ISSN: KEYWORDS: Nickel; biosorption; mine effluent drainage; green alga, adsorption kinetics

Transcription

1 ISSN: A COMPARATIVE STUDY OF THE KINETICS OF NICKEL BIOSORPTION BY RIVER GREEN ALGA OBTAINED FROM DIFFERENT ENVIRONMENTS; MINE EFFLUENT DRAINAGE STREAM AND NATURAL RIVER SYSTEM. C. Mahamadi a*, N. Torto b a,* Chemistry Department, Bindura University of Science Education, P. Bag 1020, Bindura, Zimbabwe. b Department of Chemistry, University of Botswana, P. Bag UB 00704, Gaborone, Botswana. ABSTRACT: A basic study on nickel biosorption properties of nonliving, dried river green alga sampled from a stream polluted with mine effluent discharges and from a natural river source (Bindura, Zimbabwe) has been investigated. The effect of alga dose, solution ph, temperature, and initial nickel concentration on nickel biosorption was studied for a nickelcobalt mixture. Maximum uptake capacities were recorded using 1.0 g of biomass and a 250 mg L -1 nickel solution at uncontrolled ph and these were about and mmol L -1 for samples from the mine drainage stream and natural river system respectively. Nickel biosorption by green alga occurred rapidly, with 80% of total uptake occurring within 50 minutes. The uptake was found to greatly depend on initial ph, uptake being apparently negligible at low ph values and increasing with an increase in ph.the adsorption was described by a pseudo-second order rate model and the rate constant and equilibrium sorption capacity are reported. KEYWORDS: Nickel; biosorption; mine effluent drainage; green alga, adsorption kinetics 1. INTRODUCTION Technological processes involved in extraction of metals and their productive cycle generate significant amounts of effluents contaminated with metal cations. These include mining, mineral processing and extractive metallurgical operations, electroplating, painting, dying, surface treatment industry and many others. The environmental damage produced by this type of effluents has created a sharp awareness of environmental crisis and the need for achieving a balance with nature.

2 Some metals (Fe, Zn, Cu, Ni) can be harmful above certain limits and certain metals (Hg, Pb and Cd) are toxic and very harmful to living beings [1]. These pollutants are nonbiodegradable, prompting the need for their removal from effluents or, at least, the reduction in their concentration to limits allowed by current regulations. To this effect, a number of methods exist, including precipitation, evaporation, electroplating, ion-exchange, and membrane processes [2]. However, besides being expensive, these processes have their own shortcomings, such as limited tolerance to ph change, incomplete removal of metals, moderate or no metal selectivity, and very high or low working levels of metals [3]. Biosorption and bioaccumulation of heavy metals by different biomasses (such as algae, fungi, and bacteria), dead or alive, can be considered as alternative technology in industrial waste treatment [4]. The technology is based on the ability of biological materials to accumulate heavy metals from wastewater by either metabolically mediated, physicochemical pathways of up-take. Various bio-materials have been examined for their bisorptive properties and different types of biomass have shown levels of metal up-take high enough to warrant further research [4-8]. In a recent paper, we reported on the modelling of equilibrium and kinetic properties of cadmium biosorption by river green alga and water hyacinth weed [9]. In the current work, an investigation on the comparative uptake of nickel by green alga obtained from different sources was carried out by characterising the sorption kinetics of inactive biomass. Generally samples of alga were placed in known volumes and concentrations of nickel in a nickel-cobalt mixture. The alga growing in the mine effluent stream should be exposed to high levels of both nickel and cobalt, and many other metals. Cobalt was chosen to be the matrix element because it is also produced together with nickel at the mine. The influence of different parameters on the kinetic and equilibrium of nickel biosorption processes from a nickel-cobalt mixture were evaluated. These included sorption time, initial ph, temperature and initial nickel concentration. 2. EXPERIMENTAL 2.1. Materials: The green river algae, was collected from two different sources; one source being a shallow stream that periodically receives inflows of mine effluent loaded with elevated levels of heavy metals (mine effluent source), and the other source being a naturally flowing river with minimum level of heavy metals (river source). Trojan Nickel mine, Bindura Zimbabwe, which produces nickel and cobalt at international production rate, also operate a smelter and extractive metallurgy. Effluent periodically released from the smelters flows along a shallow stream into Pote river. Samples of alga were collected from the effluent stream and the control sample from Pote river, up-stream. The alga was washed three times with running water and once with de-ionized water. After washing, it was oven-dried at 65 0 C for 24 Hr, crushed and ground with a motor and a pistol, sieved (size fraction of 2.5 mm), and stored in polythene bottles until use. 2166

3 2.2. Equipment Shimadzu AA 6601 F Atomic Absorption and Flame Emission Spectrophotometer, KDB temperature bath supplied by Memmert Holdings, Pvt Ltd, Stuart Scientific shaker and magnetic stirrer hot plate, Vacuum suction pump- KnF neuberger Labort Chemicals Chemicals used in this work were analytical reagent quality. NiCl 2.6H 2 O, CoCl 2.6H 2 O, NaOH, supplied by Associated Chemicals (Pvt) Ltd, HCl and HNO 3 supplied by Sky Labs (Lenasia, South Africa) 2.4. Methods For each of the following experiments, 1ml of sample was periodically removed for analysis, and ph was adjusted using 0.1 M NaOH and 0.1 M HNO 3. After immediate filtration using cellulose nitrate membrane filters under vacuum suction, the aliquots diluted to 50 ml and stored under refrigeration for eventual analysis by atomic absorption spectrophotometry. Calibration standards were matrix-matched by spiking with controlled amounts of cobalt to minimise interference effects Effect of temperature: A sample of alga (0.35 g) was added to nickel-cobalt mixture (100 ml, 250 mg L -1 ) at natural ph. The experiments were carried out in a constant temperature shaker bath at 298, 310 and 320 K respectively Effect of solution ph: A sample of alga (0.35 g) was added to the nickel-cobalt solution (100 ml, 250 mg L -1 ) at constant temperature ( o C). The experiments were carried out at ph 2.61, 3.66, and 4.81 in 100mL Erlenmeyer flasks placed on a magnetic stirring plate. PH adjustments were made using 0.1 M HNO 3 and 0.1 M NaOH Effect of Initial Metal Concentration: A sample of alga (0.35 g) was added to the nickel-cobalt solution (100 ml) at constant temperature ( C) and natural ph. The initial concentrations of the metal solution tested were 10, 100 and 250 mg L -1 of nickel Effect of Alga Dose; Samples of alga (1.0, 2.5, and 5.0 g) were added to nickel-cobalt solution (100 ml, 250 mg L -1 ) at constant temperature and at uncontrolled ph. 3. RESULTS AND DISCUSSION Pseudo-second order kinetic model The following equation was assumed in the kinetic treatment [10,11] t q t 1 = 2 kq e + t q e (1) where; k (g mg -1 min -1 ) is the pseudo-second order rate constant of sorption, q e (mg g -1 ) is the amount of metal ion sorbed at equilibrium and represents the metal uptake, q t is the amount of metal ion adsorbed at time t (min). Equation (1) shows that if second order kinetics is applicable, the plot of (t/q t ) against t should be linear, from which the constants q e and k can be determined. 2167

4 3.1. Effect of ph on nickel biosorption The results of the effect of initial ph of the solution on kinetics of adsorption are shown in Fig. (i). The corresponding values of k, q e, and the correlation coefficients, r 2, at several ph values of the nickel-cobalt solution are listed in Table (i). These values were averaged from three replicate experiments. Generally, the results show that significant nickel uptake occurred with increasing ph values. Similar results were reported for Cu (II)-biomass systems [1,2]. Working over ph 6.0 was avoided to prevent possible precipitation of nickel hydroxide. The minimum adsorption taking place at low ph values indicates competition of excess protons for the same binding sites on the algal cell wall [11]. As the ph increased, the ligands such as carboxylate groups in the river alga would be exposed, increasing the negative charge density on the biomass surface, thus enhancing the biosorption onto the cell surface. A comparison of the experimental maximum q t values for the alga from the river source (69.71 mg g -1 ) and alga from the mine effluent source (53.35 mg g -1 ) suggests that samples from the river showed greater absorption capacity. This could be explained in terms of the low ph environment (3.11) to which the alga growing in the stream receiving the mine effluent is exposed to, compared to the river source ph (6.98). A significant proportion of protons could be trapped within the alga biomass chemically and physico-chemically, making the alga generally acidic thus limiting the amount of available adsorption sites. Therefore the alga growing in low ph environment showed low adsorption capacity. Fig. (i). Kinetics of nickel up-take by river green alga at several ph values, at temperature of C, initial nickel concentration 250 mg / L, and alga dose 1.4 g / L. River source, ph 4.81: ( ); River source, uncontrolled ph: ( ); Mine effluent source, ph 4.81: ( ). 2168

5 Initial [Nickel] C i [mg L -1 ] PH M alga / g T / K Table (i): Kinetic parameters of nickel uptake by river green alga. q e (mg g -1 ) k (g mg -1 min -1 ) r x x x x x x x x x x x x Effect of Initial Metal Concentration The results obtained for the effect of initial concentration on algal metal-uptake are shown on Fig. (ii) and in Table (i). All fits showed good correlation coefficients. The results showed that the equilibrium biosorption capacity q e, increases with increasing initial nickel concentration. Samples from the river source showed greater adsorption capacity than those from the mine effluent source. The plots shown in the figures above indicate that the kinetics of absorption by alga from the mine effluent stream is fairly complex. The mine effluent should contain high levels of metal ions such as Na +, K +, Ca 2+, Mg 2+, Cu 2+, Ni 2+, Co 2+, Pb 2+, etc. Some of these metals could be adsorbed on the algal cell walls. Varying initial metal concentration sets up competing equilibria among the various metal ions and protons, the effect of which is reflected in the kinetic constant Effect of Alga Dose The results for the effect of green river alga dose, m alg, [g L -1 ], are shown in Fig. (iii). The equilibrium biosorption capacity, q e, and the rate constant, k, were determined using equation (4). The data showed very good correlation. Nickel uptake values increased with a decrease in alga mass. Generally the results show that biosorption kinetics of nickel by green river alga is fairly fast, with 80% of the total absorption taking place within the first 50 minutes. 2169

6 Fig.2 Kinetics of nickel biosorption by river green alga at C, 1.4 g L -1 alga dose, and at natural ph as function initial nickel concentration. 250 mg L -1, River source: ( ); 100 mg L -1, River source: ( ) ; 10 mg L -1, River source: ( ) ; 250 mg L -1, Mine effluent source: (x) ; 100 mg L -1, Mine effluent source: ( ) ; 10 mg L -1, Mine effluent source: ( ) Fig. (iii). Kinetics of nickel uptake by green river alga at several alga doses (Pote river source), C, initial nickel concentration 250 mg / L, and uncontrolled ph. Biomass dose, 1.0 g/l: ( ); 2.5 g/l: ( ); 5.0 g/l: ( ) Metal ion competition In a mixture, such as the stream water polluted with mine effluent discharges, contaminants have a synergistic effect towards one another in competing for binding sites. The synergistic interactive effect of the two metals on the biomass was evaluated using a modified Suzuki equation [12]; ' 1 $ 2U! C0 = k% " exp (2) & 1(! # RT which was linearised to the form: 1 ' 1 $ 2U! ln C 0 = ln % " + (3) K & 1(! # RT where 2U represents pair interaction energy and R is the universal gas constant. The quantity 2U is positive for repulsion and negative fro attraction. The data obtained from the plots of 1 C 0 against ln is shown in Table (ii). (1 "! ) 2170

7 Table (ii). Pair interaction energy (JK -1 mol -1 ) for multiple media effluent. C 0, mg L -1 Ni 2+ Ni 2+ + Co 2+ Natural river source Mine effluent source Natural river source Mine effluent source The data in Table (ii) obtained show that the pair interaction energy between the two pairs of metal ions on the green alga biomass is positive, thus suggesting that the two metals would not be effectively bound to the green alga biomass if present in the same waste effluent. Furthermore, the results show that this effect is more pronounced for alga biomass sampled from the stream polluted with mine effluent discharges. This could be due to greater repulsion caused by previously adsorbed ions from the effluent. CONCLUSION The biosorption kinetics by green alga is a first process, with 80% of total absorption taking place in the first 50 minutes. The process can be described by pseudo-second order model, assuming that the rate-limiting step may be chemisorption. The biosorption equilibrium capacity was found to depend on initial nickel concentration, ph, alga dose and temperature. The alga samples from the source polluted with mine effluent generally showed lower biosorption equilibrium capacity, compared to samples from a less polluted river source, suggesting strong influence of the chemical environment on the biochemical modification of the alga. The data confirm that mass transfer is important in determining the adsorption and that its relative significance depends on initial metal concentration. The results also show that competitive adsorption reduces the uptake of nickel by green alga in the presence of cobalt, and that adsorption is further reduced in the presence of other adsorbed ions for the mine effluent source. ACKNOWLEDGEMENT This research was supported by the International Foundation for Science, Stockholm, Sweden, through grant No. W/ to Mr C. Mahamadi. REFERENCES: 1. M.A. Antunes, S.A. Luna, C.A. Henriques, A.C. da Costa. Electron. J. Biotechnol., (2000) 2. J.T. Matheickal, Q. YU. Bioresource. Technol., 69(3), (1999) 3. F. Eccles. Trends in Biotechonol. 17(12), (1999) 4. F. Penche, A. Fraile, F. Gonzalez, M.A. Blazquez, J.A. Monoz, A. Ballster. Chemistry & Ecology. 21(1), (2005) 2171

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