International Journal of Enviornmental Science and Technology, Vol. 3, No. 2, Spring 2006, pp. 131-139
Lead remediation of contaminated water using Moringa Stenopetala and Moringa oleifera seed powder
L. M. Mataka, *E. M. T. Henry, W. R. L. Masamba and S. M. Sajidu
Chemistry Department, Chancellor College, University of Malawi, Zomba, Malawi
Received 29 October 2005; revised 7 March 2006; accepted 23 March 2006; available online 20 April 2006
*Corresponding Author, E-mail:firstname.lastname@example.org
Code Number: st06016
The increasing influx of heavy metals into water bodies from industrial, agricultural, and domestic activities is of global concern because of their well documented negative effects on human and ecosystem health. A recent study of streams in Blantyre and Zomba, Malawi revealed lead levels of up 0.118 mg/L, exceeding the World Health Organisation acceptable level of 0.01 mg/L. Our ongoing study on low cost effective heavy metal remediation techniques in developing countries has already demonstrated that Moringa oleifera, the well known source of natural water clarifiers, is effective in heavy metal detoxification of water. This paper presents the first reported use of a related species, the African moringa, Moringa stenopetala for lead detoxification and preliminary investigation of the interaction of the metal with the polyelectrolytes of M. oleifera and stenopetala. The potential of M. stenopetala for lead removal was tested by means of jar tests. With an initial lead concentration of 7 ppm, M. stenopetala seed powder, at doses of 0.50, 1.00, 1.50, 2.00 and 2.50 g/100mL, reduced the concentration of lead by 20.00 ± 0.00, 46.19 ± 2.06, 71.19 ± 2.06 and 89.43 ± 0.60 and 96.23 ± 0.12 % respectively. M. stenopetala was more effective than M. oleifera in removing lead from water (p=0.001 at 95% confidence level). For oleifera, lead levels decreased exponentially during the first 5 h. of the reaction and then equilibrium was established; for stenopetala, a linear decrease was observed. The pH of the mixture rose from 2.30 to a maximum of 2.53 and 2.57 and then fell to an equilibrium value of 2.30 and 2.29 for oleifera and stenopetala respectively. Lead removal was also affected by pH, ionic strength, and water hardness. Our results show that M. stenopetala has potential in lead remediation of contaminated waters. Further studies are being carried out on remediation of other metals and the mechanism of the metal moringa interaction.
Key words: Lead, Moringa oleifera, Moringa stenopetala, heavy metals, water pollution, remediation.
The United Nations Environment Programme’s (2004) Water Policy and Strategy identifies several water focal areas including fresh water scarcity, land based pollution sources, aquatic biological diversity, resource use and management, and knowledge and technology transfer in integrated water management. The Malawi State of the Environment Report notes that water degradation is a major environmental problem that threatens the health and well being of humans and ecosystems (Malawi Government, 2002). Improper disposal of various types of waste, deforestation, and poor agricultural practices that encourage soil erosion and deposition of sediments into the water bodies were identified as the major causes of water degradation in Malawi. Studies of Malawian urban water bodies and wastewater treatment plants revealed that some water quality parameters including heavy metals exceed the World Health Organization (WHO, 2004) acceptable limits (Matope, 2002; Banda et al., 2001 and Sajidu et al., 2005). Of particular concern are the high levels of lead in some of the water bodies. The Sanitation Master Plan for the City of Blantyre (Matope, 2002) reported lead levels of 0.73-0.96 mg/L in Mudi and Limbe streams and Banda et al., (2001) recorded 0.23 ± 0.00 mg/L lead for the Lunyangwa river basin, all above the WHO acceptable limits of less than 0.01 mg/L. Our recent quality inventory compilation of Blantyre streams (Limbe, Nasolo, and Mudi) and wastewater treatment plants (Limbe and Soche) recorded levels of lead from 0.027 to 0.118 mg/L with most values exceeding the WHO limit (Sajidu et al., 2005). Lead is a pollutant of global concern with well known toxic effects (Alloway and Ayres, 1990). Major sources of lead pollution in water are lead-acid batteries, lead pipes, solders, agricultural chemicals, lead mining, and vehicle exhausts especially where, as in Malawi, leaded petrol has not been completely phased out. Lead is a powerful neurotoxin and a range of pathological conditions is associated with acute lead poisoning, the most characteristic being cerebral oedema. Clean-up technologies for the removal of lead and other heavy metals from water include chemical coagulation using aluminium and ferric salts (Fatoki and Ogunfowokan, 2002) and cationic surfactants (Evans, 2003); physical precipitation using ion exchange and adsorption (Singh et al., 2001) and phytoremediation that includes rhizofiltration, phytostabilisation, phytoextraction (Lyte et al., 1998 and Lambert et al., 2003). However, water and wastewater treatment in Malawi does not include heavy metal removal or monitoring due to the high costs and/or the lack of technical expertise. Recently there has been increased interest in the subject of natural coagulants for treatment of water and wastewater in developing countries (Jahn, 1986; Ndabingesere et al., 1995; Sutherland et al., 1994; Gebremichael, 2004 and Henryet al., 2004). The tropical plants of the family of Moringaceae are amongst some of the natural coagulants that have been studied for clarification of turbid water. Moringa oleifera is the most widely distributed, well-known and studied species of the family Moringaceae because of its previous economic importance as a source of the commercially important ‘Ben oil’ and morerecently, as a multipurpose tree for arid lands and a source of water purifying agents for developing countries (Morton, 1991). M. oleifera is native to sub-Himalayan Northwestern India and Pakistan but the plant was distributed to other areas of tropical Asia in prehistoric times and to other parts of the world including Malawi during the British colonial era. M. stenopetala, often referred to as the African Moringa Tree, originates from southern Ethiopia and Kenya (Jahn, 1991). The food, fodder, water clarifying and medicinal uses of the Moringaceae, especially oleifera are well documented and the trees are recommended for live fencing, intercropping, and pollution control (Morton, 1991; Moges, 2004; Coote et al., 1997; Pratt et al., 2002; Williamson, 1975; Palgrave, 1983 and van Wyk and van Wyk, 1997). The water soluble Moringa seed proteins possess coagulating properties similar to those of alum and synthetic cationic polymers. Jahn (1981) first studied and confirmed the coagulating properties of Moringa seeds after observing women in Sudan use the seeds to clarify the turbid Nile waters. The active agents in Moringa extracts responsible for coagulation are dimeric cationic proteins with molecular weight of 13,000 Da and isoelectric points between 10 and 11 (Jahn, 1981 and Ndabigengesere et al., 1995). The mechanism of coagulation was suggested to be adsorption and neutralisation of charges, or adsorption and bridging of destabilised particles, the two assumed to take place simultaneously. Gassenschmidt et al., (1995) reported the isolation from M. oleifera of a flocculating protein of 60 residues with molecular mass of about 6.5 kDa, isoelectronic point above pH 10, high levelsofglutamine, arginine andproline with the amino terminus blocked by pyroglutamate, and flocculant capacity comparable to a synthetic polyacrylamide cationic polymer. However, a non-protein coagulant has also been reported but not characterised (Okuda et al., 2001). Our earlier studies showed that M. oleifera seed powder is effective in heavy metal remediation of water (Sajidu et al., 2005). The African moringa, M. stenopetala is a quick growing tree 6-10 m tall with a trunk 60 cm in diameter at breast height, strongly branched, thick at base with a bark white to pale grey or silvery, smooth wood and soft leaves up to 55 cm long (Jahn, 1991 and Moges, 2004). The species is known by different vernacular names such as Kallanki and Haleko (Ethiopia), and Cabbage Tree (English). M. stenopetala is less widely distributed than M. oleifera but stenopetala is reportedly more resistant to insect pests than other members of the family and its seeds are larger and easier to process than those of oleifera (Kayambazinthu, 2003). Although the water clarifying properties of M. stenopetala have not been as extensively studied as those of M. oleifera, Jahn (1986) reported that 100-150 mg/L of M. stenopetala was as effective in water clarification as 200 mg/L of M. oleifera which indicates that stenopetala is more effective than oleifera. However, no work has been reported previously on the potential of M. stenopetala to remove heavy metals from water. The objectives of this study, therefore, were to evaluate the potential of M. stenopetala in removing lead from water and compare its effectiveness with that of M. oleifera reported earlier by our group (Sajidu et al., 2005) and therefore this is the first report on the potential of Moringa stenopetala on heavy metal removal.
MATERIALS AND METHODS
Seeds were identified by the Forestry Research Institute of Malawi. Moringa stenopetala seeds were obtained from Kenya (Whizpop Products Ltd., Nairobi) in 2005. Moringa oleifera seeds were collected from Chikwawa district in southern Malawi in 2004 and 2005. Seeds were deshelled by hand and the deshelled seeds were ground in a coffee mill (National MX-J210PN), until a consistent powder was obtained. Defatted cakes were prepared by cold solvent extraction of the powdered seed with hexane fraction and traces of fat were removed from the residue by washing with diethyl ether until the residue was confirmed as fat free using a paper test and then dried in a vacuum oven (Gallenkamp OVL 570 010 J) at 40 ÚC and 600 mbarsfor 24-48 h. The following analytical grade chemicals were used: leadnitrate (BDH Chemicals Limited, Poole, UK); sodium carbonate, magnesium sulphate, sodium bicarbonate, hexane fraction (Sarchem (Pty) Krugersdorp Ltd, RSA); sodium chloride, sodium hydroxide, calcium chloride and nitric acid (Associated Chemical Enterprises (Pty) Ltd, RSA); hydrochloric acid, diethyl ether (Glassworld, RSA). pH was determined using a pH meter (Metrohm 744) checked with buffers at pH 4 and 7; constant temperature treatments were done in a constant temperature water bath (Bath: Haake Type 000-5584, Thermostirrer: Gallenkamp No. 85) and shaking was done using a Griffin shaker. Microsoft Excel was used for descriptive statistics and plots of treatment data. Analyses of variance were carried out using GenStat Discovery Edition.
Synthetic lead water
Synthetic lead water was obtained by dilution of a stock lead solution (1000 ppm) prepared by dissolving lead nitrate in de-ionised water or sodium chloride solution as described by the American Public Health Association (APHA, 1990).
Determination of lead content
Lead concentration was determined using atomic absorption spectroscopy (Shimadzu AA-680G V-5) at 283.3 nm with an air-acetylene flame as described by APHA (1990).
100 mL of synthetic lead water containing 7 ppm Pb was prepared using deionised water and appropriate masses of whole seed or defatted powder were added and the mixture stirred for 1 h. The mixture was filtered by gravity through Whatman No.1 filter paper and the lead concentration of the filtrate was determined.
Kinetics of lead removal
Synthetic lead water containing 7 ppm Pb was treated with 1.0 g of Moringa oleifera and 0.5 g of Moringa stenopetala whole seed cake and the residual Pb concentration of the solution quantified after different times of treatment.
7 ppm synthetic lead water was treated with 1.0 g M. oleifera or stenopetala at different temperatures. The moringa suspension was immersed in a constant temperature water bath with shaking for 1 h. and the residual lead concentration quantified.
pH changes during treatment
2.5 g portions of M. stenopetala or oleifera were added to 100 ml of 7 ppm or 3 ppm Pb in deionised water or 0.1 M sodium chloride and the pH recorded at intervals.
Effect of pH on lead removal capacity
Synthetic lead water of different pH’s were prepared by adjusting the pH of a solution of 7 ppm Pb in deionised water using 1 M sodium hydroxide or hydrochloric acid and the resulting lead solutions treated with moringa whole seed cakes for 1h.
Effect of water hardness
7 ppm Pb solutions were treated with 1.5 g of moringa whole seed powders suspended in different concentrations of Mg2+/Ca2+ or HCO3-/CO32- mixtures. The Mg2+/Ca2+ was prepared as described in Texas specification No. 485-54-09A. The HCO3-/CO32 mixture was prepared by dissolving 1.3768g NaHCO3, and 1.7662g Na2CO3 to make a 1000 ppm mixture and this was diluted to different appropriate concentrations.
Effect of sodium chloride
Solutions containing 7 ppm Pb in 0.0, 0.2, 0.4, 0.6, and 0.8 M sodium chloride were prepared by combining and diluting the stock lead solution (1000 ppm) and a 1 M stock solution of sodium chloride. The solutions were treated with 1.5 g of M. oleifera or stenopetala whole seed cake and the residual lead concentration determined.
Lead ion removal
Lead treatment was done using different doses of moringa whole seed powder. The results indicate that M. stenopetala has the capacity to remove heavy metals from water (Table 1) and the effectiveness of removal increased with increasing dosage of whole seed powder with p <0.001 at 95% CL. There was no significant difference for lead removal at 1 h and 24 h. (p = 0.348). Similar results were obtained using defatted cakes (Table 2). Metal ion removal increased with increasing dosage of M. stenopetala defatted seed cake (p <0.001) and there wasnosignificantdifference for Pbremoval at1 h. and 24 h. (p=0.55). The effectiveness of Pb removal ofthe defatted cakes (Table 2) and the whole seed powder (Table 1) was also compared and the defatted cakes were more effective than the whole seed cakes (p <0.001). Comparison of the removal effectiveness of Moringa oleifera and Moringa stenopetala (Table 3) using ANOVA showed that there was a significant difference in the effectiveness of removal with stenopetala being more effective than oleifera (p = 0.001).
Kinetics of lead removal
Time dependencestudies of lead removal bystenopetala gave a straight line plot for lead concentration versus time suggesting that removal is zero order with respect to metal ion (Fig. 1). For oleifera, a plot of natural log of lead concentration versus time gavea straight line indicatingthat removal is either first order or pseudo first order (Fig. 2).
Effects of other ions on metal removal
(a) Sodium chloride
Fig. 3 shows the effect of increasing NaCl ionic strength on lead removal using Moringa. Metal ion removal decreased with an increase in the concentration of NaCl from 0.0 to 0.8 M. The percentage decreased from 72.00 to 16.44 % for oleifera and from 65.76 to 34.84 % for stenopetala.
(b) Water hardness ions (Mg2+/Ca2+ and HCO3-/CO32-)
Removal of lead ions in different concentration of magnesium/calcium or carbonate/bicarbonate mixtures was carried out to investigate the effects of increasing water hardness on the metal ions removal (Fig. 4). Percentage removal increased with the increase in carbonates/bicarbonates concentration and there was no general trend for magnesium/calcium.
(a) pH changes in during treatment
Lead treatment was carried out in deionised water or 0.1 M sodium chloride solution while recording pH at different time intervals to investigate the effects of treatment timeon pH of the mixture (Fig. 5). The removal was done for different initial lead concentration of 3 ppm for deionised water and 7 ppm for both deionised and salt water. The variation in pH was more pronounced for 3 ppm Pb2+ solution in deionised water and least pronounced in 0.1 M NaCl solution.
(b) Effect of initial pH
Treatment was carried out at different initial pH (pH 2–10) to investigate the effects of initial pH on lead ions removal (Fig. 7). The percentage removal increased with an increase in initial pH. The maximum removals for treatment with powders, 93.99 % and 98.39 %, for oleifera and stenopetala respectively, were observed at pH 10. However significant change in percentage removal was observed between pH 2 and 3 (29.21 – 87.42 and 54.31 – 89.07 for oleifera and stenopetala respectively).
Effect of temperature on lead removal
To investigate the effects of temperature on lead removal using M. oleifera and stenopetala whole seed powders lead treatment was done at different temperatures, 0, 25, 40, 60, 80, and 100 oC (Fig. 7). The percentage removal of lead ions increased with temperature from 36.69 to 57.76 and 36.07 to 61.57 for oleifera and stenopetala respectively.
DISCUSSION AND CONCLUSION
Our previous studies have shown that M. oleifera is effective in removal of heavy metals including lead from water (Sajidu et al., 2005). This present study indicates that M. stenopetala seed powders (whole seed and defatted seed) are more effective than oleifera in lead removal (Table 3). These results agree with Jahn’s (1986) observation that stenopetala is a better water clarifier than oleifera and suggests that the same agents, the polyelectrolytes, are responsible for water clarification and lead removal. Our observed increase in removal with increasing dosage of seed powders (Tables 1 and 2) is probably due to increasing concentration of the polyelectrolytes. Similarly the superiority of the defatted cake over whole seed cake may be due to the removal of lipids from the whole seed cake, thus increasing the relative concentration of the polyelectrolytes, the likely agents of metal ion removal. Increase in sodium chloride ionic strength caused a decrease in lead removal (Fig. 3) suggesting that the lead removal reaction involves electrostatic interaction (Krishnan and Anirudhan, 2003). At high ionic strength the sodium ions can compete for binding sites with Pb2+ ions on the polyelectrolytes and hence reduce lead removal. Furthermore Pb2+ions form stable complexes with chloride ions, which inhibit interaction between Pb2+ and the polyelectrolyte sites. The effect of water hardness on lead removal varied depending on the ions involved (Fig. 4). Generally increase in concentration of carbonates/bicarbonates enhances lead removal; this effect can be explained by the formation of sparingly soluble lead carbonates that precipitate out of the solution (McGinnes, 2002). However there was no noticeable trend observed for the effect of the concentration of Mg/Ca on lead removal and further studies are being carried out. For the variation in pH during the course of treatment (Fig. 5), there is a rapid increase in pH during the first few minutes as the seed powder dissolves to give free moringa polyelectrolyte in solution followed by a drop in pH as the dissolved polyelectrolyte interacts with the Pb2+ions. Therelative maximum pHvaluesfor the different treatments reflect the amount of free moringa polyelectrolyte in solution. Therefore at low concentration (3 ppm) of Pb2+ the highest concentration of free polyelectrolytes is expected due to less interaction between the sites leading to the highest maximum pH. At 7 ppm more interaction is expected due to a higher amount of Pb2+ hence the observed lower maximum pH. The lowest maximum pH was obtained for the treatment in 0.1 M sodium chloride solution and may be explained by the presence of a greater amount of metal ions which have a salting in effect on proteins and inhibit the extraction and dissolution of the free polyelectrolytes. The observed increase in lead ion removal with increase in initial pH, may be attributed to the formation of insoluble lead hydroxides at higher pH. The lower efficiency in lead removal at low pH probablymay arise from the presence of a large amount of H+ ions in the mixture, which compete with Pb2+ ions for the binding sites (Krishnan and Anirudhan, 2003and Raji et al, 1997). The increase in lead ion removal with temperature is possibly due to the breakdown of the protein chain, at higher temperatures to expose more sites for binding with metal ions. In conclusion the study has shown that Moringa stenopetala seed powder can be used as an effective heavy metal purifier in water and that Moringa stenopetala is more effective than oleifera in heavy metal removal.
The authors acknowledge the material support of the International Programme in Chemical Sciences (IPICS) through the University of Uppsala, Sweden, the technical advice of the ForestryInstitute of Malawi and the use of the atomic absorption spectrophotometer of the Geological Survey Department, Zomba.
© 2006 Center for Environment and Energy Research and Studies (CEERS)
The following images related to this document are available:
Photo images[st06016t2.jpg] [st06016f2.jpg] [st06016f6.jpg] [st06016t3.jpg] [st06016t1.jpg] [st06016f3.jpg] [st06016f5.jpg] [st06016f1.jpg] [st06016f4.jpg] [st06016f7.jpg]