Journal of Materials Science and Engineering B 1 (2011) 843-853 Formerly part of Journal of Materials Science and Engineering, ISSN 1934-8959

D

DAVID

PUBLISHING

Heavy Metal Removal in Aqueous Systems Using Moringa oleifera: A Review Peter Papoh Ndibewu, Robert L. Mnisi, Sharon N. Mokgalaka and Rob I. McCrindle Department of Chemistry, Faculty of Science, Tshwane University of Technology, Arcadia Campus, Pretoria, South Africa Received: June 29, 2011 / Accepted: July 20, 2011 / Published: November 25, 2011. Abstract: Currently, no literature extensively reviews the availability and the potential of Moringa oleifera parts as a non-competitive low-cost value-added renewable source for biosorbents, particularly for bioremediation and water purification usage in poverty threatened areas of developing nations. The present review article discusses the potential of this plant as a substitute to expensive and synthetic technologies for the removal of toxic metals from contaminated aqueous systems while critically evaluating research results presented in rather too few articles that exist. The main biosorption processes linked to heavy metal sequestration is assessed. The coagulation properties of Moringa oleifera for organic contaminant removal are analyzed and linked to their metal biosorption mechanism. The removal of heavy metals and its potential accumulation in granular medium of M. oleifera are also discussed. Current advanced models used in bioadsorbent studies are presented as a powerful tool for understanding multiple interactions occurring in aqueous systems during the removal of contaminants using M. oleifera. On account of its enormous pharmacological significance, water coagulation properties and potential for heavy metal sequestration, M. oleifera, which can be easily be domesticated in areas where they lack versatility in growth such as Southern Africa, can be a good candidate as a biosorbent in bioremediation of hazardous aqueous systems (contaminated environments or effluents) and water purification. Key words: Heavy metals, Moringa oleifera, aqueous systems, adsorption, biosorption.

1. Introduction Biosorption of heavy metals from aqueous solutions is a relatively new process that is proven very promising in the removal of contaminants from aqueous effluents. Adsorbent materials derived from low-cost agricultural wastes can be used for the effective removal and recovery of heavy metal ions from wastewater streams [1]. Seeds of Moringa oleifera have been traditionally utilized in many rural areas of Africa [1] and Asia [2] for drinking water purification as they possess strong coagulation properties for sedimentation of suspended mud, turbidity and exert a disinfecting effect on pathogens. This is particularly true for remote areas of West and East Africa. The unique property of this tree Corresponding author: Peter Papoh Ndibewu, professor, research fields: materials science and nanoscience, environmental chemistry, spectroscopy. E-mail: [email protected].

is still to be fully exploited in the rest of the continent. So is the case in Southern Africa. The question why it is less known and used in one part of Africa and not the other is a subject that research must holistically address. Of the thirteen species of Moringa trees in the family Moringaceae, Moringa oleifera Lam. (synonym: Moringa pterygosperma Gaertn) is the most widely known species and the others still deserve further research into their use [3]. Because of its coagulating property, the plant is called a clarifier tree [4]. On account of the enormous pharmacological significance and water coagulation properties of this plant, efforts have been made by various groups of workers to isolate its constituents responsible for these activities while much less research has focused on the study of its metal biosorption capacity [5]. This explains why literature on heavy metal removal in aqueous systems using Moringa oleifera is very scanty. Also, no research has been done too to elucidate and understand the

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Heavy Metal Removal in Aqueous Systems Using Moringa oleifera: A Review

particular active chemical ingredients that make this plant suitable to adsorb and sequester heavy metals from aqueous systems. It is generally being speculated that the flocculant agents which are polypeptides found in the seeds and responsible for clarification also play the main role of metal sequestration through complexation processes. There is too few or little literature documentation on the compositional distribution of these molecules in different parts of the part. Compositional information for species grown in the rather difficult and diversified Southern African sub-ecologies is completely absent [6]. Other compounds suspected to be active in metal adsorption include the flavonoids and phenolic compounds. Understanding the multiple interactions occurring in an aqueous system during the removal of contaminants using M. oleifera could be critical for several reasons. Firstly, it is a multipurpose tree as its various parts are used as fodder [7] herbal medicine [8] spices, food and natural coagulants [9], thus, lending itself as a non-competitive low-cost value-added renewable source for biosorbents. Secondly, since attempts to isolate Moringa seed flocculants showed that they are basic polypeptides with molecular weights ranging from 6,000-16,000 daltons [10], these compounds can be modified in-situ to attain efficiency comparable with available synthetic cationic polyelectrolytes [10]. Finally, an in-depth understanding of the chemical structure of the polypeptides and glycosides, the predominant active chemical constituents in M. oleifera seeds and leaves, respectively, is important in assessing the type of chemical modification in enhancing the adsorption capacity of Moringa oleifera. Demirbas [11] has recently reviewed the removal of heavy metal ions using adsorbents from a diverse range of renewable sources. Currently, low-cost abundantly available sources for biosorbents include agricultural wastes such as rice husk and maize cobs [12], tea waste and coffee [13], hazelnut shells [14-16], peanut hull [17, 18] red fir [19] and maple [20] sawdusts [21], pinus bark [22-25] and different bark samples [26-32], palm

kernel husk [33] and coconut husk [34, 35], peanut skins [36], modified cellulosic materials [37, 38], chemically modified cotton [39], corncobs [40] and modified corncob [41], rice hulls [42], apple wastes [43], coffee grounds [44], bark [45, 46], modified bark [47], wool fibers [48], tea leaves [49], and wool, olive cake, pine needles, almond shells, cactus leaves, charcoal [50], modified lignin [51-53], banana and orange peels [54], modified sugar beet pulp [55], modified sunflower stalk [56], palm fruit bunch [57], maize leaf [58] and different agricultural by-products [59-65]. The use of inexpensive and effective metal ion adsorbents from many of the agro waste materials to offer these adsorbents as replacements for existing commercial materials is still under research scrutiny and may not yet be commercially available [66, 67]. One major setback with fiber-based agro-waste biosorbent is their relatively low capacity for heavy metals removal [68, 69]. Apart from the cost of production, perhaps, one of the reasons why agro-based biosorbent materials are not widely available is that agricultural production must be on a large scale. But, more conveniently so, where the agroor agricultural based wastes are not competing with other value-added usage such as for animal feed, soil supplementation or secondary fuel sources. Chemical modification improves the adsorption capacity and stability of biosorbents. Biosorption experiments over Cu(II), Cd(II), Pb(II), Cr(III), and Ni(II) demonstrated that biomass Cu(II) adsorption ranged from 8.09 to 45.9 mg/g, while Cd(II) and Cr(VI) adsorption ranged from 0.4 to 10.8 mg/g and from 1.47 to 119 mg/g, respectively [69-71]. The aim of this paper is to review reports in the literature on use of Moringa oleifera for sequestration of heavy metals in aqueous solutions and to explore possibilities for further applications and study.

2. Chemical Properties of Moringa oleifera Moringa fresh leaves contain between 19.3% and 26.4% crude protein (CP) in dry matter (DM) [72-74].

Heavy Metal Removal in Aqueous Systems Using Moringa oleifera: A Review

The leaves have a negligible content of tannins, a saponin content similar to that of soybean meal and no trypsin and amylase inhibitors or cyanogenic glucosides [72]. Hemagglutinating proteins were identified in M. oleifera tissue extracts and a seed coagulant lectin isolated by affinity chromatography and partially characterized. Literature reports that M. oleifera Lam. seeds present a protein mass composition of 29.36% [75]. The

shelled

and

non-shelled

seeds

contain

approximately 37% and 27% of protein, respectively. A flocculating protein from the seeds of M. oleifera Lam. was isolated by the same authors and its molecular mass was found to be around 6.5 kDa and the isoelectric point was above pH 10. Flocculation activity could be explained by the charge-patch mechanism due to low molecular weight and high charge density proteins. The amino acids detected are mostly glutamic acid, proline, methionine and arginine. The main amino and carboxylic groups present in the M. oleifera seeds have been characterized to be connected

to

this

protein,

some

fatty

acids,

carbohydrates and the lignin units. The adsorptive capacity of Moringa oleifera is highly favored since most of them contain considerable quantities of cellulosic interlinked with lignin in their structure. Lignin is a complex biopolymeric heterogeneous molecule which is endowed with many different functional groups, such as methoxyl, hydroxyl-aliphatic, carboxyl and phenolic [76, 77]. It has an aromatic, three-dimensional polymer structure with an infinite apparent molecular weight, thus, favoring biosorption as one of the promising techniques to treat chromium-containing wastewaters through the use of M. oleifera as a low-cost adsorbent [78]. This structure makes lignin quite insoluble in water. Studies have revealed that lignocellulotic plants can be used to remove a wide range of heavy metals [80-82] from aqueous systems with high removal efficiencies. Still from reports of the few literatures available, not much research has been done to understand fully the

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active chemical compounds responsible for biosorption in M. oleifera [78, 80, 82, 83]. Most of the studies surveyed give specific attention to such properties as water coagulating, nutritional, therapeutical, and pharmacological properties. This is a glaring gap that scientists need to fill for a holistic understanding of the chemistry of M. oleifera. The bioactive chemical constituents of M. oleifera responsible for the observed activities are presented in Table 1.

3. Theoretical Adsorption Modeling Adsorption processes taking place in biosorbents are rather difficult to model mathematically, due to two main reasons: (a) the complex nature of the phenomenon, which implies physicochemical interaction through molecule-molecule (van der Waals and hydrogen bridges forces) [83] and (b) the fact that the intrinsic composition of the biosorbent that forms the bioactive principle in the material is not completely known. After literature survey, the hypothesis that metal adsorption onto Moringa oleifera may involve two phases was discovered: The first is a destabilization of colloids, that may be ruled by chemical interactions between biosorbent molecules (anionic, negatively charged) and metal ions (cationic, positively charged). Once the complex biosorbent metal ion is formed, adsorbates begin to grow by sorption mechanisms. This should be the controllant stage, so the whole process can be simulated as an adsorption phenomenon. The Moringa oleifera is known to form bridges and netlike structures during adsorption and coagulation processes may be considered in this theoretical model [84]. First, adsorption capacity q has been determined, defined as: ( C o  C 1 )V (1) q  W where Co is initial metal ion concentration (mg·L-1), C1 is equilibrium metal ion concentration in bulk solution (mg·L-1), V is the volume of solution (L), and W is biosorbent mass (mg). Two main adsorption models have been widely considered in literature: the

846

Heavy Metal Removal in Aqueous Systems Using Moringa oleifera: A Review

Table 1 Summary of bioactive chemical compounds of Moringa oleifera responsible for the observed nutritional, therapeutical, pharmacological, nutritional, coagulating and biosorption properties of the plant parts. Plant part Leaves

Pods/seeds

Property Nutritional Pharmacological Therapeutical Pharmacological Therapeutical Therapeutical Pharmacological Pharmacological Pharmacological Coagulating Coagulating Coagulating Biosorption of Cd Biosorption of organics Coagulating Coagulating Biosorption of Ag Coagulating Biosorption of As Biosorption of Zn Biosorption of Cd2+, Cr3+ and Ni2+

Class of bioactive compounds Amino acids Glycosides Phenols Glycosides Phenolic Phenolic Glycosides Glycosides Glycosides Amino acids Amino acids Amino acids Amino acids Not mentioned Amino acids Not a protein Amino acids Amino acids Amino acids Amino acids Amino acids Glycosides Hydroxyl and carboxyl groups Alkaloids

Pharmacological Biosorption of Pb

Bark

Langmuir and Freundlich models. The first of them assumes that the metal ions striking the surface have a given probability of adsorbing. Metal ions already adsorbed similarly have a given probability of desorbing. At equilibrium, equal numbers of metal ions desorb and adsorb at any time. The probabilities are related to the strength of the interaction between the adsorbent surface and the adsorbate [85]. That is the physical meaning of Eq. 2: C1 (2) q  k 11

1  k 12 C 1

where kl1 is the first Langmuir adsorption constant [L (mg of biosorbent)-1] and kl2 is the second Langmuir adsorption constant [L (mg of metal ion)-1] [85, 86]. The Freundlich model was derived from empirical data [87] and assumes that q capacity is an exponential function of the equilibrium metal ion concentration (Cl). That is what Eq. (3) expresses: q  k

f

C

n 1

f

(3)

Source [72] [120] [121] [122] [123] [124] [125] [126] [125] [127] [128] [129] [112] [130] [131] [9] [132] [133] [114] [113] [111] [125] [110]

where nf is the Freundlich adsorption order (dimensionless) and kf is the Freundlich adsorption constant [(Lnf) · (mg of biosorbent)-1 · (mg of metal ion1-nf)]. By combining data series, it is possible to look for a theoretical model that fits rather well to experimental data. However, linear expressions of Freundlich and Langmuir models give a criterion to discriminate the goodness of these data adjustments. A linear fitting graphic for the Langmuir hypothesis can be included. Dye removal caused by Moringa oleifera seed extract coagulant seems to work according to the Langmuir hypothesis. Through the linearization method, it is shown that the Langmuir equation fits better to the experimental data than Freundlich’s proposal, according to r2 values. These are equal to 0.97 in the case of Langmuir and 0.78 in the case of Freundlich. The values of the different parameters involved in these two models in each case are k12 = 2.24 × 10-2 L (mg of coagulant)-1 and k12 = 3.29 × 10-2 L (mg

Heavy Metal Removal in Aqueous Systems Using Moringa oleifera: A Review

of dye)-1 in the case of Langmuir adjustment and kf = 1.38 × 10-1 Lnf (mg of coagulant)-1 (mg of dye1-nf) and nf = 2.76 × 10-1 in the case of Freundlich [85].

4. Mechanism of Metal Adsorption 4.1 Mechanism of Metal Biosorption Metal biosorption is a rather complex process affected by several factors. Mechanisms involved in the biosorption process include chemisorption, complexation, adsorption-complexation on surface and pores, ion exchange, microprecipitation, heavy metal hydroxide condensation onto the biosurface, and surface adsorption [88, 89]. In order to understand how metals bind to the biomass, it is essential to identify the functional groups responsible for metal binding. Most of the functional groups involved in the binding process are found in cell walls. Plant cell walls are generally considered as structures built by cellulose molecules, organized in microfibrils and surrounded by hemicellulosic materials (xylans, mannans, glucomannans, galactans, arabogalactans), lignin and pectin along with small amounts of protein [90-92]. The behavior of cellulose as a substrate is highly dependent upon the crystallinity, specific surface area, and degree of polymerization of the fibers being studied [93]. The cellulose is located predominantly in the secondary cell wall. Bundles of cellulose molecules are aggregated together in the form of microfibrils in which highly ordered (crystalline) regions exist with less ordered (amorphous) regions. The proportions of crystalline and amorphous regions in cellulose vary depending upon the type of the sample and the method of measurement. Cotton cellulose is usually more crystalline than wood cellulose [94]. Laszlo and Dintzis [95] have shown that lignocellulosics have sorption capacity, which are derived from their constituent polymers and structure. The polymers include extractives, cellulose, hemicelluloses, pectin, lignin and protein. These are adsorbents for a wide range of solutes, particularly divalent metal cations [95]. Lignocellulosics are hygroscopic and have an

847

affinity for water. Water is able to permeate the non-crystalline portion of cellulose and all of the hemicellulose and lignin. Thus, through absorption and adsorption, the aqueous solution comes into contact with a very large surface area of different cell wall components. Because of its disordered structure, amorphous cellulose should be much more accessible to reagents than highly structured crystalline cellulose. Cellulose can also sorb heavy metals from solution [96]. The molecular structure and supramolecular structures have a strong influence on sorption properties. Water adsorption of fibers, orientated in one particular direction, invariably causes swelling. The bigger the amount of water adsorption is, the bigger is the swelling. Swelling also depends on the fiber’s structure, on the degree of crystallinity and on the amorphous and void regions [97]. Swelling occurs when polar solvents such as water and alcohols are contacted with wood [98]. Wood swelled extremely fast at high temperatures. The polar solvent molecules are attracted to the dry solid matrix and held by hydrogen bonding forces between the –OH or –COOH groups in the wood structure. The adsorption mechanisms of heavy metals onto the adsorbent vary widely and depend upon the heavy metals under investigation, the degree of functionalization and the type of adsorbent [99]. Most agro-based adsorbents interact with metallic species through binding of the metal ion and cellulose/lignin units in the active sites [100]. This can be through binding two hydroxyl groups in the cellulose/lignin units or through binding the cellulose units together. Hydrogen bonding has also been found to be responsible for the adsorption mechanism [101] in some systems of heavy metal removal, and Hashem et al. [102], has also highlighted that chelate formation between the adsorbent and adsorbate can immobilize a wide range of metallic species in water. It has also been found [102] that the larger the ionic valence number of the metal, the stronger the attractive forces, suggesting that, for instance, Fe3+ will be strongly adsorbed than a Cd2+,

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Heavy Metal Removal in Aqueous Systems Using Moringa oleifera: A Review

when they are together in a multi-element aqueous medium. Ion-exchange has also been suggested [102] as one the mechanisms for heavy metal removal from aqueous systems. Other authors [103-105] have also applied ion-exchange to study its efficiency in removing different heavy metals from aqueous systems. 4.2 Metal Adsorption and Biosorption onto M. oleifera Many studies involving metal pre-concentration have been reported, mostly with the use of commercially available sorbents. However, other materials known as “natural adsorbents” have recently been successfully employed in metal adsorption processes [106-108]. The term “natural adsorbent” is assigned to any material that is not synthetically produced and has adsorptive properties in terms of chemical species of inorganic and organic origins. Moringa oleifera has been applied in many water purification studies to remove heavy metals such as Ni, Cu and Zn [109], Pb [110], Cd, Cr and Ni [111], Cd [112] and Zn [113] with success. It appears like the different plant parts have different active chemical constituents which are also responsible for different functions. For instance, among the different parts of the plant, many authors [114, 115] have preferred to use the seeds in heavy metal sequestration from water. The reported recoveries are generally high for the specific targeted metals. For instance, Kumari et al (2006) [114] reported recoveries of up to 60.21% for As (III) and 85.60% for As (V), Sharma et al. (2007) [111] reported 76.59% for Cd (II), 68.85% for Cr(III) and 60.52% for Ni (II). These seeds have also been used even for the removal of low-concentrations of Cd in alcohol [112] and Ag in water [116] after a pre-concentration step. This shows the versatility of M. oleifera seeds in heavy metal adsorption. The same cannot be said of the different parts, such as the leaves, the wood, the bark and the root. These are mostly used for their therapeutic properties. Apart from the seeds, the leaves have also been

applied in heavy metal sequestration studies, though to a lesser extent. Reddy et al. (2010) [117] modified the leaves of the plant to improve the removal efficiency for Pb (II) aqueous systems. The removal efficiency of the bark of Moringa oleifera was also investigated for Pb (II) from aqueous systems [117]. It was revealed that the bark is endowed with hydroxyl and/or carboxyl functional groups as the Pb (II) ions were observed to be chelated with these.

5. Conclusions and Future Perspectives The Moringa oleifera plant is a natural biosorbent that is used for a wide range of applications utilizing such properties as water coagulation, nutritional and pharmacological. Research into the active chemical compounds has been done to some extent. However, those active chemical compounds responsible for the removal of heavy metals from aqueous systems are still widely unknown and research to understand the clear biosorption mechanism ongoing. The active sites hypothesized to be responsible for the observed adsorptive capability of M. oleifera contain functional groups such as hydroxyl, carboxyl, amines, phenolic, methoxyl, hydroxyl-aliphatic groups. The plant is generally chemically composed of a large protein molecule with a molecular mass of 6.5 kDa and an isoelectric point above pH 10. The protein occurs in large concentrations in the seeds, leaves, stem and bark. Further research, detailed indeed to answer specific chemistry questions on the adsorption behavior and nature (coagulation and flocculation properties) [118], would be required to assign specific molecules to specific compositional functions. For future studies, it would be much more comprehensive to investigate the chemistry of this “miracle” plant per species per biodiversity [119].

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