about the strong force of attraction operating during the adsorption.

Figure 4. Variation of log k₁ vs 1/T for adsorption of Hg (II) on hydrous ferric oxide and hydrous tungsten oxide [Initial concentration of Hg (II): 1.0 x 10^-5 mol dm^-3; pH : 8.68 for HFO, pH : 4.42 for HTO )].

The change in standard enthalpy ΔH⁰during the adsorption process has been evaluated using van't Hoff equation (3);

log K_D= + Constant (3)

Where, K_D,ΔH⁰, R and T have their usual meaning. The values of ΔH⁰ (at 303K) found from the slopes of straight lines (Fig. 5) obtained by plotting log K_Dvs 1/T are found to be 17.72 ±0.041 kJmol^-1 for Hg-HFO and 10.06 ±0.036 kJmol^-1 for Hg-HTO systems respectively. The positive value of ΔH⁰confirms the endothermic nature of the adsorption process²⁵ and the numerical value indicates an ion exchange type mechanism²⁶for the uptake.

Figure 5. Variation of log K_D and log k₁ vs 1/T for adsorption of Hg (II) on hydrous ferric oxide and hydrous tungsten oxide [Initial concentration of Hg (II): 1.0 x 10^-5 mol dm^-3; pH : 8.68 for HFO, pH : 4.42 for HTO )].

Desorption study:

HFO and HTO with preadsorbed Hg (II) was washed with double distilled water to ensure the removal of adhering species and subsequently the adsorbent was dried in an electric oven at 383K. The desorption of pre-adsorbed Hg (II) on these solids was studied in Hg (II) solution (1.0 x 10^-5mol dm^-3)at different temperatures (i.e., 303 to 333 K). The very low values of percentage desorption at different temperatures (cf Table 2) indicates that the desorption process is almost independent of temperature. Thus a low value of desorption, unaffected by increase of temperature shows that the process of adsorption for Hg (II) is irreversible in nature. It also indicates that a major part of Hg (II) was probably bound to the HFO or HTO surface through strong interaction and converted to a final stable adsorption phase.

Table 2: Temperature dependence of equilibrium adsorption/desorption of Hg (II) ions on HFO and HTO systems, [initial adsorptive concentration = 1.0x10^-5 mol dm^-3, pH = 8.68 (for HFO) and pH = 4.42 (for HTO)].

Temp. (K)	Percentage adsorbed at equilibrium (%)		Desorption in equilibrium bulk concentration (%)
Temp. (K)	Hg – HFO	Hg - HTO	Hg – HFO	Hg – HTO
303	46.5	81.5	2.6	7.6
313	52.1	86.4	2.6	8.0
323	57.2	90.9	2.9	8.2
333	62.0	93.5	3.3	8.6

Effect of pH:

Mercury (II) occurs with its stable +2 oxidation state and exists with its various forms on varying the pH of solution. The important species in aqueous solutions are Hg(NO₃)₂ (at lower pHrange) and Hg(OH)NO₃ (at moderate pHrange)^27-28, and such species would greatly affect the adsorption behavior of Hg(II) on solid sorbent surfaces on variation of the solution pH.

In order to find out the mechanism involved at the solid/solution interface, the study has been extended for the adsorption of Hg (II) on the surface of HFO/ or HTO as a function of adsorptive pHat a constant temperature (303 K) and using Hg (II) solution of 1.0 x 10^-5 mol dm^-3.The increase in the pH from 3.4 to 6.4 results in sharp increase in Hg (II) uptake. Similar type of results observed for HTO and beyond pH6.4 to pH 8.8^29-30. On the other hand HFO has a reported P_ZPC at 8.4²⁹. From pH 4.6 to pH 8.6 the uptake of Hg (II) ions is low but beyond pH8.6 to 10.6 there is a sharp increase in the uptake, because for the pHbeyond P_ZPC the solid adsorbent is negatively charged, and hence attracts Hg (II) ions.

Effect of added cations:

The adsorption capacity of any adsorbent is highly dependent upon the individual nature of adsorbent and adsorbate species and the pHof the bulk solution. In addition, adsorption depends more or less significantly on the characteristics of added species in the bulk of the system.

The effect of six fold concentrations (6.0 x 10^-5 mol dm^-3) of some cations (viz. Na⁺, K⁺, Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺) towards the uptake behavior of HFO/ or HTO for the removal of Hg (II) through independent experiments (Table 3) clearly indicates that the presence of Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺do hamper the uptake of Hg (II) ions to some extent. However, the presence of Na⁺ and K⁺ did not affect the removal of toxic metal ion markedly. The suppression in the adsorption of Hg (II) ions on the HFO/ or HTO surface in the presence of several cations is explicable on the basis of competition of metal ions for the solid surface²¹.

Table 3 Influence of added cations (six fold) on the adsorption of Hg (II) on HFO and HTO, [initial concentration of adsorptive solution 1.0x 10^-5 mol dm^-3, pH = 8.68 (for HFO) and pH = 4.42 (for HTO) systems].

Added Cation	Amount of metal ion adsorbed± 3 s (mol g^-1x 10^-6)		Percentage adsorbed
Added Cation	Hg - HFO	Hg – HTO	Hg –HFO	Hg - HTO
Nil	0.463 ± 0.002	0.816 ± 0.007	46.3	81.6
Na⁺	0.442 ± 0.004	0.808 ± 0.008	44.2	80.8
K⁺	0.447 ± 0.005	0.814 ± 0.007	41.7	81.4
Ca²⁺	0.410 ± 0.006	0.802 ± 0.005	41.0	80.2
Mg²⁺	0.383 ± 0.006	0.774 ± 0.008	38.3	77.4
Sr²⁺	0.375 ± 0.004	0.761 ± 0.009	37.5	76.1
Ba²⁺	0.452 ± 0.003	0.796 ± 0.008	45.2	79.6

Effect of irradiation:

The effect of irradiation on radiation stability of HFO and HTO towards removal behavior of Hg (II) has been assessed by carrying out the experiments using a 11.1 x 10⁹Bq (Ra-Be) neutron source having an integral neutron flux of 3.9 x 10⁶ n cm^-2 s^-1 and associated with a concomitant ¡ - dose rate of 1.72 Gy h^-1 (total dose: from 41.28 Gy to123.84 Gy for 24-72 h of irradiation). It is evident from the results (Table 4) that irradiation [upto the doses used] shows some sintering effect but do not appreciably affect the adsorption of Hg (II) ions. Results of decrease in the adsorption of Sr²⁺ ions on irradiated alumina surface was reported by Newton³¹ and the present results are also in line to those of Leavy³² where no appreciable decrease in the uptake was reported. Decrease in the uptake of methyl red on irradiated silica and alumina was also reported by Zofia³³.

Table 4: Effect of irradiation of HFO and HTO by neutron (Ra-Be) source on the adsorption of Hg(II) ions, [initial concentration of adsorptive solution 1.0x 10^-5 mol dm^-3].

Time of irradiation (h)	Total Dose (Gy)	Amount of metal ion adsorbed (mol g^-1x 10^-6)
Time of irradiation (h)	Total Dose (Gy)	Hg – HFO	Hg - HTO
Nil	1.72	0.465	0.816
24	41.28	0.412	0.750
48	82.56	0.383	0.645
72	123.84	0.346	0.615

Values of 3 s varies between ± 0.003 to ± 0.006

CONCLUSION:

HFO and HTO are found to be effective in rapid and efficient removal of micro concentrations of Hg (II) toxic ions from aqueous solutions. The adsorption process follows Freundlich isotherm with endothermic/irreversible nature. The uptake of metal ions is affected to varying degrees by adding diverse ions to the bulk solution prior to the adsorption process. The adsorbent shows good radiation stability towards ionizing radiations and may have potential use in radiotoxic waste management.

ACKNOWLEDGEMENT:

I wish to thank the Head of Department of Chemistry, BHU. Varanasi, for providing necessary facilities

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Received on 18.10.2009 Modified on 09.01.2010

Asian J. Research Chem. 3(3): July- Sept. 2010; Page 591-595

Temperature (K)	Distribution Coefficient K_D(cm³g^-1)		Rate Constant k₁ (min^-1)x10²		Enthalpy Change (DH⁰) kJmol^-1		*Activation energy (E_a)* kJmol^-1
Temperature (K)	Hg - HFO	Hg - HTO	Hg - HFO	Hg - HTO	Hg - HFO	Hg - HTO	Hg - HFO	Hg -HTO
303	86.23	432.02	2.36 ± 0.03	1.62 ± 0.02	17.72 ±0.041	10.06 ± 0.036	9.11 ± 0.042	10.46 ± 0.084
313	107.96	655.54	2.64 ± 0.05	1.85 ± 0.03
323	133.84	987.80	2.95 ±0.03	2.00 ± 0.06
333	163.41	1417.14	3.36 ±0.04	2.30 ± 0.06