(329c) Adsorption of Nickel on Coarse Loamy Mixed Hyperthermic Fluventic Haplustept Soil of Punjab, North-West India

Abstract: Nickel (Ni) release into the environment has been increasing continuously as a result of man's industrial activities and posses a significant threat to the environment and public health because of its toxicity, bioaccumulation in the food chain and persistence in nature. Nickel input in the environment is mainly derived from phosphate fertilizers, pesticides, bio-solid sources, nickel electroplating, producing & using nickel catalysts, fabricating parts and structures by welding, flame spraying, cutting, grinding, and polishing of nickel containing alloys, manufacturing nickel cadmium batteries, constructing nickel molds in glass bottle factories and spraying nickel containing paints. Nickel causes gastrointestinal irritation and lung cancer. Adsorption reactions occur at the solid/liquid interface and are the most important mechanisms for controlling the activity of metal ions in soil solution. In a complex system with amphoteric behavior, the comprehension of the mobility, availability and fate of pollutants in the soil system is crucial for the prediction of the environmental consequences and for development of prevention/remediation strategies. The most common remediation techniques are based on adsorption and precipitation phenomena. The present investigation was undertaken to employ alkaline soil of north-west
India as a cheapest adsorbent to remove nickel from the water bodies having elevated nickel content.  The soil collected from the region for Ni adsorption experiment was coarse loamy mixed hyperthermic Fluventic Hyplustept. 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 gkg^-1 and CaCO₃ 3.8 %_. The soil contains 54 % sand, 28 % silt and 18 % clay. For Ni adsorption, 0.5 gm soil in quadruplicate was equilibrated for 24 hours in 200 ml plastic bottles containing 100 ml of 0.01 M NaNO₃ solution with varying Ni concentration up to 50 µg Ni ml^-1. Different Nickel concentration solutions were prepared from Ni(NO₃)₂ 6H₂O analytical reagent salts 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 280 and 318 ⁰K. After equilibration period, soil solution suspensions were separated through filtration. Nickel in the filtrates solution was determined on atomic adsorption spectrometer. Nickel (II) 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 Ni concentration (µg Ni cm^-3), b is the adsorption maxima (µg Ni g^-1 soil) and k is a constant related to the energy of adsorption (cm³ µg^-1 Ni). The differential isosteric hest of adsorption,  Δ H was obtained by collecting nickel adsorption data for the same soil at  280 and 318⁰ K  and applying the Clausiues Clapeyron equation to the system. For a given amount of Nickel adsorbed, Ø, Log [C₂/C₁] = -Δ H/ 2.303 R [1/T₁-1/T₂], Where C₁ and C₂ are the equilibrium Ni concentration (µg Ni cm^-3) at temperature T₁ (280⁰ K) and T₂ (318⁰ K) respectively and R is the molar constant (1.985 Kcal mol^-1). Adsorption of Ni on a coarse loamy mixed hyperthermic soil determined at 280 and 318 ⁰K showed one break in the slope of Langmuir plots (Fig.1). Thus Nickel adsorption on this soil is described by two regions Langmuir isotherm equation, i.e. the plots showed two distinct linear portions. In the Langmuir plot for the region I and II, the   bonding energy of nickel adsorption on soil increased with temperature from 280 to 318 ⁰K (Table1). The bonding energy for Ni adsorption for region I enhanced from 0.78 to 1.00 cm³ µg^-1 Ni and for region II enhanced from 0.04 to 0.06 cm³ µg^-1Ni with increase in temperature of the equilibrated soil solution suspension from 280 to 318 ⁰K. However, the bonding energy for Ni adsorption onto soil was higher for region I than region II. In region I of Langmuir plot the adsorption maxima for Ni adsorption enhanced from 1429 to 1667 µg g^-1 soil with increased in temperature from 280 and 318 ⁰K. However, the adsorption maxima of 5000 µg Ni g^-1of soil in the region II remained unaffected by temperature. The heat released or absorbed during adsorption of Nickel is the differential molar heat of adsorption, Δ H. The value of Δ H between 280 and 318 ⁰K were computed using Clausiues Clapeyron equation and are plotted as a function of Nickel adsorption (Fig. 2). The Fig. 2 shows that the adsorption process is energy consuming (endothermic), within Δ H varying between 0.52 and 0.99 kcalmole^-1 for a constant surface, Ø, of 1000 to 2000 µg of Ni gm^-1 soil. Thus it evident that adsorption process was predominant due to chemisorptions or precipitation of nickel compounds on the surface of soil matrix. It is evident from this investigation that the surface properties of soils available near the polluted water bodies can be utilized for remediation   work of the nickel pollution. Mole fraction diagram (Fig.3) illustrate the aqueous species of Nickel (II) in soil solution suspension of 0.01 M NaNO₃ after equilibration for 24 hours containing 0.69 mM total Ni (II), 30 mM Cl-, 0.15mM CO₃^2- and 0.33 mM SO₄^2- . Ni²⁺ aqueous species is predominantly found in equilibrated solution below pH 7.0 When the pH is increased above 7.0, Ni²⁺ species is precipitated in the form of Ni(OH)₂ (c) compound and predominantly occurred above pH 7.50. Nickel carbonate (NiCO₃) in aqueous form was present between pH 6 to 8 and its maximum amount about 10 percent found at pH 7.0, however, its mole fraction decline with increase or decrease in pH from 7.00. The results of the present investigation suggest that local soil could be employed for removal of Ni from contaminated water bodies.

Table 1. Langmuir parameters for Nickel adsorption on coarse loamy used mixed hyperthermic Fluventic Hyplustept soil

k = bonding energy, cm³ µg^-1;  b = adsorption maxima, µg g^-1 soil