Characteristics Of Rice Husk Adsorbent

Published Date: 02 Nov 2017

Introduction

This section of the report provides the results of the different experiments carried out. The section also brings into focus the behaviour of the sorption process with varying time, pH, dosage, and chemical activation of adsorbents. Results and explanations are supported by past researches for these observed behaviours. The detailed calculations are annexed in APPENDIX E.

Rice husk characterisation

Table : Characteristics of rice husk adsorbent

Bulk density, g/L

154.08

Moisture content, %

8.05

Ash content, %

20.92

Ismail and Lokuliyana (1983) carried out studies on the physical and chemical composition of 6 varieties of rice hulls. The bulk density of the rice husk approximated the results obtained for the variety named ‘BG 11-11’. BG 11-11 was found to have a bulk density of 152.4 mg/ml and a water content of 8.7079 %. The results obtained for the adsorbent used closely matched that particular variety of rice hull. Chemical analysis of rice husk indicated that silica was the major component, about 9.94% by weight.

Rice husk is described as having an ash content which varies between 15-18% whereby 90% is SiO2, a moisture content of 5-10% and a volatile matter content of 65-70 % (Rayaprolu, 2012). FTIR technique used to study the structure of rice husk indicated the presence of the following bonds which can be related to certain functional groups: C-H which can indicate presence of alkane, C=O relating hemicelluloses and lignin aromatic groups, C=C of alkenes and aromatic functional groups, aromatic CH and carboxyl-carbonate structures, CO of lactones, CHOH, Si-O-Si and Si-H groups (Daffalla et al., 2012). Vieira et al. (2012) qualifies rice husk as a mesoporous material.

Textile wastewater

pH of wastewater

The pH of the textile wastewater was found to be 10.64. The pH of the effluent from a cotton dye house ranges from 8-12 (Choudhury, 2006). The results obtained were in the range stated by Choudhury (2006).

4.3.2 Optimal Wavelength

The absorbance for textile wastewater, undiluted and diluted by a factor of 10, was read off at different wavelengths. The readings were plotted against their respective wavelengths, yielding three graphs, illustrated by Figure 4.2, 4.3 and 4.4. It was observed that the wavelengths for both wastewaters converged to an optimum wavelength of 588 nm. This optimal wavelength was used to take the absorbance in all the experiments that are carried out.

Figure 4.2: Variation of absorbance with wavelength from 400nm to 700nm at interval of 50 nm

Figure 4.3: Variation of absorbance with wavelength from 575 nm to 610nm at interval of 5 nm

Figure 4.4: Variation of absorbance with wavelength from 585 nm to 595 nm at interval of 1 nm

Optimum pH

Batch adsorption was carried with a constant adsorbent dosage of 20 g/L at different pH for a constant time period of 120 minutes. The batch adsorption was started with the pH of the textile wastewater which was 10.64 and then decreased until 2.07. The results for the absorbance are resumed below.

Table : Absorbance readings at before and after experiment with a dosage of 20 g/L at different pH

pH

Absorbance at start of experiment

Absorbance at 120 minutes

2.07

0.719

0.032

4.24

0.719

6.05

0.719

8.11

0.719

10.64

0.719

0.390

Figure 4.5: Percentage decrease in absorbance against pH of solution

As observed from Figure 4.5, the percentage decrease in absorbance increased with decreased of pH value. At pH 10.64, the absorbance decreased from 0.719 to 0.390. This represented a percentage decrease of 45.76%. For pH 8.11, 6.03 and 4.24, the percentage decrease in absorbance was of 43.11 %, 38.66% and 84.86% respectively. The highest percentage decrease in absorbance was obtained at pH 2.07 whereby the absorbance decreased from 0.719 to 0.032. This represented a percentage decrease of 95.55%.

pH 10.64 and 8.11 can be considered as alkaline medium while pH of 4.24 and 2.07 can be considered as acidic medium. It can be deduced that acidic medium promotes adsorption process to a greater extent than alkaline medium. The lowest percentage decrease was at pH of 6.03 which is nearest to neutral pH of 7. A neutral pH of the wastewater will thus be least efficient for adsorption process.

pH of the aqueous solution is an important factor that influencing the adsorption process at the solid-liquid interfaces. pH of the solution may affect the degree of ionization of the adsorbate specie and the surface charge of the adsorbent (Al-Anber, 2011).

DeSilva (2000) attributed the increase in adsorption on activated carbon at lower pH to the decrease in solubility of most organic compounds in a solution. The decrease in solubility of the organic compounds increases the rate of adsorption on the adsorbent.

The increase in adsorption rate at lower pH can also be related to the zero point charge of the adsorbent. For sawdust, Badot and Crini (2010) explained effect of pH of solution on lignocellulosic materials derived from wood in terms of pH of zero point charge (pHZC). Kumar et al. (2003) state that the pH at which the surface of the adsorbent is zero is known as the zero point of charge (pHZPC).When pH is less than pHZPC, the adsorbent becomes positively charged while at pH, higher than pHZPC, the adsorbent becomes negatively charged. Vieira et al. (2012) reported that the pHZPC of rice husk is 6.45.

Based on the literature data and observations made, the following deductions can be made. Lowering the pH implied increasing the positive charge on the rice husk. The dye can thus be assumed to be composed of negative species which are more readily adsorbed at low pH. At pH higher than pHZPC of rice husk, interaction of other components such as electrolytes could have increased adsorption of dye.

Effect of time

Figure 4.6: Variation of absorbance with wavelength with time at constant rice husk dosage of 20 g/L and different pH

The variations in removal of dye with contact time at different pH ranging from 2.07 to 10.64 for an adsorbent dosage of 20 g/L are illustrated in Figures 4.6. It is observed that the absorbance decreases with time. As time increases, removal efficiency also increases. The maximum amount of dye adsorption took place within the 60 minutes of the experiment and the rate to which absorbance decreases is decreased from 60 to 120 minutes. Data has been taken up to 120 min of agitation and equilibrium can be said to happen after 120 mins for a dosage of 20 g/L at different pH. The decrease in removal efficiency of dye molecules from the wastewater could be due to saturation of dye adsorption. As adsorption process proceeds, there is a decrease in the number of vacant sites on the adsorbent available on the adsorbent. It can also be concluded that along with its dependence on pH, adsorption process is also dependent on contact time.

4.6 Optimum dosage

Figure 4.7: Variation of absorbance with time with different rice husk dosage at optimum pH

The variations in absorbance at different adsorbent concentrations ranging from 20 to 100g/L at pH 2 are illustrated Figures 4.7. Decrease in absorbance indicates removal of dye from the wastewater. It can be observed that the lowest reading obtained for absorbance was 0.019. At 40 mins, absorbance of 0.019 was reached for a dosage of 100g/L of rice husk. The absorbance remained unchanged until 120 minutes. This was an indication that the absorbance could not go below that value. Comparing the different dosage of 20g/L, 40g/L, 60g/L, 80g/L and 100g/L, it can be concluded that increasing dosage of sorbent reduces the time taken to remove the dye from the textile wastewater. 100g/L was found to be the most efficient dosage. Due to the high bulk density of the rice husk, dosage concentrations above 100g/L were not investigated. Apart from the shorter lapse to time taken to reach an absorbance of 0.019, no visible change was noted when dosage was increase. Increase in dosage concentration increases adsorption sites. Increase in rice husk increases surface area where adsorption can occur

The results obtained can be compared to those obtained by Abdel Wahab et al. (2005) where the increase in dosage of modified rice husk caused an increase in adsorption of Direct F. Scarlet dye removal. This increase was attributed to the increase in availability of surface area for adsorption due to increase of rice husk biomass. Research carried out by Safa and Bhatti (2011) on adsorption of Direct Red – 31 and Direct Orange – 26 from aqueous solution by rice husk showed a decrease with increase in rice husk dose. Amount of dye adsorbed per unit mass may be reduced when adsorbent dosage is increased as unsaturation of adsorption sites occurs during the adsorption process. Another cause mentioned for the decrease in adsorption was particle interaction, such as aggregation. Aggregation increases diffusion path and decreases total surface area of rice husk adsorbent.

Plate4.: Decrease in colour intensity

The ‘yellowish’ colour remaining in the water could be due to the chemicals added during processes. Adsorption process on rice husk could not remove the colour from the water.

A dosage of 100mg/L was found to be the optimal dosage due to the high bulk density of rice husk. Dosage of rice husk adsorbent should also avoid aggregation problem.

4.7 Effect on chemical oxygen demand

Initial COD of the textile wastewater was found to be 1051.47 mg/L. After adsorption process, the COD value was reduced to 453.68 mg/L. This represented a decrease of 56.85%.

The initial COD value was in the range of COD mentioned by Choudhury (2006) for a cotton dye house. Yeh et al. (1993) investigated the dye removal capacity together with COD removal capacity of PAC, activated alumina, molecular sieves, GAC and sawdust. The efficiency of adsorbent was as follows PAC>activated alumina>molecular sieves>GAC. Sawdust on the contrary removed the dye to a small extent only and causes the COD to increase. The COD increase was explained by the probable leaching of hemicellulose and cellulose from the sawdust.

Untreated rice husk showed satisfactory removal of COD. The impurities present in the rice husk even after washing and drying could have decreased the efficiency of the rice husk.

4.8 Effect of chemical activation

Comparison was made between acid treated rice husk and untreated rice husk. Both were used as adsorbent with an optimum dosage of 100 g/L and pH of 2. It was observed that textile wastewater decolourisation occurred more rapidly with RHA. The wastewater was decolourised within 15 minutes when RHA was used while the time taken for RHU was about 25 minutes.

Daffalla et al. (2012) explain the possible changes that happened during the acid treatment of rice husk. Impurities present on the surface of RHU affect the adsorption rate of the dye both chemically and physically. Chemical treatment of the rice husk is a way of increasing adsorption properties of the rice husk. Subjecting the rice husk to an acid treatment helps in achieving the following:

Reduction of impurities at the surface of the rice husk. As a result, the roughness of the RH is improved. The availability of hydroxyl groups and reactive functional groups, found on the surface of the RH and which are generally involved in adsorption processes, increases.

Acid treatment of RH is an effective method that can be used for structural break-down. Along with the increase in porosity of the surface area, lignin and hemicelluloses are removed and cellulose crystallinity is reduced.

Since the knowledge of the chemical structure of rice husk is an important parameter for the study of adsorption process, Daffalla et al. (2012) used FTIR technique to identify the difference in chemical composition between untreated rice husk and rice husk treated with 1M HCL. After treatment with acid, it was observed that reduction occurred in O-H and C-H groups present in untreated rice husk. The Si-O-Si group was present to a lesser intensity. The changes could be explained by the washing out of inorganic materials such as silica and carbonate from rice husk’s surface.

Considering removal of dye, the increase of adsorption capacity using rice husk, HCL treated, can be explained by possible of protonation of the rice husks’ surfaces. The adsorbent surfaces become positively charged. Electrostatic attraction is developed between positively charged surface and negatively charged dye molecule which causes an increase the amount of dye adsorbed. The increase in efficiency of rice husk after chemical treatment was reported by Abdel Wahab et al. in 2005 using citric acid and Safa and Bhatti in 2011 using nitric, sulphuric and HCL acid.

It can be concluded that acid treatment of adsorbents increase specific surface area, micropores area and causes protonation of rice husk compared to non – activated adsorbent.

4.9 Effect of other components in textile wastewater

Considering processes in a dye house, the presence of salts and electrolytes in the final effluent is inevitable. Safa and Bhatti (2011) reported that investigation was done on the effect of salts on the biosorption of acidic dye on wood shaving. The results demonstrated that at adsorption of dye decreased with decrease in concentration of salts. The reason behind this observation was that salts played a role in the electrostatic interactions between the dye molecule and adsorbent’s surface. Increase in salt concentration increased those electrostatic interactions.

Effect on the adsorption process of Direct Red-31 and Direct Orange-21 on rice husk due to presence different salts such as NaCl, CaCl2 at varying strength was investigated by Safa and Bhatti (2011). Increased in various salts concentrations increased dye adsorption. This was explained by the possibility of salting out phenomena. The solubility of the dyes was reduced due to presence of the salts and thus adsorption of dyes increased. Also there was an increased in electrostatic interaction between biosorbent sites and dye molecules.

Badot and Crini (2010) reported that presence of salts in low or high concentrations did not significantly affect adsorption of methylene blue and Egacid orange on treated wood shavings. The little change in adsorption was explained by the fact that there could have been competition between dye cations and salts cations for the active sites on the wood shavings’ surfaces. Zou et al.(2010) also studied effect due to presence of salts on adsorption of Natural Red on oxalic acid modified rice husk. Since most used salts in dyeing process contained Na+, Ca2+ and Cl-, the use of NaCl and CaCl2 was favoured as background electrolyte. The results indicated that adsorption efficiency of the adsorbent decreased with increase in NaCl or CaCl2 concentration. This observation was also explained by the competition for vacant site.

Considering the adsorption of dye from the textile wastewater three approaches could be possible. Firstly a low pH could have affected the effect of the salts or electrolytes in the solution. Low pH could have made the salts and electrolytes play a role in increasing the electrostatic interactions between the dye and rice husk. Thus more dye was adsorbed at low pH. Secondly there could have been the possibility of competition between dye cations and cations of salts for vacant active sites on rice husk at pH greater than pHZC. This would explain the low adsorption rate of the dye at higher pH. However it should be noted that even if there is competition between the dye molecules and electrolytes, adsorption of dye still occurred. A third possible case is that at higher pH than pHZPC electrolyte actions could have favoured dye adsorption. This would also explain why at pH 6.03, which is close to a neutral pH, the adsorption process was least.

4.10 Purity of sorbent

Since the rice husk was obtained directly from the mills, it contained a considerable amount of impurities. Washing and oven drying process might not have fully removed the impurities. This could have affected adsorption process by providing less adsorption site on the rice husk and the impurities might have contributed to increase the COD of the wastewater.