REMOVAL OF DYES FROM WASTEWATER BY ACRYLIC ACID GRAFTED PINE CONE: EFFECT OF TEMPERATURE ON GRAFTING AND ADSORPTION PERFORMANCE Ngema, S.L., Ofomaja, A.E., Naidoo, E.B., Biosorption and Water Treatment Research Laboratory, Department of Chemistry, Vaal University of Technology P. Bag X021, Vanderbijlpark, 1900, E-mail: bobby@vut.ac.za ABSTRACT In this research, the effect of temperature on the grafting of acrylic acid onto pine cone and its adsorption efficiency for the removal of cationic dyes from aqueous solution was examined. The grafting experiments were performed using KMnO 4 at an established optimum concentration of 0.020M as radical initiator for the grafting of acrylic acid on to pine cone powder at temperatures ranging between 50 and 90 C. The efficiency of KMnO 4 as a radical initiator was followed by measuring the change of oxidation reduction potential (ORP) and the change in solution ph. 10 g of the biosorbent was added to the 0.020M KMnO 4 at different temperature varying from 50 C, 70 C, 80 C, and 90 C. Acrylic acid solution was mixed with the activated pine cone to initiate the polymerization reaction. The efficiency of the grafting process was followed by monitoring the total monomer conversion, the percentage grafting, and the rate of homopolymer formation and rate of graft polymerization. The results obtained showed that it was possible to follow the efficiency of KMnO 4 initiator by monitoring the ORP and the change in solution ph. Maximum values of ORP (67 mv) and change in solution ph (0.0049) were obtained for the optimum KMnO 4 concentration at 90 C and a total monomer conversion of 91.97% was obtained. As the temperature of grafting reaction increased, the total monomer conversion and weight of grafted product increased then decreased afterwards. These results indicate that increasing the temperature of the grafting reaction affects the formation of the grafted product, grafting efficiency and the formation of the homopolymer. The Dye removal was also found to improve with increase temperature of grafting. Keywords: Temperature; Acrylic acid; Oxidation reduction; change in solution ph
INTRODUCTION Although biomaterials applied for adsorption dyes and metal pollutants from aqueous solution have been shown to be very effective adsorbents, some draw backs are still encountered with these materials. For example, mechanical strength, degradation properties and availability of sufficient amounts of the desired functional groups needed for the removal of targeted pollutants are some limitations of the application of biomaterials in adsorption. In this regards, researchers have investigated the use of grafting vinyl functional polymers onto biomaterials with the aim of improving their mechanical and degradation properties as well as increase the amounts of the desired functional groups on their surfaces. The product of grafting copolymerizing these vinyl monomers onto biomaterials are composites which may be used for membrane production [1]. However, the new biomaterials must not only be mechanically but also biologically compatible. Adequate surface modification of biomaterials can enhance its biocompatibility without sacrificing its key physical properties. Temperature is one of the important factors that control the kinetics of graft copolymerization. It is known that as temperature of the grafting system increases, the rate of monomer diffusion through the process to the backbone also increases [2]. Samal et al. [3] reported that in the grafting of methyl methacrylate on silk, the graft yield increases significantly with increasing temperature due to greater swelling of silk, and a corresponding enhanced rate of diffusion of the monomers in the vicinity of silk, whereas sun et al. [4] attributed the same phenomenon to increased thermal decomposition rate of initiator and the initiator efficiency in producing free radicals on base polymer with increasing temperature, resulting in increased polymer macro radicals concentration, and thus enhanced the graft polymerization. In the present study, the effect of temperature on the graft copolymerization of acrylic acid onto pine cone powder was examined. The grafting process will be carried out using KMnO 4 initiator in the presence of dilute nitric acid at temperatures between 50 and 90 o C. The variation in surface properties such as surface negative charge and MnO 2 deposited on the pine cone surface with temperature will be observed. Finally the grafting parameters at different temperatures will be determined to predict the effect of temperature on grafting parameters and rates. METHODS Sample preparation Pine tree cones were collected from a plantation in Sasolburg, Free State, South Africa. The cone was washed to remove impurities such as sand and leafs. The washed cones were then dried at 90 o C for 48 hr in the oven. The scales on the cones were removed and crushed using a pulveriser. The pine cone powder was then sieved and particles between 90 and 45 µm were collected and used for analysis.
Grafting procedure Grafting was determined by mixing 20 g of the treated pine cone with 750 cm 3 of 0.020 mol/dm 3 KMnO 4 solutions at room temperature for 45 min. The pine cone powder was filter and washed with distilled water and dried. 10 g (m o ) of pine cone powder was transferred into 500 cm 3 round bottom flask containing 10cm 3 acrylic acid in 125 cm 3 of hexane. Graft co-polymerization was carried out by mechanical stirring for 2 hr at 50, 70, 80 and 90 C respectively. The mixture was then filtered on a Buchner funnel, washed with 50 cm 3 acetone dried and weighed (m 1 ). To remove unreacted chemical and homopolymer, the resulting pine cone (m 1 ) was mixed with 250 cm 3 of hot water, stirred for 2 hr at room temperature. The washed solid was then stirred in 0.1 mol/dm Na 2 CO 3 solution, filtered on a Buchner funnel and washed with 10 cm 3 acetone before drying to constant weight (m 2 ) at 70 C. The solution ph and ORP were measured when pine cone powder was contacted with KMnO 4 solution and after stirring for 45 min using a ph meter Hanna HI2550 model ph meter. Determination of MnO 2 deposited The amount of MnO 2 deposited onto pine cone powder was determined by adding 10 cm 3 of 0.2 mol/dm 3 oxalic acid and 10 cm 3 of 4 mol/dm 3 sulfuric acid to the pine cone powder treated with potassium permanganate in a conical flask. The mixture was gently heated to about 60 ºC and then titrated against a KMnO 4 solution of 0.05 mol/dm 3. Vx0.2. x100 The amount of MnO 2 deposited = (meq/100g) (1) W Where, V is the volume of KMnO 4 equivalent to the MnO 2 in the sample and W is the weight of the sample used. Surface Negative Charge One-half gram of pine cone powder, which had ph values < 3.0, was suspended in 25 cm 3 of 0.10 mol/dm 3 NaOH and stirred at 300 rpm for 16-20 hr in a glass stopped Erlenmeyer flasks. The flasks were kept stoppered during stirring to minimize the dissolution of carbon dioxide gas in the NaOH and the subsequent formation of Na 2 CO 3. The flask contents were filtered by vacuum filtration through Whatman #4 filter paper and 10 cm 3 of the filtrate was added to 15.0 cm 3 of 0.10 mol/dm 3 HCl. The addition of excess HCl prevented any possible adsorption of carbon dioxide by the base and was particularly important if the solutions were required to stand for extended time periods before analysis. The solution was titrated with 0.10 mol/dm 3 NaOH until an end point. The results were expressed in mmoles H + neutralized OH - per gram of pine cone powder. RESULTS AND DISCUSSION Determination of MnO 2 deposited The grafting of vinyl monomers using KMnO4 as initiator proceeds by the formation of MnO 2 from the MnO 4 - ions under acidic conditions.
MnO4 3e 4H MnO2 2H 2O E o = 1.68 V (2) The MnO 2 produced can either react with more HNO 3 in solution to produce NO 3. radical which initiates a radical site on the biomaterial or be deposited on the biomaterial surface. Mn 4 HNO 3 Mn H NO 3. 3.. 3 PineCone O HNO3 PineCone OH NO (4) (3) Fig 1 shows the plot of MnO 2 deposited on the pine surface at different temperatures when acrylic acid was grafted on pine cone using 0.020 mol/dm 3 KMnO 4 as radical initiator. Higher amounts of MnO 2 were deposited at lower grafting temperatures than for higher temperatures. As temperature increased from 50 to 90 o C, the amount of MnO 2 deposited decreased from 2.10 to 1.09 meq/100 g. These results imply that increasing grafting temperature will favor Eq. (3) i.e., the conversion of MnO 2 to Mn 3+, H + and NO 3 radical. It has been shown that MnO2 deposited on the surface of the biomaterials during grafting may also initiate radical sites on monomers and increase polymerization. Fig. 1: Relationship between MnO 2 deposited on pine cone and Temperature of graft copolymerization. Surface negative charge The aim of grafting acrylic acid onto pine cone was to increase the amount of carboxylic acid functional groups on the pine material. Therefore, as the carboxylic acid functions increased, they will also be an increase in surface negative charge. The results of the surface negative charge experiment revealed that the surface negative charge of the raw pine cone was found to be 0.0012 mol/g but when acrylic acid was grafted onto the
pine cone surface at 50 o C, the surface negative charge was increased to 0.0020 mol/g (Fig. 2). When the temperature of grafting was varied between 50 and 90 oc, the surface negative charge was found to increase from 0.0020 to 0.0027 mol/g. The implication of this result is that the amount of grafting of vinyl monomer containing carboxylic acid function increased as the temperature of grafting increased. Fig. 2: Relationship between surface negative charge of pine cone and Temperature of graft copolymerization. Effect of temperature on grafting parameters The grafting parameters monitored in the graft copolymerization of acrylic acid monomers onto pine cone includes the monomer conversion, homopolymer conversion, percentage grafting, rate of polymerization and rate of grafting. These parameters were monitored at different temperatures so as to observed the effect of temperatures on the grafting parameters. The results of the effect of temperature on grafting are shown in Table 1. It will be observed that the total monomer conversion was found to increase from 80.14 to 83.52 %. This result indicates that as temperature increased from 50 to 90 o C, the percentage of monomer in solution converted was found to increase. Monomer molecules may be converted in two ways; they may be grafted onto the pine cone and increase in their chain length, or they may be polymerized onto another monomer molecule carrying an active radical site. The latter is usually referred to as homopolymer and may increase in chain length by further addition of monomer units, while the latter is referred to as the grafted copolymer (which is our targeted product). The result for homopolymer conversion displaced in Table 1. The result shows that homopolymer conversion reduced with increasing temperature from 50 to 90 o C. Therefore, the amount of unwanted product (side reaction product) decreased with
Table 1: Relationship between Grafting parameters and reaction temperature. Temp. o C Total monomer conversion Homopolymer conversion % Grafting Rate of polymerization Rate of grafting 50 80.14 62.24 20.80 0.0158 0.0103 70 81.45 60.06 22.45 0.0159 0.0105 80 83.30 59.61 24.85 0.0161 0.0107 90 83.52 58.28 26.49 0.0161 0.0108 Rate of polymerization: mol/dm 3 min; Rate of grafting: mol/dm 3 min
increasing temperature. The implication of this result is that increase in temperature increase the conversion of MnO 2 (Mn 4+ ) to Mn 3+ (in Eq.(3)) which brings about the formation of NO 3 radical in the presence of dilute HNO 3 (in Eq.(4)) and consequently the formation of radical sites on the pine cone which leads to monomer grafting. The rate of polymerization and the rate of grafting were also determined at different reaction temperatures. The rate of polymerization is defined at the rate at which monomer molecules are grafted onto the pine cone and also to the homopolymer chain, while the rate of grafting measures the rate at which monomer molecules are grafted to the pine cone biomaterial. The results indicate that in general the rates of polymerization and grafting increased with temperature from 50 to 90 o C. As reaction temperature increased the percentage homopolymer conversion reduced and this can only be attributed to the increased rate of grafting of monomers to the biomaterial. It will be observed that as the temperature of the grafting reaction was increased from 50 to 90 o C, the percentage grafting increased from 20.80 to 26.5 %. This result indicates that the grafting temperature affects the percentage grafting of monomer onto the pine cone positively. This increase in percentage grafting has been attributed to difference reasons by several authors including the increased migration of the monomer molecules to the biomaterial surface or the swelling of the pine cone particles allow for increase surface contact. But in this study, the increase in percentage grafting is attributed to increase in the conversion of MnO 2 to Mn 3+. Methylene blue removal capacities The methylene blue removal capacities of the grafted materials produced were compared in a plot of percentage methylene blue removal versus sample type in Fig. 3. Fig. 3: Relationship between percentage methylene blue removal and biomaterial type. The result shows that the percentage methylene blue removal increased with the grafting temperature. The biomaterial grafted at 50 oc gave the least percentage
methylene blue removal while that grafted at 90 oc gave the highest. This may be due to the higher surface negative and the higher conversion of MnO2 into Mn 3+ yielding higher amounts of radical sites for grafting. Conclusion The results obtained showed that it was possible to follow the efficiency of KMnO 4 initiator by monitoring the ORP and the change in solution ph. Maximum values of ORP (67 mv) and change in solution ph (0.0049) were obtained for the optimum KMnO 4 concentration at 90 C and a total monomer conversion of 91.97% was obtained. As the temperature of grafting reaction increased, the total monomer conversion and weight of grafted product increased then decreased afterwards. These results indicate that increasing the temperature of the grafting reaction affects the formation of the grafted product, grafting efficiency and the formation of the homopolymer. The Dye removal was also found to improve with increase temperature of grafting.
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