(11c) Adsorption of Arsenate on Course Loamy Mixed Hyperthermic Fluventic Haplustept Soil of Punjab, North-West India

Arsenic (As) is a toxic element for animals including humans. Agricultural drainage of irrigated waters from soils especially in arid region of south-western districts of Punjab, north-west India elevated arsenic concentration in shallow alluvial aquifer water.  Long term intake of water with high As concentration causes serious diseases such as cancer, degenerative effects on circulatory body system and neurotoxicity. The World Health Organization (WHO) gave As limit for safe drinking water is 10 µgL^-1 (WHO, Guidelines for drinking-water quality, 1993) while in India the limit is 50 µgL^-1. Large rural and urban people in south-western areas of Punjab are dependent on groundwater that exceeds also the Indian limit. Thus the symptoms related to arsenic related diseases especially cancers are very common in the local populations. Immobilizations of arsenic in the environment occur through adsorption and precipitation of low solubility salts on the surface of soils and sediments. Remediation processes will also follow the same principles and the most common techniques are based on adsorption and precipitation phenomena. The objective of the present investigation is to elucidate/ show that soils available around rural settlements in the affected areas could be used as adsorbent with some locally available cheap materials for removal of arsenic oxyanions from shallow alluvial aquifers waters to safe limits before it can be used for drinking by humans and domestic animals.  The soils collected from the region for arsenates adsorption experiment was course loamy mixed hyperthermic Fluventic Haplustept. 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. Iron fillings were also collected from a disposal waste of cutting and turners industry located near by site and then washed with distilled water to remove dust particles, air dried and stored in polyethylene bags for subsequent used in the present investigation. For arsenate adsorption, 2.5 gm soil in duplicate was equilibrated for 7 days in 200 ml plastic bottles containing 50 ml of 0.01 M NaCl solution with varying As concentration up to 250 µg As ml^-1. Different arsenic concentration solutions were prepared from Na₂HAsO₄. 7H₂O analytical reagent salts and their pH were adjusted to 8.3 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 305 ⁰K. For remediation of arsenic from soil solution suspension of varying As concentration, 100 mg of iron filling were also added in the soil and then equilibrated at 305 ⁰K for the same period. After equilibration period, soil solution suspensions were separated through filtration. Arsenic in the filtrates solution was determined on hydride generation atomic adsorption spectrometer. The arsenic 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 As concentration (µg As cm^-3), x is the adsorption maxima (µg As g^-1 soil) and k is a constant related to the energy of adsorption (cm³ µg^-1 As). The differential isosteric hest of adsorption, Δ H was obtained by collecting arsenate adsorption data for the same soil at  280 and 305⁰ K  and applying the Clausiues Clapeyron equation to the system. For a given amount of arsenate adsorbed, Ø,              Log [C₂/C₁] = -Δ H/ 2.303 R [1/T₁-1/T₂], Where C₁ and C₂ are the equilibrium As concentration (µg As/cm³) at temperature T₁ (280⁰ K) and T₂ (305⁰ K) respectively and R is the molar constant (1.985Kcal/mol.) Adsorption of arsenate on a course loamy mixed hyperthermic Fluventic Haplustept soil determined at 280 and 305 ⁰K showed one break in the slope of Langmuir plots (Fig.1). Thus arsenate adsorption on this soil and soil+ iron fillings are described by two regions Langmuir isotherm equation, i.e. the plots showed two distinct linear portions. The   bonding energy and adsorption maxima for arsenate adsorption by soil increased slightly at higher equilibrium temperature of  305⁰ K relative to 280⁰ K in the Langmuir plot for the region I, but followed by appreciate decline in both parameters for region II (Table1).The addition of iron fillings considerably enhanced the adsorption maxima of arsenic by soil-iron filings mixtures suspension. Thus the results of the present investigation suggest that water withdrawn from shallow aquifer containing  elevated  As concentration should be equilibrated with mixtures of soils and iron fillings for removal of As. After equilibration period, separation of water by decantation or filtration could be used for humans and domestic animals for drinking purposes.   The heat released or absorbed during adsorption of arsenate is the differential molar heat of adsorption,   Δ H. The value of Δ H between 280 and 305 ⁰K were computed using equation (2) and are plotted as a function of arsenate adsorbed (Fig. 2). The Fig. 2 show that the adsorption process is energy producing (exothermic), within Δ H varying between -4 and 0 kcal/mole for a constant surface, Ø, of 250 to 410 µg of As gm^-1 soil. Thus it evident that physical (exothermic process) adsorption was predominant below 410 µg As gm^-1 adsorbed by soil, due to electrostatic force of attraction between the negatively charged arsenate oxy-anions and the existence of positive charge on surface of soil matrix of hydrous-oxides of iron and alumina and aluminosilicate minerals   present in the soils. However, when the surface coverage, Ø, of arsenate adsorption lies between 410 to 650 µg of As gm^-1 soil, the adsorption is energy consuming (endothermic process), with Δ H varies from 0 to 1.5 Kcal mole^-1. This shows that at higher arsenate concentration, adsorption process was predominant due to chemisorptions or precipitation of arsenate minerals on the surface of soil matrix. Nevertheless, it is true that one can not differentiate between the physical adsorption and chemisorptions process, since both occurred simultaneously. However in the present investigation, one can conclude that at lower equilibriums As concentrations, the negative values (-4 and 0 Kcal mole^-1) of Δ H were due to the predominance of physical adsorption process, but at higher As concentration, the positive (0 to 1.5 Kcal mole^-1) value of Δ H elucidate that the sorption occurred mainly due to chemisorptions or precipitation mechanism. The surface chemistry of arsenate anion adsorption on the surfaces of soils and soil minerals is analog to phosphate oxy-anion. Similarly phosphate anion also adsorbed with different energies of binding on homogenous soil mineral surfaces. At relatively low P concentration P adsorption exclude precipitation whereas at high concentration adsorption mainly due to nucleation followed by precipitation of amorphous P compounds. It is conclusively evident from this investigation that the surface properties of soils available at the sites having elevated arsenic ground water can be utilized for remediation   work. The prevalent temperature during winter and summer is not going to effect on the removal of arsenic by the soils from elevated As groundwater. Iron fillings produced by cutting and turner Industries considerably enhanced the As removal capacity of soils form water of higher As content. Thus Iron fillings waste produced by local industries near the rural settlements in the affected areas could be mixed with soil for removal of arsenic oxyanions from shallow alluvial aquifers waters to safe limits before it be used for drinking by humans and domestic animals. Table 1. Langmuir parameters for arsenate adsorption on course loamy used mixed

hyperthermic Fluventic Hypluostept soil

k = binding energy, cm³ µg^-1; b = adsorption maxima, µg gm^-1 soil