Biosorption of Chromium by Agricultural Waste, Anacardium Occidentale Testa Powder: Characterization, Kinetic, Isotherm and Thermodynamic Studies

The study on low-cost biosorbent such as Anacardium occidentale testa powder was used to remove chromium from aqueous solutions. Based on R 2 values, the Langmuir (R 2 = 0.9927) model fitted the equilibrium biosorption data best, confirming monolayer adsorption of chromium on to the biosorbent surface. The biosorption of chromium was best described by pseudo-second order (R 2 =0.9902) kinetics since at all concentrations, the R 2 values were higher than the corresponding pseudo-first order (R 2 =0.9278) values. Based on thermodynamic parameters the biosorption of chromium by Anacardium occidentale testa powder was found to be spontaneous, endothermic and feasible under optimized conditions.


Introduction
Industries like fuel production units, atomic energy stations, electroplating and fertilizer industry, leather and electrical appliance manufactory, and iron enterprises generate enormous wastes containing large amount of toxic heavy metals discarded into the environment resulting in ecological imbalance (Kümmerer 2009). Numerous disorders and diseases are caused by the deposition of heavy metals like lead, mercury, cadmium and chromium at the top of the toxicity list (Fu and Wang 2011) Chromium exists in several oxidation states, but the most stable and common forms are Cr (0), the trivalent Cr (III), and the hexavalent Cr (VI) species. Cr (VI) in the forms of chromate (CrO4 2 − ), dichromate (Cr2O7 2 − ), and CrO3 are considered the most toxic forms of chromium, as it presents high oxidizing potential, high solubility, and mobility across the membranes in living organisms and in the environment. The main source of Cr (VI) ions are produced from industries like plating, alloying, tanning, water corrosion, textile, pigments, ceramic glazes, refractory bricks, and pressure-treated lumber industries (Avudainayagam et al.2003). Cr-VI ions are more toxic than Cr-III ions, Cr (VI) ions are powerful epithelial irritant and also considered as a human carcinogen. Cr (VI) ions are toxic to many plants (Shanker et al. 2005) aquatic animals (Velma et al. 2009), and microorganisms (Petrilli Flora De 1977). It also been reported that human kidney and liver are damaged due to high chromium dosage (Mungasavalli et al. 2007) and in low doses it causes skin irritation and ulceration (Karthikeyan et al. 2005) Many studies identified that 10 ppm Cr (VI ) ions as the threshold at which not more than 10% of exposed individuals developed skin sensitization. Chronic health effects are observed in human population when exposed to approximately 20,000 µg Cr-VI/L in drinking water contaminated by a ferrochrome plant caused mouth sores, diarrhea, stomach pain, indigestion, vomiting, and higher levels of white blood cells than the reference population. Environmental Protection Agency has concluded that the only chromium (VI) should be classified as a human carcinogen. Exposure to chromium (VI) ions resulted in complications during pregnancy and child birth. In aquatic ecosystems chromium causes hardness, rise in temperature, pH, and salinity of water; and biological factors such as species, life stage, and potential differences in sensitivities of local populations (Marchese et al. 2008;Dwivedi et al. 2010). In animals Cr +6 produced cancers, reproductive harm, behavioral changes, reduced growth and survival using experimental doses through food, water or injections. Metallic Cr and Cr +3 are non-toxic (Gale 1978). When plants are exposed to excess chromium, toxic effects are reduced growth, decreased chlorophyll production causing yellow leaves, narrow leaves, small root systems, decreased or complete inhibition of seed germination, delayed growth, decreased seed yield, wilting and death (Dube et al. 2003;Zayed and Terry 2003).
EPA has a drinking water standard of 0.1 milligrams per liter (mg/l) or 100 parts per billion (ppb) for total chromium. Among many conventional techniques (Nguyen et al. 2013), adsorption is very attractive owing to its easy operation and high efficiency to treat water contaminated with low concentrations of heavy metals, i.e.,<100mg/L. Continuous attempts were developed to produce low-cost adsorbents derived from industrial, agricultural and aquatic wastes e.g., cucumber peel (Basu et  Anacardium occidentale testa powder can be used as a potential source of biosorbent. It belongs to the family Anacardiaceae, native to Northeast Brazil, India, Southeast Asia and Africa. The Cashew nut is a high value product from this tree but the testa is one of the by-product obtained from processing of cashew nuts, and it is not been considered as animal feed. The cashew nut testa is a reddish-brown skin that covers the cashew kernel. This skin is reported to be rich in hydrolysable tannins and polyphenols (Donkoh et al. 2012). Polyphenols, extracted from plants, are characterized by the abundant phenolic hydroxyls that are capable of chelating with transitionmetal ions, especially for those metal species with d-orbits (Zhang et al.2017). Because of the strong chelating ability of phenolic hydroxyls towards transition metals, the polyphenol-based cashew nut testa powder showed to be potential and efficient sorbent for the removal of chromium from aqueous solutions.

Preparation of Anacardium occidentale testa powder
Anacardium occidentale testa was collected from Vetapalem, Prakasam, Andhra Pradesh. The testa were cleanly washed with water and then with distilled water, dried in sun until it became colorless and crispy. They were grinded to powder, separated into desired sizes using BSS sieves and stored in air tight plastic bags.

Scanning electron Microscope (SEM) studies
The pretreated biosorbent samples were examined in Scanning Electron Microscope and electron probe micro analyzer. The samples were coated with ultra-thin film of gold by an ion sputter JFC-1100 and exposed under SEM.The working height was 15 mm with a voltage ranging from 10 to 15 kV.The compositional image analyses of untreated and treated samples as shown in the Figures 1&2 were taken using equipment at 15 kV and 40-100 nA beam current.

Biosorption studies
The initial concentrations of chromium in the aqueous solutions were analyzed in an Atomic absorption spectrophotometer (Perkin Elmer A Analyst 200 model) and found to be 20 mg/L, wave length is 357.87 nm, and sensitivity check is 4.0 mg/L. The procedures adopted for the bio sorption of chromium is optimized by using following parameters.

Effect of agitation time
50 mL of aqueous solution (initial concentration of chromium was 20 gm/L) was taken in each 250 mL conical flasks and 10 g/L of 53 μm size biosorbent was added and exposed to varying agitation times (3,5,10,15,20,25,30,40,50, 60, 90, 120, 150 and 180 min). These samples were shaken on an orbital shaker at 180 rpm at 30 o C for 1 min. These samples were filtered separately with Whatman filter paper and the filtrates were analyzed in AAS to obtain final concentrations of chromium and the equilibrium agitation time was calculated and the data is shown in Figure 3. flasks containing each 50 mL of aqueous solution and the contents were agitated in an orbital shaker.
The optimum biosorbent size was determined from the data as shown in Figure 4.

Effect of pH on the aqueous solution
To study the influence of pH on chromium biosorption, 50 mL of aqueous solution was taken in 250 mL conical flasks. The pH values of the solutions were adjusted to 2, 3, 4, 5, 6, 7 and 8 in separate flasks by adding required amounts of 0.1 N H2SO4 or 0.1N NaOH. 10 g/L of 53 μm size biosorbent was added separately to these flasks. The samples were shaken on an orbital shaker at 30 o C for equilibrium agitation time and the results are depicted as shown in the Figure 5.

Results and discussion
The potential of dry Anacardium occidentale testa powder as a biosorbent for the biosorption of

SEM analysis for untreated and chromium treated Anacardium occidentale testa powder
The analysis of untreated Anacardium occidentale testa powder as shown in Figure1 indicates that the powder is devoid of chromium. The Figure 2 shows the analysis of Anacardium occidentale testa powder loaded with chromium. The SEM analysis shows variation in the surface structure compared to unloaded one. Irregular spikes are accumulated on the surface and porous structure is also noticed indicating biosorption of chromium by Anacardium occidentale testa powder.

Effect of biosorbent size
The variations in % biosorption of chromium from the aqueous solution with biosorbent size are obtained. The results are drawn in Figure 4 with percentage biosorption of chromium as a function of biosorbent size. The percentage biosorption is increased from 42.72 (2.136 mg/g) to 59.08 (2.954 mg/g) as the biosorbent size decreases from 150 to 53 μm. This phenomenon is expected, as the size of the particle decreases, surface area of the biosorbent increases; thereby the numbers of active sites on the biosorbent are better exposed to the biosorbate.

Effect of Temperature
The effect of temperature on the equilibrium metal uptake was significant. The effect of changes in the temperature on the chromium uptake is shown in Figure 8. Isotherms for biosorption of chromium using Anacardium occidentale Testa Powder

Langmuir isotherm
Langmuir isotherm (Langmuir 1918) is drawn for the present data and shown in Figure 9. The equation obtained 'n' Ce/qe = 0.06843 Ce + 5.05471 with a good linearity (correlation coefficient, R 2~0 .9927) indicating strong binding of chromium ions to the surface of Anacardium occidentale testa powder.    Table.1.