(329b) Adsorption of Mercury on Coarse Loamy Mixed Hyperthermic Fluventic Hyplustept Soil of Punjab, North-West India

Adsorption is an important process that affects the mobility and fate of contaminants in soil. An understanding of this process and its mechanism is crucial to the assessment of potential groundwater contamination and to the development of cost-effective soil remediation strategies. Generally, soil properties, environmental conditions and the amount of contaminants in soils, affect adsorption. Remediation processes, usually, also follow the same principle and the most common techniques are based on adsorption and precipitation phenomena. Studies on animals have reported effects such as alterations in testicular tissue, increased resorption rates, and abnormalities in development. The major effect from chronic exposure to inorganic mercury is kidney damage. Therefore, the present investigation was carried out to understand whether an alkaline soil could be used as effective adsorbent material for removal of mercury from contaminant water. For this purpose, a Coarse Loamy Mixed Hyperthermic Fluventic Hyplustept Soil of Punjab, North-west
India was collected. The soil has pH (H₂O) 8.3 and EC 8.7 dSm^-1 for a 1:2 extract. It has cation exchange capacity 6.8 cmol (+) kg^-1, organic C 2.7 g kg^-1 and CaCO₃ 3.8 %_. The soil contained 54 % sand, 28 % silt and 18 % clay. For mercury adsorption, 0.5 gm soil in quadruplicate was equilibrated with 100 ml of 0.01 M NaNO₃ solution containing a range of Hg concentration (up to 45µg Hg ml^-1) for 20 hours in 200 ml plastic bottles. Different mercury concentration solutions were prepared from HgNO₃ of analytical reagent salt and their pH was adjusted to 7.0 by addition of 0.01 M HCl or NaOH.

            Adsorption isotherms were determined by incubating soil solution suspension at two different temperature of 278 and 318 ⁰K. After equilibration period, soil solution suspensions were separated through filtration from Whatman no 42 filter paper. Mercury in the filtrates solution was determined on GBC make Atomic absorption spectrometer (Model Avanta PM) using cold vapours technique on hydride generation assembly. The mercury adsorption was computed from the difference between initial and final concentrations. The Langmuir equation (C/x = 1/ kb + C/b) was used to interpret the equilibrium adsorption data. Where C is the equilibrium Hg concentration (µg Hg cm^-3), x is the adsorption maxima (µg Hg g^-1 soil) and k is a constant related to the energy of adsorption (cm³ µg^-1 Hg). The differential isosteric hest of adsorption, Δ H was obtained by collecting mercury adsorption data for the same soil at  278 and 318⁰ K  and applying the Clausiues Clapeyron equation to the system. For a given amount of mercury adsorbed, Ø,              Log [C₂/C₁] = -Δ H/ 2.303 R [1/T₁-1/T₂], Where C₁ and C₂ are the equilibrium Hg concentration (µg Hg/cm³) at temperature T₁ (280⁰ K) and T₂ (305⁰ K) respectively and R is the molar constant (1.985 E^-3 Kcal/mol.) Adsorption of mercury on a course loamy mixed hyperthermic Fluventic Hyplustept soil determined at 278 and 318 ⁰K showed single linear Langmuir plots (Fig.1). The adsorption maxima for Hg adsorption by soils remained constant (5000 µg g^-1 soil) at both temperature of equilibration of soil: solution suspension at 278 ⁰ K and 318 ⁰K. The bonding energy of Hg adsorption by soils suspension was higher (0.0526 cm³ µg^-1 Hg) at 278 ⁰ K than (0.08695 cm³ µg^-1 Hg) at 318 ⁰K.  

            The heat released or absorbed during adsorption of mercury is the differential molar heat of adsorption,   Δ H. The value of Δ H between 278 and 318 ⁰K were computed using equation (2) and are plotted as a function of mercury adsorbed (Fig. 2). The Fig. 2 show that the adsorption process is energy consuming (endothermic), within Δ H varying between 6.26 and 0.36 kcal/mole for a constant surface, Ø, of 1100 to 2500 µg of Hg gm^-1 soil. This shows that at adsorption process was predominant due to chemisorptions or precipitation of mercury minerals on the surface of soil matrix. The differential isosteric heat of adsorption for Hg adsorption markedly decline from 6.26 to 1.44 kcalmole^-1 as the amount of Hg adsorbed decreased from 1100 to1200 µg Hg g^-1 soil.As the amount of Hg adsorbed by soil enhanced from 1200 to 2500 µg Hg g^-1 soil, the differential isosteric heat of adsorbed change slightly from 1.44 to 0.36 kcalmole^-1. The higher magnitude of Δ H below 1100 µg Hg g^-1 soil might be due to the interaction of Hg²⁺cations with the hydrous oxide site of iron and manganese minerals  or clay particles in  the soil. When the amount of  Hg²⁺cations adsorbed enhanced from 1100 to 3000 µg g^-1 soil, there was very little decline of Δ H (1.44-0.58 kcalmole^-1). This might be due to interaction of adsorbate to adsorbate i.e. between the mercury atoms rather than with the surface sites of soils with mercury atoms. Thus, the surface chemistry of mercury adsorption on the surfaces of soils and soil minerals is different from other heavy metal cations. In the present study, pair interaction between mercury atoms are most likely to dominate on the surface of the adsorbate. Hg₂ is a weekly bound molecules, which require lesser binding energy and eventually very little amount of Δ H is involved during mercury adsorption due to pairing process.  

To understand the aqueous species and solid phase of Hg as a function of pe-pH, the Pourbaix diagrams (Fig.3a and b) were drawn for soil- solution containing equilibrated 0.14mM Hg, 30 µM Cl^-1, 0.15mM CO₃^2- and 0.33 mM SO₄^2-. The mercury in soil solution predominantly exits as Hg, Hg(OH)₂  and HgS(SH)^-1 in aqueous forms (Fig. 3a). In a pH range of 5 to 9 and at highly oxidized condition, the Hg(OH)₂ is predominant aqueous species. When the pH moves below 5 and at highly oxidized condition (pe ³10), HgCl₂ becomes predominant aqueous species. At elevated oxidized condition between pH 4 to 9, the HgCO₃(c) was identified as dominant solid phase (Fig. 3b). In moderately reduced to oxidize condition adsorption of mercury is mainly due to pairing of mercury atoms. In reduced environments, removal of mercury from water is mainly due to precipitation of HgS (c). It is conclusively evident from this investigation that the surface properties of soils available at the sites having elevated mercury ground water can be utilized for remediation   work.