Low-cost Rice Husk Ash and Silica for Chromium Ion Sorption from Aqueous System: Characterisation and Kinetics

: Several materials have been developed and used to remove Chromate (VI) from industrial wastewater before discharge into the environment because of its toxicity and lethality. In this study, low-cost adsorbents (rice husk ash (RHA) and silica (RHS)) were prepared from rice husks purchased locally from a rice mill industry and characterised. While the crystallinity and mineralogy of the adsorbents were analysed using powdered XRD crystallography, analysis of the physicochemical properties was performed using standard procedures. Elemental analysis (CHN) was done using Perkin Elmer CHN elemental analyser and FTIR Spectrometer was used to determine the functional groups on the surface of the adsorbents at room temperature. The prepared samples were used as adsorbents for the removal of chromate (VI) ion in a batch sorption process with reaction conditions vis–a-viz adsorbent dosage = 0.5 g, adsorbate volume = 500 ml, adsorbate solution concentrations: 10-200 mg/l, time = 4 h, temperature = ambient temperature. The residual Cr (VI) concentration in the solution was analyzed spectrophotometrically at λ = 540 nm following the 1,5–diphenylcarbazide procedure. Results obtained for the characterisation of the adsorbents were similar and comparable with what obtained in literatures. Physico - chemical analysis revealed a high ash content of 89% and silica content of 95.83%. XRD analysis for the rice husk ash samples reflected the presence of cristobalite (SiO 2 ) in amorphous form and a characteristics broad peak at 2θ angle = 26.5°. FTIR analysis revealed the presence of silanol groups (Si-OH), silicic acid (Si-O-Si-OH) in the ash and Si-O-Si and Si-O) in the silica. The results of effect of time-concentration experiments of chromium ion sorption by the adsorbents revealed that Cr (VI) ion sorption was both time and initial solution concentration dependent with over 80% removal achieved within the first 30 min of contact for both adsorbents as the initial chromate concentration was increased. Kinetic modeling of the process showed up for pseudo second order, hinting on chemisorption as the mechanism of interaction.


Introduction
Chromate (VI) amelioration in industrial wastewater before discharge into the environment has become mandatory due to its lethality. The extensive use of chromate compounds in mining operations, electronic device manufacturing, power generation, tannery, metal plating and production of stainless steel has been reported to generate process water containing up to 10-100 mg/L of chromate [1]; much higher than limit of 0.1 mg/L stipulated for industrial wastewater, thus requiring specialized or advance treatment for its removal. Chromium exhibits +3, +6 and 0 oxidation states in environmental matrix.
Hexavalent chromium is very mobile in the environment, speciated as dichromate, hydrogen chromate and chromate and a confirmed carcinogen and mutagen. Technological options of removal include chemical precipitation, membrane filtration, electrolysis and ion exchange. Conventionally, Cr 6+ is removed by chemical reduction to the Cr 3+ which is of lesser toxicity, and subsequent precipitation with a precipitant e.g. NaOH. The aforementioned technologies while able to effectively meet the standards, are hampered by various factors such as consumption of chemical agents, huge cost, generation of large amount of sludge requiring disposal and ineffective for the removal of low concentration of pollutants. Adsorptive method Aqueous System: Characterisation and Kinetics for Cr (VI) abstraction in waste streams has gained acceptance by researchers in recent times due to its simplicity of operations, economics, high efficiency and availability of many arrays of cheap environmental -friendly adsorbents [2]. Many agro-based adsorbents have been applied with success for the treatment of chromium contaminated wastewater. Rice husk (RH) is an agricultural wastes obtained during rice milling, and composed of both mineral and organic materials including cellulose, lignin silica, alkalis and trace element. Traditionally, rice husk is considered a waste material and generally disposed off by dumping or burning, although some are used as a low-grade fuel. However, the amounts of rice husk available are so far in excess of any local use and have posed environmental problem. The unique physical and chemical properties of RH such as granular structure, chemical stability, high ash and silica content, has promoted its use in domestic and industrial application [9]. Rice husk has been applied (directly or as rice husk ash) as a resource in the industry as a value added material or in manufacture and synthesis of new materials [10]. Controlled burning of rice husk at moderate temperature followed by acid leaching treatment yields rice husk ash (RHA) of high silica content (> 92%) of very high purity [11,12]. The high silica content in RHA has been employed for the production of silicon carbide, sodium silicate, zeolite and silica gel. Silica gels have been employed to remove heavy metals and organic pollutants from aqueous system [13][14][15]. Silica is a polymer of silicic acid made of inter-linked SiO 4 repeating units in tetrahedral arrangement. The active silica surface with large specific surface area and surface hydroxyl groups have been credited to its importance in adsorption and ion exchange processes. On the outer surface of the silica structure, there are free silanol groups, referred to as isolated silanol groups, and hydrogen bonded to adjoining silanol groups referred to as vicinal or germinal silanol group [16]. In this work, we report the preparation and characterization of rice husk ash and silica from rice husk as well as the kinetic of the adsorption of chromate VI ion from aqueous solutions by these materials in a batch reactor. Elucidation of mechanism of interaction was undertaken through kinetic modeling of the data obtained.

(a) Preparation of Rice Husk Ash (RHA)
Rice Husks (RH) was obtained from a nearby rice mill, sieved to remove very fine particles, washed with deionized water and dried in an open air. Rice husk ash was prepared from the dried rice husk by acid leaching using nitric acid (Analar grade) followed by calcination in a muffle furnace (CARBOLITE, 5220). Acid leaching of the dried rice husks was done to remove soluble elemental impurities and to increase the purity of silica content. 50 g of the dried rice husks was soaked in 200 ml of 0.1M nitric acid solution and heated at 80°C for 15 min. Rice husk was filtered and rinsed several times with distilled water until neutral pH and dried at 100°C for 2 hr. The rice husk ash was obtained by pyrolysis of the acid leached rice husk in a muffle furnace at 700°C for 6 h in the form of a white-greyish powder. (

b) Preparation of Rice Husk Silica (RHS)
The chemicals and reagents used in the process were of analytical grade. Preparation of the rice husk silica was carried out thus: 35g of rice husk ash was mixed with 250 ml of 1N sodium hydroxide aqueous solution. The mixture was refluxed for 3 h at 110°C, filtered and stored in a refrigerator as sodium silicate. 50 ml of sodium silicate solution was poured into 15 ml distilled water at a time and stirred. This was followed by drop wise titration with 3M H 2 SO 4 until a final pH 5 under stirring speed to form rice husk silica. The acidic condition of pH 5 indicates approximately the complete precipitation of silica from sodium silicate by the following reaction: © Characterisation of adsorbents The physico-chemical characterisation and proximate analysis of the samples (RHA and RHS) was carried out by following approved procedures. Elemental analysis (CHN) was done using Perkin Elmer CHN elemental analyser. FTIR Spectrometer (IR PRESTIGE 21, SHIMADZU) was used to determine the functional groups on the surface of RHA and RHS. Pellet pressed technique was used for this purpose at the spectral range of 4000-400cm -1 with resolution of 4 cm -1 . The adsorbents crystallinity and mineralogy was assessed using powdered XRD crystallography (XRD D8, ADVANCE BRUKER), using copper as the target, with Ni as the filter media and K radiation maintained at 1.54A. Diffraction patterns of the samples were recorded in the range of 10-90° at a scanning rate of 2°/min and step size of 0.01.

(a) Preparation and analysis of adsorbate
A stock solution of hexavalent chromium (1000 mg/L) was prepared in a double distilled deionized water using potassium dichromate (K 2 Cr 2 O 7 ). Working solutions of varying concentrations (10-200 mg/L) were obtained by serial dilution of the stock solution. The residual Cr (VI) concentration in the supernatant was determined using a UV -visible spectrophotometer (UV-1601, Shimadu) at λ max = 540 nm, following the 1, 5 -diphenylcarbazide procedure.
(b) Data analysis The amount of chromium removed by the RHA and RHS samples (mg/g) was calculated using the equations: Amount of Cr (VI) sorbed per mg of adsorbent = Where c o is the initial concentration of adsorbate (mg/L), c e is the equilibrium concentration of adsorbate (mg/L), v is the volume of the solution (ml) and w is the mass of the adsorbent (g).
© Adsorption Experiments All sorption studies were done and carried out at 25°C. Adsorbate solution pH was corrected to 5.3 using 0.1M HNO 3 and NaOH for the entire reaction.
Effect of Initial Solution Concentration This was done by contacting 0.5 g of the RHA and RHS samples with 500 ml of varying concentrations of the adsorbate (Cr 6+ ) solution that ranged between 10-200 mg/L in a batch reactor. The system was agitated on a magnetic stirrer at a speed of 200 rpm. Solution samples were withdrawn with a syringe after 4 hr of contact filtered using 0.45 µm membrane filter centrifuged and the supernatant was analysed for residual Cr (VI) as previously described.
Effect of Time The time-concentration profiles of the sorption of the adsorbate by the samples was studied to obtained the kinetic parameters by contacting 2 g of adsorbent sample to 1000 ml of adsorbate solution of varying initial concentrations (10-200 mg/L) in a batch reactor. The system was agitated on a magnetic stirrer at a speed of 200 rpm. Solution samples were withdrawn with a syringe at intervals between 0 and 4 hr of contact, filtered using 0.45 µm membrane filter, centrifuged and the supernatant was analysed for residual Cr (VI) as previously described.

Physical and Chemical Characterisations of RHA and RHS Samples
Results of the proximate analysis of RHA presented in Table 2 showed its high ash content thus indicating the highly inorganic nature of RHA with silica content of 95.83%. The values obtained for the physicochemical parameters for the RHS prepared in this study are similar and compare favourably with what obtained in literatures [16,17].

Instrumental Characterisation via FTIR and XRD Analysis of RHA and RHS Samples
In the FTIR spectral of the RHA (Figure 1), the peak between 2830 -3720 cm -1 and 3440 -2900 cm -1 were ascribed to the presence of OH and Si-OH groups on the RHA surface. The band at 3430 cm -1 pertained to the silanol group and water adsorbed on the surface of RHA [18,19]. The structure of the silanol groups are similar to that of silicic acid and are in form of silicon dioxide structure (Si-O-Si-OH) [16]. The weak broad peaks around 1600 cm -1 corresponded to stretching of ketonic (C=O) and aldehydic (C-OH) groups. The presence of other organics groups were indicated at 2940 -1300 cm -1 , for instance, the bands observed between 2920 -2855cm -1 and at 1423 cm -1 corresponded to the presence of methyl groups and aromatic rings due to the presence of lignin, respectively [18]. The peak at 1100cm -1 is ascribed to Si-O-Si and -C-O-H stretching and OH deformation while peaks at 793 cm -1 and 469 cm -1 suggest the presence of Si-H [17,19]. FTIR analysis of the RHS presented in Figure 1 showed a little broad band between 3380 cm -1 and 3240 cm -1 is ascribed to the hydroxyl group of both the silanol groups and water molecules bonded to the silica surface of RHS. Characteristic peak detected at 1647 cm -1 pertained to the bonding vibration of water molecule trapped in the SiO 2 matrix. Notable silica peaks were detected at 1014 cm -1 , 989 cm -1 for (Si-O-Si) and 866 cm -1 and 478 cm -1 for (Si-O) groups, respectively.  The XRD spectra of the RHA and RHS are presented in Figure 2a and 2b, respectively. The spectrum of the RHA indicated a highly amorphous material with the broad peak spanning 2θ angle range of 18-30˚ confirms an amorphous form of silica. RHA presented lower crystallinity than RHS. The spectrum of the RHS indicated a crystalline material with a sharp notable characteristic silica peak at 2θ = 26.5°. This change in texture from amorphous to crystalline phase is attributed to the calcination of the material in temperature above 500°C [17,19]. The value of the distance between the basal interlayer (d) was calculated using Bragg's law: d is the interlayer distance between planes of atoms; θ is diffraction angle; λ (1.542 Å) is wave length. The incidence angles (2θ) of 22.75° and 26.50° for RHA and RHS, respectively gives basal interlayer distances (d) of 4.23 and 4.99 Å, respectively. The d-spacing depends upon size and orientation of the charge balancing ion. The mineralogical classification of the RHS indicated the presence of cristobalite [20] and presence of cristobalite (SiO 2 ), Magaritasite and Macedonite for RHA. The result of effect of initial solution concentration on hexavalent chromium ion removal (%) by RHA and RHS sample is shown in Table 3. It was observed that chromate removal from the aqueous system is dependent on and increases with initial solution concentration. That is, the amount of Cr (VI) ions sorbed per unit mass of the adsorbents increases with increase in initial adsorbate concentration. As the C o was increased from 10 -200 mg/L, the amount (%) of Cr 6+ removed increased from 5% to 75% for RHA and 17% to 90% for RHS. The reason given for this observed trend is that the driving force needed to overcome resistance to the mass transfer of Cr ions between the aqueous and solid phases was increased by the increase in adsorbate initial solution concentration. Thus, as the C o increases, it enhances interaction that led to the adsorption uptake of the chromate ions by the adsorbents. Consequently, an increase in the initial chromate concentration increased the amount of Cr 6+ removed per gram of the RHS/RHA samples. At the initial concentration of 10 mg/L, the efficiency ranged between 3.67 -5.15% whereas at 200 mg/L initial chromate concentration, efficiency ranged between 32.56 -74.87% for RHA while it was 17.62 -36.46% for 10 mg/L and 39.10 -90.91% for RHS after 4 h of contact time. Effect of Contact Time on sorption of Cr (VI) This was conducted between a time range of 0-4 h by contacting 2 g of the adsorbent samples with 2 L of adsorbate solution of different concentrations (10-200 mg/L) in a batch reactor. The results of the time-concentration profiles presented in Figure 3 and Figure 4 showed that chromate (VI) ions removal by RHA and RHS, respectively were clearly time dependent. The sorption rate of the adsorbate by the adsorbents was very rapid during the initial 15 min of contact, and thereafter slowed down. This was seen in the gradual approach of the state of equilibrium after the first 120 min period of the process. No appreciable adsorbate sorption was observed thereafter. Values up to 80% of the total chromate removed was obtained within the first 60 min of the process followed by an additional 20% removal after 90 min of contact time and then values less than 3% additional Cr (VI) removal occurred in the remaining 120 min of contact time. At the commencement of the process, there existed large number of vacant active sites on the adsorbent surface available for adsorption, these accounted for the initial rapid uptake of Cr observed at the inception of the sorption process. These active sites become saturated after the lapse of some time which results in the slowing down of the adsorbate removal during the later period of adsorption and eventual repulsion between the solute molecules on the solid surface and the bulk solution [21]. For the present study the equilibrium time of 120 min could be considered economically favourable for the sorption process.  Kinetic Modeling of the Sorption Process Kinetics of adsorption as a dynamic process is usually used to describe rate as well as mechanism of adsorbate uptake from the solution by the adsorbent. Data obtained from the kinetic studies of adsorbate uptake is used in selecting optimum operating conditions for the full scale batch process. Elucidation of the mechanism of Cr (VI) sorption by the RHA/RHS was studied with the aid of frequently used kinetic models. The data obtained from the time-concentration profile were fitted into the equations of the model: Lagergren pseudo-first order and pseudo-second order. The Lagergren pseudo first-order kinetic model is represented as:

Effect of Initial Chromate Solution Concentration (C o )
On integration gives the linear form expressed as: Where q e and q t are the sorption capacity at equilibrium and time t respectively (mg/g). k 1 is the rate constant of pseudo -first order adsorption (min -1 ). A plot of log (q e -q t ) against t gave a linear relationship from where the pseudofirst order parameters (k 1 and q e ) were determined from the slope and intercept, respectively. The pseudo -first order parameters obtained from the plots for Cr (VI) sorption by RHA are presented in Table 4. The pseudo second order kinetic model considers the rate -limiting step as the formation of a chemisorption bond as well as exchange or sharing of electrons between the sorbate and sorbent [22]. The non linear form is expressed as: The linear form is expressed as: Where k is the overall rate constant of pseudo-second order sorption ((g/mg)/min). The plot of t/qt against t gives a linear relationship, from which qe and k could be determined from the slope and intercept, respectively. The pseudo -second order parameters obtained from the plots for CR (VI) sorption by RHS are presented in Table 5. Establishment of the better model amongst the two was carried out thus: i. From the results presented in Tables 4  and 5, it could be deduced that values of q e calc obtained for the pseudo second order were closer in magnitude to the experimental data than what was obtained for the pseudo first order model ii. the values of R 2 obtained for the pseudo second order plot were very high, (R 2 > 0.99) compared to pseudo first order. Thus, it could be concluded that the pseudo second order model describes adequately the sorption process. This therefore implies that the mechanism of sorption of chromium (VI) on the adsorbents (RHA and RHS) was chemisorption. Estimation of Sorption Performance A parameter k 2 qe (min -1 ) defined as the second order rate index was used to define the kinetic performance as described by Wu et al., [23].
At the half-life of the sorption process, q t = 0.5qe (10) and Using equation (11), results obtained presented in Table 6 showed that increase in the initial concentrations of chromate ion in solution led to a reduction in the values of the half-life of the sorption process. This indicates therefore increase in initial adsorbate concentration brings about a lowering of the time needed to reduce it to half its initial amount.

Conclusion
Based on the findings in this work, it is justifiable that both RHA and RHS can be used for the abstraction of Cr (VI) ions from contaminated effluents. The following deductions were drawn: XRD analysis revealed the presence of cristobalite (SiO 2 ), Magaritasite and Macedonite in RHA and cristobalite in RHS, respectively. FTIR analysis revealed that functional groups such as -OH, -C-OH, C=O, Si-O-Si and Si-H are present on the RHA and RHS surfaces. Kinetics study showed that sorption is favourably influenced by increase in initial adsorbate concentration and time.
Kinetic modeling followed the pseudo second order rate expression, a hint on chemisorption as mechanism of interaction.
The equilibrium time of 120 min is favourable economically for hexavalent chromate adsorption by RHS and RHA from aqueous system.
The adsorption rate of RHS is higher than that of RHA. Both RHS and RHA are effective adsorbents for the removal of Cr (VI) from aqueous solution.