Milind R.Oidde 1, Julie Dutta', Snehal Jadhav 3. Abstract

INTERNA TIONAL CONGRESS ON ENVIRONMENTAL RESEARCH AT BITS PILANI GOA, 08-09 Comparative adsorption studies on Activated Rice Husk and Rice Husk Ash by using Methylene Blue as dye Milind R.Oidde 1, Julie Dutta', Snehal Jadhav 3 Abstract Activated Rice Husk (ARH) and Rice Husk Ash (RHA) were used as adsorbents for decolourisation. ARH was prepared from rice husk treated with nitric acid and RHA was collected directly from mill. There adsorption capacity was evaluated for the decolourisation of wastewater containing methylene blue. The effect of system variables such as ph, contact time, initial concentration and adsorbent dose were investigated. The result shows that efficiency varies with the variation in adsorbate concentrations and adsorbent. Colour removal efficiency was found to be 88 % to 94 % at the dose of 20 g/l for ARH and 80 % to 95 % at the adsorbent.dose of 2.5 g/l for RHA. The studies were carried out at methylene blue concentration of 50 mg/l, 30 mg/l and 10 mg/l. On the basis of adsorption isotherm graphs, R-square values were determined and found to fit the adsorption data. The Linear, Langmuir and Freundlich adsorption isotherms are good fitted for the experimental data. Keywords: Rice husk, ARH, RHA, Methylene blue, Adsorption isotherms 1. Professor, Department of Civil Engineering, Bharati Vidyapeeth University College of Engineering, Pune - 411043, Maharashtra, India. Email-milindgidde@yahoo.co.in 2. PO students, Vasant Dada Sugar Institute, Manjari, Pune. 1. INTRODUCTION Pollution is a serious problem ever since sewage and industrial effluents are disposed into the water bodies and on land. The wastewater contain a wider variety of materials of both organic and inorganic nature including toxic substances and are usually discharged with or without treatment into the surface water such as rivers, streams, lakes or into oceans or on land or in sewers. One of the most common water pollutant is colour. They find there way into the water by the discharge of dyes from paper and pulp industries, textile industries, tanning 1

r: industries and many other industries. Colors in the water affect the nature of water, inhibit sunlight penetration and reduce photosynthetic action. Some of the dyes cause rapid depletion of dissolved oxygen affecting aquatic life adversely. Some of the dyes are toxic and carcinogenic. Thus, uses of dye contaminated water without any treatment may cause adverse effect on human health, domestic animals, wildlife and on the environment. So it is necessary to treat or remove color from the wastewater before discharge. Various treatment methods for removal of colour and dye are co-agulation using alum, lime, ferric chloride and ferric sulphate, oxidation, flocculation, ozonation, biological treatment, adsorption and membrane processes. Among these methods, adsorption method appears to offer the best prospect for overall treatment of colour removal. Use of Granulated Activated Carbon (GAC) or Powdered Activated Carbon (PAC) is more common. However, they are expensive and the regeneration or disposal of it has several problems. Thus, to make the process customer friendly, the use of several low cost adsorbents has been studied. Locally available natural material can minimize or avoid the concerns and significantly reduce treatment cost. Being India an agricultural country, a large amount of agro-waste is generated every year. The use of these agricultural solid wastes such as coir pith, banana pith, coconut shell, rice husk and straw, bagasses, saw dust, bamboo dust, groundnut shell, etc. as an adsorbent for removal of colour from wastewater will be customer friendly. Rice is the second largest produced cereal in the world. Worldwide, India stands first in rice area and second in rice production, after China. During milling of paddy about 78 % of weight is received as rice, broken rice and bran. Rest 22 % of the weight of paddy is received as husk. The total quantity of rice husk produced annually in the country is about 19.5 million tones. The possible utilization of rice husk and rice husk ash as an adsorbent for methylene blue dye or other colour from aqueous solutions can be investigated because of its effective adsorption properties. 2. MATERIALS AND METHODS a. Collection and Preparation of Adsorbents Rice husk and rice husk ash were collected from Roha rice mill situated in Raigad district of Maharashtra state. The rice husk was screened and washed with water to remove the dirt and was sun dried for a day. 2

Then the dried rice husk was soaked in 2.0 mol/l of nitric acid for an hour. It was then rinsed with distilled water for 2-3 times and oven dried at 105 C for 2 hours. The oven dried rice husk was ground and sieved through BSS-30 mesh size particle. The name given to the adsorbent was Activated rice husk (ARH). Another adsorbent rice husk ash (RHA) was directly used without treatment. Only fine ash was used as adsorbent. b. Preparation of dye solution Methylene blue is a heterocyclic aromatic chemical compound with molecular formula: C16HlSCIN3S. It was chosen because of its known strong adsorption onto solids. The dye is regarded as acutely toxic, but it can have various harmful effects. The structure of methylene blue is as given below- Figure 2: Structure of Methylene blue Methylene blue (S.D. fine chern. Ltd) was collected from laboratory. The stock solution of dye was prepared by dissolving 10 g of methylene blue in 1000ml of distilled water. The working solutions were prepared by serial dilution of this stock solution. c. Equipment r: The Spectrophotometer (HACH DR/2000) was used for methylene blue analysis. The rotary incubator shaker (model RIS, manufactured by Steelment industries), Microprocessor ph meter (CERT), Centrifuge machine (REMI R8C laboratory centrifuge), etc. were used for all adsorption experiments. d. Experiments The working solution of different concentrations 50 mg/l, 30 mg/l, and 10mg/1 were prepared by serial dilution of stock solution. The four factors initial concentration of dye, ph, contact time and adsorbent dose were varied. The batch adsorption tests were carried out by shaking 100ml working dye solution in a stoppered conical flask. The conical flasks were placed on rotary shaking machine for one hour at 150 rpm. The progress of adsorption during the experiment was determined by removing the flask after desired contact time, centrifuging and analyzing the supernatant solution spectrophotometrically at 610nm. Adsorption tests were performed at temperature 29 C ±2. The spectrophotometric readings were recorded and further calculations were done to see the removal efficiency of the adsorbents. The removal efficiency was calculated using following formulae: 3

% Removal efficiency (11) = (Ci-Ct) x 100 Cf Where, Ci is the initial concentration of methylene blue in solution and Cf is final concentration of methylene blue in solution. Adsorption isotherms were used to model colour adsorption. The adsorption isotherms were tried to fit to the experimental adsorption data. The isotherms used in this study were: i. Linear Adsorption Isotherm q=k*ce 1 Where, q is the amount of methylene blue adsorbed on adsorbents, Ce is the amount of Methylene blue present in the solution and K is the constant given by the slope. For Linear adsorption isotherm, graphs of q vis Ce were plotted. ii. Langmuir Adsorption qm* K* Ce q = -------------- 1 + K*Ce Isotherm... 2 Where, q is the amount of methylene blue adsorbed per unit weight of adsorbent, qm is the maximum amount of methylene blue adsorbed per unit weight of adsorbent, K is the adsorption equilibrium constant and Ce is the concentration of methylene blue in liquid phase at adsorption equilibrium. Evaluation of the coefficients qm and K can be obtained using linearized form of equation 2 asliq = (l/b*k) (lice) + lib.3 In the case of Langmuir adsorption isotherm, graphs of II q vis lice were plotted. Isotherm iii. Freundlich Adsorption q = K*Ce l / n... 4 Here, K is the measure of the capacity of the adsorbent (mass adsorbatel mass adsorbent) and n is a measure of how affinity for the adsorbate changes with changes III adsorption density. Evaluation of the coefficient K and n can be accomplished using linearized form of equation 4. log q = log K + (lin) log Ce 5 For Freundlich log Ce were plotted. isotherm, graphs of log q vis The adsorption isotherms were tried to fit to the experimental adsorption data. Linear regression analysis was conducted to determine the goodness of fit for the different adsorption isotherms. For Linear adsorption isotherm, a plot of q vis Ce was plotted. Linear regression analysis was performed to determine whether the given isotherm is a good fit for experimental adsorption data. Based on the R-square values, the isotherm that explains the experimental adsorption data the best was selected. 3. RESULTS AND DISCUSSIONS A. Effect of ph 4

a) For different concentrations The graph (a) shows that as ph increases the removal efficiency increases. Considering 50 mg/l of methylene blue solution 93 % efficiency is achieved at ph 11 for ARH. At ph 7, 78 % efficiency is achieved. From graph (b), the maximum efficiency is 91 % at ph 7 for RHA. Therefore by observing both the graphs ph at 7 was optimized for solution having concentration 50 mg/l of methylene blue. For concentration 30 mg/l of solution, the efficiency is 87 % for ARH and 96 % for RHA at ph 7. Thus colour removal efficiency is quite high at ph 7 and as the discharge standard for wastewater is neutral ph at 7 was optimized for concentration 30 mg/l of solution. From graph (a) and graph (b), the efficiency is 88 % and 79 % at ph 7 for concentration 10 mg/l of methylene blue for ARH and RHA. After ph 7 the efficiency is nearly equilibrium. Therefore for all concentrations of methylene blue solution the ph is optimized at 7. b) For different adsorbents By comparing graph (a) and graph (b) we have observed that at low ph i.e. 3 the efficiency is 91 to 93 % for RHA whereas for ARH it is only 36 to 73 %. Considering all concentrations of solution, at ph 7 the efficiency is 78 % to 88 % for ARH and 79 to 96 % for RHA If the solution is acidic and have ph 3 to 5 then RHA can prove good adsorbent for all concentration. And if the nature of solution is neutral or alkaline and the concentration of solution is 10 mg/l then, ARH is applicable. For high concentration of solution RHA is a good adsorbent even if the ph is low or high. B. Effect of Contact Time a) For different concentrations From graph C and D for concentration 50 mg/l of methylene blue solution it is observed that the efficiency increases with increase in contact time. The efficiency is 84 % at contact time 60 minutes for ARH and 91 % for RHA. After doubling the contact time i.e. 120 minutes the efficiency is reached upto 88 % for ARH and 94 % for RHA. The difference is about 3-4 % only. Therefore, contact time was optimized at 60 minutes for 50 rng/l of methylene blue solution. The efficiency is same for the concentration 30 mg/l of solution as that of 5

r-- concentration 50 mg/l of solution. Therefore optimum contact time is 60 min for concentration 30 mg/l of solution for ARH. From graph (d), the efficiency for 30 mg/l of solution is 92 % at contact time 40 minute and after 40 min it attains equilibrium. Therefore, optimum contact time is 40 min for RHA for 30 mg/l of methylene blue solution. The equilibrium attained at 40 min for concentration 10 mg/l of solution for ARH. Therefore, considering concentration 10 mg/l of solution, the optimum contact time is 40 minute for ARH. From graph (d), the efficiency is 91 % for concentration 10 mg/l of solution at 40 min. And equilibrium is attained after it. Therefore, the optimum contact time is 40 min for concentration 10 mg/l of solution for RHA. b) For different adsorbents The graph (c) shows optimum contact time for Activated Rice Husk and graph (d) shows that of Rice Husk Ash. By comparing both the graphs, it is observed that ARH can remove colour to about 84 to 89 % from the solution in 60 minutes and RHA can remove colour about 88 to 92 % from solution in 40 min. Thus, by considering all concentrations of solution RHA can prove a good adsorbent to remove colour in less time i.e. in 40 min. e. Effect of Adsorbent Dose a) For different concentrations From graph (e), it is observed that for concentration 50 mg/l of methylene blue solution the efficiency increases with increase in adsorbent dose. The efficiency is 65% to 93 % when the adsorbent dose varied from 2.5 g/l to 40 g/l for ARH. The efficiency is about 88 % for 20 g/l of ARH. From graph (f), it is observed that 95 % efficiency is achieved at the dose of 2.5 g/l of RHA. And equilibrium is attained after this. Therefore, optimum adsorbent dose is 2.5 gll of RHA and 20 g/l of ARH for concentration 50 mg/l of solution. From graph (e), as compared to concentration 50 mg/l, for 30 mg/l of solution efficiency about 86 % is achieved at dose 10 g/l of ARH. At 20g/1 dose the efficiency is 91 %. The difference is of 5 %. Therefore optimum dose can be considered as 10 g/l of ARH. From graph (f), 95 % efficiency is achieved at dose 5 gll of RHA. Therefore, optimum adsorbent dose for concentration 30 mg/l of solution is 10 g/l of ARH and 5 gll ofrha. For concentration 10 mg/l of solution the efficiency is 92 % for 5 g/l of ARH as shown in graph (e). The increase in efficiency is about only 1 % after doubling the adsorbent. While from graph (f), it is observed that the efficiency is about 93 % at dose 10 g/l of RHA. Highest efficiency is 6

about 97 % at dose 20 g/l of RHA. Therefore, optimized adsorbent dose is 5 gll of ARH and 10 g/l of RHA for concentration 10 mg/l of solution. b) For different adsorbents The removal efficiency form 88 % to 94 % is achieved at the dose of 20 g/l of adsorbent having concentration of 50 mgll, 30 mg/l and 10 mg/l. For low concentration of methylene blue (i.e. 10 mgll), 92 % efficiency can be achieved for 5 g/l of ARH dose. Considering all concentration of methylene blue, optimum adsorbent dose is 20 g/l of ARH. The removal efficiency from 80 % to 95 % is achieved at the dose of 2.5 gll of RHA for all concentration of methylene blue. With increase in adsorbent dose the efficiency up to 98 % can be achieved. To minimize the volume of waste, it is necessary to use less amount of adsorbent. Also the efficiency is high at low adsorbent dose. Therefore, the optimum adsorbent dose is 2.5 g/l of RHA. By comparing graph (e) and graph (f), it is observed that for high concentrations that is 50 mg/l and 30 mg/l, RHA can prove good adsorbent. But for low concentration of 10 mg/l of solution ARH is a good adsorbent. By considering all concentrations of solution RHA can prove to be good adsorbent. D. Application of Adsorption isotherm The R-square values of the linear regression performed were used to determine whether the isotherm was a good fit for the given experimental adsorption data. The R-square value close to 1 indicates a good fit by the model for the given experimental data whereas R-square value near 0 indicates that the model is not a good fit for the given experimental data. Linear Adsorption Isotherm From the Graph No.1 and 4 for adsorption data, it can be seen that the R2 values are greater than 0.7 for adsorption of methylene blue on ARH and RHA. These indicate that the linear adsorption isotherm is good fit for experimental adsorption data. Langmuir Adsorption Isotherm Langmuir adsorption isotherm graph is plotted with l/q vis lice. Trend lines for the adsorption data of different concentration of methylene blue using different adsorbents are plotted in graph (2) and graph (5). The linear regression was conducted using plot liq vis lice, it was found that R2 values are closer to 1 (i.e. 0.9), indicating that the Langmuir adsorption isotherm is a good fit for both of the adsorption data. Freundlich Adsorption Isotherm From graph 3 and 6 the Freundlich adsorption isotherm can be said to be good 7

.~ fit for the given experimental adsorption data, since the linear regression of log q vis log Ce gave R2 values in the range of 0.89 to 0.92 for the different concentration of methylene blue with different adsorbents. The activated rice husk and rice husk ash can be used as good adsorbent for selected effluent having specific concentration of adsorbate (colour/organic matter). E. Application of adsorbents for different effluent samples To understand the actual application of this adsorption method on the results obtained the experiments were conducted on different waste water samples collected from different industries. The ARH did not show good affinity towards adsorption for all effluent. But for some of the effluent (sample 5) shows good removal efficiency. Activated rice husk is effective for organic dyes as it showed 86 % to 94 % efficiency. The removal efficiency for sample 3 is 22 % to 24 %. The treatment with other samples shows negative results as activated rice husk releases colour into the effluent. The rice husk ash shows good removal efficiency for the sample 1, sample 3 and sample 5 i.e. 65 to 84 %, 45 to 53 % and 75 to 98 % respectively. The colour removal efficiency for other samples are very low i.e. 1 to 4 % only. Thus, it reveals that rice husk ash can be used as good adsorbent for organic colour. 5. CONCLUSION The result of present study clearly shows that acid treated rice husk and RHA is effective in removal of methylene blue and can provide an economical solution for removal of such colour from the aqueous solution. 88 % to 94 % colour removal efficiency can be achieved at the dose of 20 g/l of ARH having methylene blue concentration of 50 mg/l, 30 mgll and 10 mg/i. 80 % to 95 % colour removal efficiency is achieved at the adsorbent dose of 2.5 g/l of RHA for methylene blue concentration of 50 mg/l, 30 mg/l and 10 mg/i. With increase in RHA dose the efficiency increased upto 98 %. RHA also shows good efficiency in less time than that of ARH. The efficiency varies with the variation in adsorbate concentrations and adsorbent dose. From the results of the experiments conducted in present investigation, it is evident that experimental adsorption data for the adsorption of colour in this research can 8

be explained by more than one adsorption isotherms. The result shows that the R- square values are closer to 1 for all adsorption isotherm plots. Thus, Linear, Langmuir and Freundlich isotherm models are good fitted to the experimental data. Thus full utilization of agro-waste and treatment of wastewater is one of the good prospective for good environment. The rice husk can be proved as good, effective and eco friendly adsorbent. 6. REFERENCES 1. Aydun A Haluk and Orner Yavuz, Removal of acid red 183 from aqueous solution using clay and activated carbon, Indian Journal of Chemical Technology., 11 (01), 89-94, (2004) 2. Balasubramanian M. R. and Muralisankar 1., Utilisation of fly ash and tea-waste ash as decolourising agents for dye effluents, Indian Journal of Technology, 25, 471-474, (1987) 3. Dhar N.R., Khoda A.K.M.B., Khan A.H., Bala P., Karim M.F., A study of Effects of Acid Activated Saw dust on the removal of different dissolved Tannery Dyes (Acid dye) from aqueous solution, Journal of Environmental Science and Engg., 47 (02), 103, (2005) 4. Gokarm A.N., S. Mayadevi, Gawande Sunayana, Jacob Nalini, A cost effective sorbent for decolorisation of treated spent wash, 8 th Joint Convention of Three Association- 2005, 137 5. Gokarn A. N., Mayadevi S., Active charcoal from Agro-wastes for colour removal of treated spent-wash, D.S.TA. Part 1" B1-B7, (2002) 6. Gokam A. N., Oswal Namita, Sankpal Narendra, Studies on the use of Natural Polyelectrolytes for treatment of distillery effluent, DSTA, Part I, BI-BI0, (1999) 7. Jadhav D. N. and Vanjara A.K., Adsorption equilibrium study: Removal of dyestuff effluent using sawdust, polymerized sawdust and sawdust carbon-i, Indian Journal of Chemical Technology., 11 (03), 194, (2004) 8. Jadhav D.N. and Vanjara A.K., Adsorption kinetics study: Removal of dyestuff effluent using sawdust, polymerized sawdust and sawdust carbon-ii, Indian Journal of Chemical Technology., 11, 42, (2004) 9. Jyoti D. Mane, Shweta Modi, Swati Nagawade, Phadnis S.P., Bhandari V.M., Treatment of spentwash using chemically modified bagasse and colour removal studies, Bioresource Technology, 97, 1752-1725, (2006) 10. Khattri S D and Singh M K, Adsorption of basic dyes from aqueous solution by natural adsorbent, Indian Journal of Chemical Technology, 6 (03), 112, (1999) 11. Mall 1.D. and Upadhyay S.N., Removal of basic dyes from waste water using boiler bottom ash, Indian Journal of Environmental Health., 37 (1), 1, (1995) 12. Manaskom Rachakornkij, Sirawan Ruangchuay and Sumate Teachakulwiroj, Removal of reactive dyes from aqueous solution using bagasse fly ash, Journal of Science Technology., 26 (1),13-24, (2004) 13. Ramteke D.S., Wate S.R. and Moghe c.a., Comparative adsorption 9

studies of Distillery waste on Activated carbon, Indian Journal of Environmental Health, 31, 17, (1989) 14. Sarioglu M. and Atay U.A., Removal of Methylene blue by using biosolid, Global NEST Journal, 8 (02), 113-120, (2006) 15. Singh D. K. and Srivastava Bhavana, Basic dyes removal from waste water by adsorption on rice husk carbon, Indian Journal of Chemical Technology,133, 03 (2001) 16. Stephen Inbaraj B. and Sulochana N, Basic dye adsorption on low cost carbonaceous sorbent - Kinetic and equilibrium studies, Indian Journal of Chemical Technology., 201, 05 (2002) Optimum ph for Adsorbent Activated Rice Husk (ARH),., 100 T'""~~~~""""~"""-"""-~--"""""""""''''.~ 80t---------~~~~~~~~~~~~ IE ~ w+---~~~----~~--------------~ > o ~ 40+-~~--~L---------------------~ 0:: ~ 20+---~----_r----,_----r_--~----~ 2 5 ph 7 9 II -It- Concentration 01 methylene blue SOmgJ1 -+- Concentration 01 methylene blue 3OmgJ1 -->+-Concentration 01 methylene blue 10mgJ1,., c Optimum ph for Adsorbent Rice Husk Ash (RHA) loor-~-';:~~~===:~~l ; W+---~T7~--------~~----~~----~ 'u s~ W'~--~-----------------------------4 > o E ~ 40+-~------------------------------~ ~ 20+-----~----~----r_----r_----r_--~ 2 3 5 ph 7 9 II Concentration 01 methylene blue SOmgJ1 -+- Concentration 01 methylene blue 3OmgJ1 Concentration 01 methylene blue 1OmgJ1 Graph (a): Optimum ph for Activated Rice Husk Graph (b): Optimum ph for Rice Husk Ash ~ 95 e.9! 90 ~ 85-80 ~ 75 & 70 ~ 65 Optimum contact time for Activated Rice Husk»> -.»: ~.r > -: /..--r" V/ 10 20 40 60 120 Time in minute Concentration of methyiene blue 50mgll -..- Concentration of methylene blue 30mgll ~ Concentration of methylene blue 10mgJl ~ 240.. 100 Optimum Contact time for Rice Husk Ash (RHA) 1:-... c: U 90 w ;; >0 E ---.»---- ~~..- W ~./.,. 70 10 20 40 W 120 240 Tlmoln mln -4.Concentration of methylene blue SOmgll Concentration of methylene blue 30mgll Concentration of methylene blue 10mgll Graph (c): Optimum contact time for Activated Rice Husk Graph (d): Optimum contact time for Rice Husk Ash I J 100 90 Optimum Adsorbent dose of Activated Rice Husk 80...--....,. 70... 60 I 2.5.>: ~.~-..",".....~.'.'"....:..~~--- 20 40 5 Adsorbent ~s. In gmll Concentration of methylene blue SOmglL -.- Concentration of methylene blue 30mg/L ~ Concentration of methylene blue 1Omg/L Graph (e): Optimum adsorbent dose for ARH I Optimum Adsorbent dose of Rice Husk Ash 100..~~~ ~~ ~, i 90 t------7"-=~-----i I 80+-~~--------------------------~... 70+----- ----~------ ----------~ 2.5 5 10 20 AdsOf"bent dose in gml1 _Concentration of meth)'tene blue 50mgll -.- Concentration of meth)'tene blue 30mgIL -i<t- Concentration of meth~ene blue 10mgll Graph (0: Optimum adsorbent dose for RHA 40 10

./ For all graphs the legend are same as given bellow: Concentration of methylene blue is 50mg/l Concentration of methylene blue is 30mg/l Concentration of methylene blue is 10mgll " - Linear!concentration of methylene blue is 10mg/ll - Linear Concentration of methylene blue is 30mg/l - Linear (Concentration of methylene blue is 50mg/l) 14.000 Linear adsorption isotherm for ARH Langmuir isotherm for ARH R" 0.9933 12.000 -. " 5.000./' R' = 0.6879 R' =0 9313 10.000 4.000 He = 0.9901 / R' = 0.9972 8.000.,. 3.000 / /".: 6.000!! I 2.000 /...... -: 4.000 1.000, I / 2.000 ~./. 0.000 1 /'-/ / 0.000 0.000 5.000 10.000 15.000 20.000 0.500 1.000 1.500 2. 00 _1.008 COO I Graph I: Linear plot for ARH Co Freundlich isotherm for ARH 1/Ce Graph 2: Langmuir plot for ARH Linear adsorption isotherm for RHA -0 R' 0.8967 R" = -~ 0.893 R.. 0.9746./ ",. /../.c / ~ ].., ~v' 7..,./ 1:' <T 20.000. 15.000 10.000 5.000 R' = 0.6103 R? ': 07565 R' = 0.9131 0.000 +----'-=;::;.<:-'---+---~-~--~-_j o. 2.000 2.500 3. -5.000 -'-- ---1 log Ce c. Graph 3: Freundlich plot for ARH Graph 4: Linear plot for RHA Langmuir isotherm for RHA ----4600. ------------------..-- ----. I n, R' 0.8429 ~O,~~R~,~=~02.9~46~7-------~--~.. / R'=0.9498 / ~OO~------------~~------~ ---2~~------_,~~----~ / -0. Freundlich isotherm for RHA -------... -- --..1,500 R' = 0.8429 Rl:;; O.g4G7 R'= 0.886 O.00. "'- -l.q!&..~h;o"' Inn 1.000 1/Ce 2.000..3.000 4. 00 "--~~-~-1.. 000-'-~~-~-~~ log Co.. Graph 5: Langmuir plot for RHA Graph 6: Freundlich plot for RHA 11