Chromate, being oxyanion, at an elevated level is a confirmed carcinogen and mutagen. The anions of Cr (VI) are epidermal irritants, corrosive and also cause nausea, vomiting, severe diarrhoea, and haemorrhage. Hexavalent chromium is toxic to plants, marine animals and micro-organisms [1-3].Chromium exists in different oxidation states ranging from 0 to VI. Trivalent and hexavalent oxidation states are the most stable forms of chromium. Hexavalent chromium exists as H2CrO4, HCrO4−, CrO42−, H2Cr2O7 HCr2O7− and Cr2O72− in aqueous solution depending upon the concentration and pH of the system . A trivalent form of chromium is an essential trace nutrient having an important role in the metabolism of glucose and cholesterol while hexavalent chromium is highly toxic for biological systems . Although Cr (III) is benign, its interconversion to Cr(VI) in the environment poses a global problem. The World Health Organization (WHO) recommend values for hexavalent chromium in drinking water of 0.05 mg/L, whereas the United States Environmental Protection Agency (US EPA) guideline set a lower value of 0.1 mg/L for total chromium in drinking water [6,7]. The oxyanionic contaminant of Cr(VI) poses a large threat in surface and ground water due to its mobility in alkaline soil. Therefore, water decontamination from oxyanionic chromium is a prerequisite before it is discharged into water bodies.
Chemical precipitation, coagulation, ion exchange, reverse osmosis, lime softening, electrodailysis and adsorption techniques are conventionally employed for the removal of oxyanions from waste and potable waters [3,6]. However, mostly these techniques are associated with a high maintenance and operational cost, incomplete metal removal, technical difficulties in the preparation of materials, and generation of toxic sludge [7, 8]. Considerable work has been done on the metals removal through adsorption because the scheme is cost-effective, most efficient and simple to operate [9-12]. Most significantly, it is a naturally available process in the geological system. Geosorption plays an important role in the migration of metal ions in natural water systems and the mobility of toxic elements in living systems.
Iron oxides present in many natural systems are the most common geosorbents for adsorption of anions from aqueous systems . Generally, iron oxides have a high surface area and high point of zero charge (PZC). This causes the surface of iron oxides to maintain a positive charge over a broad pH range. Iron oxides are non hazardous and could be safely restored. Though the sorption capacity of iron oxide is greater for several ions, it can be further enhanced by increasing it’s surface area. Iron oxide has conventionally been applied for the removal of oxyanions from drinking water; however, little attention has been paid to such type of studies on Fe2O3 nano-powder with well-defined characteristics.
2 Experimental Section
The samples of Fe2O3 micro and nano-powder were purchased from Merck. The stock solution of 1000 ppm Cr(VI) was prepared by dissolving K2CrO4 in de-ionized water. Working solutions of several concentrations of Cr(VI) were prepared from this stock solution. The concentration of Cr in the solutions was determined by a Graphite Furnace Atomic Absorption Spectrophotometer (Analyst800).
Both the nano-and-micropowder Fe2O3 were characterized by a Quanta chrome surface area Analyzer model 2200C, XRD (JDX-3532), a SEM/EDX model JSM 5910 (JEOL Japan), a SHIMADZU 8201PC FTIR spectrophotometer and a TGA/DTA analyzer, Perkin Elmer model 6300. The dissolution of both the Fe2O3 was studied in the pH range 2-10 at 308 K. PZCs of the iron oxide samples were determined by the salt addition method. A detailed description of the characterization techniques has already been presented in our earlier report [12, 13].
Batch adsorption experiments were conducted to examine the effect of initial Cr(VI) concentration, contact time, media dosage, pH and temperature on the adsorption of Cr(VI) onto micro and nano-powder Fe2O3. Forty millilitres of the Cr(VI) solution was transferred to a series of reaction flasks. 0.1 g of the adsorbent was added to each flask. The pH was adjusted by adding HNO3 and NaOH. The suspension in the flask was then shaken (120 rpm) in a thermostat shaker at the desired temperature. After 24 h, the final pH was recorded and the suspension was filtered. The filtrate was analyzed for chromium by an Atomic Absorption Spectrophotometer model AAnalyst 800. The amount of chromium adsorbed was calculated from the difference of initial and final concentration of chromium in the solution.
Similar adsorption studies were conducted at 298, 308, 318 and 328 K which corresponds to the seasonal changes in the temperature in the Indian sub-continental region. Sorption of Cr(VI) on Fe2O3 and Fe2O3 nanopowder was also examined by varying the pH in the range 3-9 at 308 K. Control experiments (without Fe2O3) demonstrated neither precipitation nor chromate sorption onto the walls of the reaction vessels and filter materials. The simple batch adsorption technique was also used for kinetic studies.
3 Results and Discussion
Both the micro-and-nanopowder Fe2O3 samples were characterized by the following physicochemical methods. The surface areas of Fe2O3 were determined using a Quanta chrome Analyzer model 2200C. The values of the BET surface areas were found to be 84 and 154 m2/g for Fe2O3 and Fe2O3 nano-powder respectively. As expected, the greater surface area of the nano-powder is due to a smaller particle size. The current values of surface area are comparable to the one reported for Fe2O3 by Tang et al..
Fig. 1 (a) illustrates the XRD profile of the commercially obtained micropowder Fe2O3. The peaks observed at d values of 3.61, 2.69, 2.51, 2.20, 1.83, 1.69, 1.61, 1.48, 1.45 Å for the Fe2O3 correspond to the rhombohedral phase of α-Fe2O3 (JCPDS card No-79-007) with the corresponding miller indices values, (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 3), (0 1 8), (2 1 4), (3 0 0).
Fig. 1 (b) illustrates the XRD profile of the commercially available nano-powder Fe2O3. The peaks observed at d values of 2.95, 2.52, 1.60, Å for the Fe2O3 nano-powder correspond to the cubic maghemite phase of γ-Fe2O3 (JCPDS card No-39-1346) with corresponding miller index values, (2 2 0), (3 1 1), (4 4 0). It is to be noted that micro-powder Fe2O3 is more crystalline than the Fe2O3 nano-powder. The XRD spectra observed in this study are consistent with those reported for Fe2O3/Fe2O3 nanopowder . The Scanning Electron Micrographs (SEM) demonstrate that the micro-powder sample of Fe2O3 is bulkier, irregular in shape and its particle size ranges from 3 to 6 μm (Fig. 2A). While the particle size of the nano-powder is in the range of 50 to 60 nm. Moreover, the particles of nano-powder Fe2O3 are generally spherical in shape and in aggregated form (Fig. 2C). These micrographs also reveal that both the oxides are highly porous in nature.
A detailed dissolution study of the Fe2O3/Fe2O3 nano-powder was conducted as a function of pH. The experimental data were plotted by plotting the Fe released versus pHi of the system. Closer inspection of the given data in Fig. 2E shows that the Fe released from the nano-powder Fe2O3 is gradually increasing with the pH of the system. It then reaches a maximum at pH 6 and decreases linearly until pH 10. While, in the case of micro-powder Fe2O3, the Fe released is higher at the initial pH values and reaches a minimum at pH 4 and then the curve plateaus, which is opposite to the behaviour mentioned in case of Fe2O3 nano-powder. The maximum Fe released from the Fe2O3 nano-powder is greater than Fe2O3 which illustrates that the Fe2O3 remains more stable than the nano-powder sample of Fe2O3.
The FTIR spectra of the Fe2O3 and Fe2O3 nano-powder before and after adsorption of Cr (VI) are shown in Fig. 3A and 3B respectively. FTIR spectra show five major peaks at 670, 1000, 1612, 3404 and 3662 cm-1 which are the characteristic peaks of iron oxide. The band at 670 cm-1 is attributed to Fe-O vibration; while 1630 cm-1 is assigned to the O-H bending vibration. A broad band at 3404 cm-1 accompanied by a shoulder at 3662 cm-1 is assignable to O-H stretching vibrations. Similar characteristic peaks for the iron oxide material have also been reported by other researchers .
The PZCs of the samples were determined by the salt addition method [17, 18]. The corresponding experimental data for both the samples are given in Fig. 2F. The PZC values for Fe2O3 and Fe2O3 nano-powder are found to be 5.6 and 7.6 respectively. This indicates that Fe2O3 nano-powder remains more positively charged than its counterpart micro-powder Fe2O3.
3.1 Adsorption Studies
The kinetic studies of the chromate adsorption by iron oxides were conducted at 298K (Fig. 4A). As can be seen, chromate adsorption linearly increases with the increase in contact time, which then changes into a curve until system equilibrium is established. The data shown in Fig. 4A further illustrates that chromate adsorption takes place quickly and then gradually slows down as it gets to the sorption equilibrium. The curves further reveal that the equilibrium in the system is established within 680 and 480 min for the Fe2O3 and Fe2O3 nano-powder respectively. The comparison of the kinetic curves further demonstrates that the affinity of chromate is higher towards the Fe2O3 nano-powder than Fe2O3 which further strengthen the result obtained from the adsorption equilibrium data.
Like other parameters, the effect of media dosage on the recovery of Cr(VI) from aqueous solution was also investigated (Fig. 4B). The % recovery of chromium increases with the increase in the dry mass of the solid and reaches a maximum at 2.5g/L. The effect of media dosage is more dominant on chromate adsorption by the Fe2O3 nano-powder than the Fe2O3 micro-powder. The decrease in the adsorption of Cr(VI) at a low adsorbent dosage may be due to the decrease in the driving force of the concentration gradient .
The adsorption capacity of both the Fe2O3micropowder/Fe2O3 nanopowder increases with the increase in concentration of chromate anions (Fig. 4C). The highest concentration of chromate is responsible for shifting the equilibrium in the forward direction . The increase in chromate concentration provides an important driving force to overcome the mass transfer resistance between the solid and aqueous phases. Moreover, the concentration effect of chromate is more pronounced in case of the nanopowder sample of Fe2O3 which is attributed to its larger surface area and PZC value.
During the course of this investigation, adsorption of Cr(VI) onto the Fe2O3 micropowder/Fe2O3 nanopowder was studied by varying the temperature from 298-328K (Fig, 4C). During the course of this investigation, Microsoft Excel was used for plotting the adsorption data. The adsorption of chromates increases with an increase in temperature, which indicates that a larger amount of heat is needed to divest the Cr(VI) from the aqueous phase onto the solid surface. The effect of temperature is more pronounced on the Cr(VI) sorption by Fe2O3 nano-powder than its counterpart Fe2O3. The present findings are consistent with the data reported for the uptake of chromate by malachite nano-particles  and are opposite to those described for the chromate adsorption on zeolite NaX .
The effect of pH on the sorption of Cr(VI) by Fe2O3 and Fe2O3 nano-powder was also examined by varying the pH in the range 3-9 at 308K. The experimental adsorption data have been compiled in Fig. 4D for nanopowder Fe2O3 and Fe2O3 respectively. As expected, the sorption of Cr(VI) is greatly dependent on the pH of the system. The anion exchange removal of Cr(VI) from aqueous solution follows the sequence: pH 3 > pH 5 > pH 8 > pH 9 for both the adsorbents. In fact, the iron oxide exhibits PZC at pH 5.6 and 7.6 for Fe2O3 and Fe2O3 nano-powders respectively. Consequently, the surfaces of these oxides remain positively charged below their PZC values while the hexavalent chromium exists in anionic form in solution. On account of the electrostatic interaction, the sorption of Cr(VI) is more facilitated below the PZC values of the corresponding metal oxides while the converse is true for the adsorption of Cr(VI) at high pH values. Several researchers have described similar results while studying the sorption of anionic species from aqueous electrolyte solutions onto various oxides/hydroxides of metals [23, 24].
Hexavalent chromium exists in aqueous systems in different speciation forms. Depending upon the pH and concentration, the major species of chromium are CrO42−, HCrO41−, HCr2O71− and Cr2O72−. Moreover, in the acidic pH range, HCrO4− is the predominant species, while CrO42− predominates in aqueous media at pH > 4. In addition, the presence of polychromate at high concentration and low pH values has been described elsewhere .
In the light of the above speciation, the following tentative mechanism may be responsible for the adsorption of chromate anions; (1) (2) (3)
Similar findings for chromate adsorption have also been reported by Di et al..
Langmuir isotherms give information about the sorption maxima and the binding energy constant. This model assumes that the surface of the adsorbent consists of energetically homogenous sites which facilitates the adsorption of aqueous Cr(VI) ions at monolayer coverage only. A linear shape of the conventional Langmuir equation is given below [26,27]. (4)
where Ce is the remaining concentration of the chromate in the solution, X is the amount of chromate adsorbed on the surface of the metal oxide, Xm is the maximum amount adsorbed and Kb is binding energy constant. The plots of Ce/X vs. Ce for sorption of Cr(VI) on nano and micro-powder Fe2O3 are given in Fig. 5A. The Langmuir parameters for the sorption of Cr(VI) at different temperatures are listed in Table 1 and Table 2. The increase in the Langmuir sorption maxima and binding energy constant with temperature indicates the endothermic nature of the process .
The values of binding energy constant and sorption maxima for Cr(VI) sorption onto the substrates followed the trend; Fe2O3 nano-powder > Fe2O3 micro-powder at all temperatures. The higher affinity of Cr(VI) towards the Fe2O3 nanopowder can be assigned to its greater surface area.
To examine the mechanism of chromate sorption onto iron oxides, the Dubinin-Radushkivech (D-R) model was applied to the equilibrium adsorption data to distinguish between physisorption and chemisorption. This model suggests the mechanism of adsorption in micropores present on the adsorbent is that of pore-filling rather than the layered surface coverage. The D-R model is mostly written in the following form ; (5)
where X is the amount of Cr(VI) adsorbed per unit mass of adsorbent (mol/g), Xm represents the adsorption capacity (mol/g) of the metal oxides, ε = [RTln 1/Ce] is a Polanyi potential, k signifies the constant related to adsorption energy. The plots of lnX vs. ε2 (Fig. 5B) give a linear relationship with R2> 0.96. The values of k obtained from the slope of the plot are given in Table 3. The mean free energy of adsorption (E) can be obtained by the following mathematical relationship; (6)
The mean value of the free energy of adsorption is helpful to probe the mechanism of the sorption reaction. E values in the range 8-16 kJ/mol indicate an anion exchange mechanism while below 8 kJ/mol represents a physisorption mechanism . The values of free energy of adsorption (Table 3) confirm the anion exchange mechanism for Cr(VI) sorption onto both of the iron oxides.
The isosteric heat (ΔH of adsorption is a specific combined property of an adsorbent-adsorbate combination. It is the heat energy released or taken up in the sorption process. The ΔH may be estimated by collecting adsorption data at different temperatures. Its values can be obtained using the Clausius Clapeyron equation in the form : (7)
where Ce is the equilibrium concentration of Cr(VI) at constant surface coverage (θ) of Fe2O3micropowder/Fe2O3 nanopowder powder, R is the universal gas constant and T represents the absolute temperature. The values of ΔH at surface converges (θ) were calculated from the slopes of the linear plots of ln [Ce]θ vs. 1/T (Fig. 5C). The ΔH values are positive for the Cr(VI) adsorption onto Fe2O3micropowder/Fe2O3 nanopowder, which shows the endothermic nature of the process .
The data presented in Fig. 5D reveals that the isosteric heat of adsorption gradually increases with the increase in surface coverage (θ) of the Fe2O3 micro-powder. However, opposite results (Fig. 5E) are observed for nano-powder sample of Fe2O3, where the isosteric heat of adsorption first decreases with increasing surface coverage up to 3x10-5 mol/g and then remains almost constant over the entire surface coverage. Moreover, the ΔH values are almost double in the case of the Fe2O3 nanopowder compared with the Fe2O3 micropowder. The greater ΔH for the nanopowder may be assigned to the greater adsorption of Cr(VI) on its surface which shows that a large amount of heat is needed to divest the Cr(VI) from the aqueous phase. The different behaviour of the isosteric heat (ΔH) of adsorption with surface coverage indicates the heterogeneous nature of the oxide surfaces. Similar findings were also reported by Unnithan and Anirudhan ) and Durano ğlu et al. .
Both the nano and micro-sized iron oxides before and after Cr (VI) adsorption were analyzed by SEM/EDX and FTIR techniques. The FTIR spectra of micro and nano-powder Fe2O3 after Cr(VI) sorption gave additional peaks at 881 and 920cm-1 (Fig. 3A and 3B). As described by Nyquist and Kagel , the bands around 880-892 and 924-966 cm–1 represent the Cr2O72- anions. Therefore, the FTIR spectra confirm the adsorption of Cr2O72- onto both types of Fe2O3 according to reaction 7. The SEM micrographs (Fig. 2B and 2D) show distinct morphological changes in the surface of the micro- and nano-powder Fe2O3 before and after Cr(VI) sorption. Thus, both the SEM micrographs and EDX spectra (Fig. 6) also corroborate the adsorption of the chromate anions onto the iron oxide surfaces.
From the above discussion, it is concluded that the surface area and PZC of nano-powder Fe2O3 are greater than the micro-powder Fe2O3. The increase in concentration and temperature has a positive effect on chromate adsorption while the converse is true for the lower pH values. The sorption maxima and binding energy constant are found to be higher in the case of Fe2O3 nanopowder than the micro-sized Fe2O3. The mean free energy confirms that the uptake of chromate onto the surface of both the iron oxides takes place through an ion exchange process. The isosteric heat of adsorption reflects the endothermic nature of chromate sorption onto the nano and micro-powder iron oxides. FTIR studies confirmed that the oxyanionic form of Cr2O72- was adsorbed onto both the iron oxides. Finally, it is inferred that nano-powder Fe2O3 behaves as an efficient adsorbent for the decontamination of water from Cr(VI) as compared to micro-sized Fe2O3.
We are thankful to the National Center of Excellence in Physical Chemistry, University of Peshawar, Pakistan for providing funding and facilities for this research work. The authors also extend their sincere appreciation to the Deanship of Scientific Research at the King Saud University for its funding of this Prolific Research Group (PRG-1437-29).
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Published Online: 2017-06-14