Application of mucilage from Dicerocaryum eriocarpum plant as biosorption medium in the removal of selected heavy metal ions

Journal of Environmental Management 177 (2016) 365e372

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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman

Research article

Application of mucilage from Dicerocaryum eriocarpum plant as biosorption medium in the removal of selected heavy metal ions Bassey O. Jones a, *, Odiyo O. John b, Chimuka Luke c, Aoyi Ochieng d, Bridget J. Bassey e a

Department of Ecology and Resources Management, School of Environmental Sciences, University of Venda, X0950, South Africa Department of Hydrology and Water Resources, School of Environmental Sciences, University of Venda, X0950, South Africa c Molecular Science Institute, School of Chemistry, University of the Witwatersrand, P/Bag 3, WITS University, 2050, Johannesburg, South Africa d Centre for Renewable Energy and Water, Vaal University of Technology, Private Bag X021, Vanderbijlpark, South Africa e Department of Biochemistry, Cross River State University of Technology, Cross River State, Nigeria b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 September 2015 Received in revised form 22 March 2016 Accepted 6 April 2016 Available online 2 May 2016

The ability of mucilage from Dicerocaryum eriocarpum (DE) plant to act as biosorption medium in the removal of metals ions from aqueous solution was investigated. Functional groups present in the mucilage were identified using Fourier transform infrared spectroscopy (FTIR). Mucilage was modified with sodium and potassium chlorides. This was aimed at assessing the biosorption efficiency of modified mucilage: potassium mucilage (PCE) and sodium mucilage (SCE) and comparing it with non-modified deionised water mucilage (DCE) in the uptake of metal ions. FTIR results showed that the functional groups providing the active sites in PCE and SCE and DCE include: carboxyl, hydroxyl and carbonyl groups. The chloride used in the modification of the mucilage did not introduce new functional groups but increased the intensity of the already existing functional groups in the mucilage. Results from biosorption experiment showed that DE mucilage displays good binding affinity with metals ions [Zn(II), Cd(II) Ni(II), Cr(III) and Fe(II)] in the aqueous solution. Increase in the aqueous solution pH, metal ions initial concentration and mucilage concentration increased the biosorption efficiency of DE mucilage. The maximum contact time varied with each species of metal ions. Optimum pH for [Zn(II), Cd(II) Ni(II) and Fe(II)] occurred at pH 4 and pH 6 for Cr(III). Kinetic models result fitted well to pseudo-second-order with a coefficient values of R2 ¼ 1 for Cd(II), Ni(II), Cr(III), Fe(II) and R2 ¼ 0.9974 for Zn(II). Biosorption isotherms conforms best with Freundlich model for all the metal ions with correlation factors of 0.9994, 0.9987, 0.9554, 0.9621 and 0.937 for Zn(II), Ni(II), Fe(II), Cr(III) and Cd(II), respectively. Biosorption capacity of DE mucilage was 0.010, 2.387, 4.902, 0688 and 0.125 for Zn(II), Cr(III), Fe(II), Cd(II) and Ni(II) respectively. The modified mucilage was found to be highly efficient in the removal of metal ions than the unmodified mucilage. © 2016 Elsevier Ltd. All rights reserved.

Key words: Biosorption Dicerocaryum eriocarpum plant Heavy metals Mucilage

1. Introduction Water is typically the prime environmental medium that has been affected by pollution mostly caused by human activities. Human activities such as; agriculture, mechanisation, urbanisation, industries and mining has impacted negatively on the environment (Saad et al., 2011). This situation impacts negatively on the aquatic ecosystem resulting in constant discharge of contaminant such as heavy metals and metallic compounds in high concentration into the environment. Primary the presence of heavy metal in waste is

* Corresponding author. E-mail address: [email protected] (B.O. Jones). http://dx.doi.org/10.1016/j.jenvman.2016.04.011 0301-4797/© 2016 Elsevier Ltd. All rights reserved.

as a result of the intended use of them in industrial products (Bellmann and Khare, 2000; Guyo et al., 2015a). Most heavy metal end up as waste after their useful life, thus becoming pollutants to the environment (Bellmann and Khare, 2000). It has been reported that industrial wastes often contain metal ions such as Zinc, Nickel, Cadmium, Iron and Chromium (Kadirvelu et al., 2001; Moyo et al., 2015b). Heavy metals toxicity is harmful and can cause diseases and disorder to organisms including humans especially when they accumulate in living tissues (Hanafiah and Ngah, 2008). Treatment of water before use still remains the best approach in overcoming related water-borne diseases often associated with untreated wastewater sources. This makes water treatment of high importance. Some of the method used in removal of heavy

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metals from wastewater includes coagulation, flotation, membrane filtration, ion-exchange and chemical precipitation (AlMasri et al., 2010; Kumar et al., 2011; Moyo et al., 2014, 2015a (Marula) Guyo et al., 2015b). The major aim of implementing all these treatment methods is to remediate and minimize pollution from leaching into the surrounding environment. Although such wastes can be stabilized by the above methods but it is often expensive and short-live (Smith and Bradshaw, 1979; Piha et al., 1995). There is an increasing application of plant materials as biosorbents in the uptake of heavy metals contaminants from water/ wastewater, as a low cost and easy method. Plants materials are considered to be environmental friendly method (Ahluwalia and Goyal, 2007) in eliminating metals and metalloid species from industrial wastes via biosorption (Ahluwalia and Goyal, 2007; Hanafiah and Ngah, 2008). Studies have reported that plants materials are able to bind with metal ions efficiently (Hanafiah and Ngah, 2008; Saad et al., 2011). The use of natural plants materials as biosorbents in treating wastewater and aqueous solution contaminated with heavy metal ions has gained more attention attributed to inexpensive cost and high biosorption efficiency. The sorption capacity of different biosorbents from plant origin whose efficiencies have been documented in literature, include, rice husk, sawdust, peanut husk, sugar beet pulp, wheat bran, groundnut shells, carrot residues, cock powder, coirpith, nipah palm, banana pith, carica papaya, etc, (Ahluwalia and Goyal, 2007; Hanafiah and Ngah, 2008). Natural plants materials mostly use for biosorption studies are bio-solids. Few literature from studies have reported the use of mucilage from Oputia species, Cactus species and Zea Mays L (Mane et al., 2011; Fox et al., 2012) as biosorbent. Diceriocaryum eriocarpum (DE) is a grassland plant from the family Pedaliaceae with a common name ‘‘Devil's thorns”. It is widely spread throughout Limpopo Province of South Africa, and in Southern Africa. It is also found in dune slopes and river banks, particularly trampled areas and abandoned fields, usually in sandy soils (Van Wyk and Malan, 1988). DE plant is considered a multipurpose plant because it can be used for various functions. Medicinally, DE plant have been used for antibacterial, antiinflammatory (Luseba et al., 2006) and for ethno-veterinary medicines (Merwe et al., 2001). Locally in Vhembe District, Limpopo Province of South Africa, DE plant parts are used to treat diseases. Most parts of the plant contain mucilage and are soapy and slimy when exposed to water. In Vhembe District, it is consumed as vegetable and also used as a substitute for shampoo and soap (Van Wyk and Gericke, 2000). Preliminary studies showed that applying mucilage squeezed from the leaves of DE into turbid raw water contributed to a decrease in turbidity of the raw water. This is an indication that DE mucilage has the potential to remove turbidity from raw water. So far no studies in literature have reported on the use of mucilage from Diceriocaryum eriocarpum for the removal of these selected metal ions (Zn2þ, Cd2þ, Ni2þ, Cr3þ and Fe2þ) from aqueous solution. The objective of this study was to investigate the application of mucilage from DE as low cost and locally available biosorbent in the removal of Zn2þ, Cd2þ, Ni2þ, Cr3þ and Fe2þ from aqueous solution. Characterisation of DE mucilage was carried out using FTIR. The effect of different parameters such as contact time, pH, initial concentration, biosorbent concentration was also investigated. Comparative studies to evaluate the biosorption efficiency of modified and unmodified DE mucilage were also assessed. Biosorption isotherm and sorption kinetics of Zn2þ, Cd2þ, Ni2þ, Cr3þ and Fe2þ onto DE mucilage were also investigated.

2. Materials and methods Fresh Dicerocaryum eriocarpum (DE) plant samples of height less than 1 m above the ground were collected from Nzhelele Village (from farm lands and nearby bushes close to human settlements) in Vhembe District, South Africa. Plant leaves were detached from the stem and washed with tap water and rinsed with de-ionised water to remove dirt, soil particles and debris that may have been deposited on the leaves from the soil and atmosphere. Fresh plant leaves were air dried in the shade and grounded thereafter. 2.1. Chemicals The chemicals including metal salts [Cr(NO3)3, Fe(NO3)2, Ni(NO3)2, Zn(NO3)2 and Cd(NO3)2], chloride salts (NaCl and KCl), HNO3 and NaOH used in this study were all purchased from Merck (Johannesburg, South Africa). No further purification was carried out on the chemicals afterward and all chemicals were of analytical grade. 2.2. Equipments The pH of the aqueous metal solution was measured using portable Thermo Scientific Orion (5 star Thermo Scientific Orion, Germany) equipped with a pH electrode. Magnetic stirrers were used to stir all the mixtures at a rotational speed of 50 rpm. Heavy metals analyses were carried out in triplicate using an Atomic Absorption Spectrometry (Perkin Elmer AAnalyst 400, Germany). Fourier Transform Infra-Red spectrometer (FTIR) (Tensor 27, Bruker, Germany) was used to characterise DE plant to identify the functional groups present in the unmodified and modified mucilage. 2.3. Extraction of mucilage from DE leaves Dried grounded leaves were suspended in boiled water, stirred, afterward, the mixture was left overnight to dehydrate under room temperature. This process was able to extract thick viscous-slimy foamy mucilage from the DE leaves. The final concentrate of pure transparent mucilage-solution was recovered by filtration using 0.45 mm with the aid of a suction pump. 2.4. Modification of DE In this study, DE mucilage was modified by introducing chloride salts into DE mucilage. This was carried out by using potassium chloride (KCl) and sodium chloride (NaCl) solely. Concentration of 1 mol/L solution of KCl and NaCl was prepared from their respective salts by weighing out 7.8 g of KCl and 5.8 g of NaCl and dissolving each salt in 100 mL of de-ionised water. This was followed by soaking appropriate amount of dried DE leaves in 100 mL of each salt solution. This was followed by filtration and the mucilage extracted from each chloride salt solution was recovered for further experiments. 2.5. Characterisation of DE mucilage The characterisation of the functional groups present in modified mucilage prepared from KClesolution mucilage, NaClesolution mucilage and unmodified mucilage from deionised water mucilage filtrate was conducted using FTIR. Each filtrate was converted to solid form for FTIR characterisation as described in Benhura and Marume (1993). The dried mucilage was ground to a fine powder prior to characterisation.

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2.6. Preparation of solutions The metal ions used in this study include Zn2þ, Cr3þ, Ni2þ, Fe2þ and Cd2þ. These metal ions were chosen in this experiment because they are often detected in industrial wastewater. Nitrate salts [Cr(NO3)3, Fe(NO3)2, Ni(NO3)2, Zn(NO3)2 and Cd(NO3)2] were used in the preparation of all metals solutions by weighing out the appropriate amount of salts and dissolving in de-ionised water. After serial dilution, 1000 mg/L volumetric flask containing each metal salt was topped to the mark using ultra-pure water. Multicomponent working standard for initial concentration of metal ions in this study were prepared carefully from the 1000 mg/L and 7.5, 12.5 and 25 mg/L initial concentrations of metal ions were achieved. Concentration of 1 mol L1 solution of NaOH and HNO3 was prepared and dissolved in deionised water. pH adjustment for the biosorption experiments was conducted using these solutions that were prepared using de-ionised water.

Fig. 1. FTIR spectra of (a) DCE, (b) PCE and (c) SCE.

3. Results and discussion 2.7. Biosorption experimental procedure

3.1. Characterisation of mucilage biosorbent

Biosorption studies were carried out by mixing a known concentration of biosorbent (mucilage) with multiple metal ions solution prepared from their respective salts. The metal ion solution contained a specific concentration of metals at uniform concentration for each metal ion. The metal ions solution pH was adjusted to the desired value using already prepared concentration of 1 mol L1 solution of NaOH and HNO3. Batch biosorption experiments were conducted using flask containing both the metal ions and the biosorbent. The mixture was stirred using magnetic stirrer at 50 rpm for specified time at room temperature. After agitation, the resultant solution was filtered using Whatman filter papers. The filtered liquid was analyse using AAS.

2.8. Optimisation of DE mucilage biosorption The extracted mucilage was studied in order to determine its efficiency as a biosorbent media. To this end the following parameters were optimised: contact time (2e30 min), pH (2e8), initial concentration of the heavy metals (7.5, 12.5 and 25 mg/L), biosorbent concentration (10e50% v/v) and comparative studies to assess the biosorption efficiency of modified and unmodified mucilage. Each parameter optimisation was carried out by varying the parameter while the others are kept constant. The influence of each of these parameters on biosorption efficiency was evaluated by calculating the removal efficiency using equation (1).

%Removal ¼

ðCo  Ce Þ  100 Co

(1)

where: Co ¼ initial concentration and Ce ¼ equilibrium metal ion concentration. The biosorption amount (qe) was calculated using equation (2).

qe ¼

ðCo  Ce ÞV M

(2)

where qe is the amount of metal adsorbed in mg/g, V is the volume of the aqueous solution in litre and M is the mass of the biosorbent used.

The FTIR spectra of DE mucilage; (a) DCE, (b) PCE and (c) SCE as displayed in Fig 1 were all measured within the range 400e4000 cm1. The characteristics of the peaks for DCE, PCE and SCE were significantly similar. The same functional groups present in the DCE were also present in PCE and SCE: carboxyl, hydroxyl and carbonyl groups. The first broad peak of DCE appeared at 3263 cm1 with a high transmittance frequency. Similar peaks were observed in PCE and SCE spectra. There were peak increases and intensity increases in PCE and SCE spectra, which appeared at 3294 and 3330 cm1, respectively, with low transmittance frequency. The above peaks correspond with eCOOH and OH stretch of carboxyl and hydroxyl groups. The low transmittance frequencies observed in SCE and PCE spectra indicate high absorption sites in SCE and PCE mucilage. The double band observed for DCE at 1724e1640, PCE at 1726e1640 cm1 and the sharp single peak observed at 1634 for SCE correspond with the C]O stretch of carbonyl groups. Generally, from the characteristics of the functional groups (frequencies and intensities); the intensities of the functional groups present in PCE and SCE are higher than DCE. The main difference observed that distinguish DCE from PCE and SCE is the change in intensity shift. More pronounced peaks were observed in PCE and SCE spectra compared to DCE spectra. These may be attributed to the increase in the intensity of the functional groups present in the mucilage and chemical interaction due to the presence of chloride salts in the mucilage.

3.1.1. Active agent of biosorption in DE mucilage The biosorption potential of DE mucilage can be attributed to the major functional groups identified in DE mucilage: carboxyl, carbonyl and hydroxyl groups. Barone et al. (1996) also confirmed that mucilage extracted from Dicerocaryum species contain carboxyl functional groups consisting of mannose and glucuronic acid as the core structure of the polysaccharide. The potential of DE mucilage to act as a biosorbent for heavy metals can be associated with the biochemical characteristics of the functional groups acting as the dominant group in the mucilage. Similar results were also obtained in different studies using natural plant biosorbent in the uptake of metal ions. Vinod et al. (2009) identified that gum kondagogu contains

B.O. Jones et al. / Journal of Environmental Management 177 (2016) 365e372

hydroxyl, acetyl, carbonyl and carboxylic groups as the major functional groups and acidic in nature. The acidic groups were responsible for uptake of multiple metal ions species (Pb, Cd, Ni, Cr, Fe, Cu, Zn, Co, Se and As) (Vinod et al., 2010) by gum kondagogu. The presence of carboxyl groups in cassava biosorbent increased the removal efficiency of cadmium in aqueous solution (Hanafiah and Ngah, 2008). Ahluwalia and Goyal (2007) documented that carboxyl groups in brown seed weed was effective in the removal of Fe2þ and Fe3þ. Evaluating the role of Zea may L. and cactus plant mucilage in the uptake of metal ions also confirmed that the binding of metal to the mucilage was due to the carboxyl functional group acting as active sites (Morel et al., 1986; Fox et al., 2012). Availability of the functional groups in DE mucilage, which are likely to be negatively charged, may be responsible for the binding and biosorption of positively charged metal ions.

Recovery %

368

90 80 70 60 50 40 30 20 10 0

Zn Ni Fe Cr Cd 0

2

6

8

10

pH Fig. 3. Effect of pH on the uptake of metal ions. [Parameters held constant: Initial metal ion concentration (7.5 mg/L), biosorbent concentration (20% v/v) and contact time (10 min)].

3.2. Biosorption studies

100 90 80 70 60 50 40 30 20 10 0

Zn Ni Fe Cr Cd 0

20

40

60

Biosorbent concentraƟon (mL) Fig. 4. Effect of biosorbent concentration on the uptake of metal ions. [Parameters held constant: pH (6), initial metal ion concentration (7.5 mg/L) and contact time (10 min)].

100 80 Recovery %

3.2.1. Effect of contact time The effect of contact time on the removal of multiple metal ions (Zn2þ, Ni2þ, Fe2þ, Cr3þ and Cd2þ) was investigated at different time intervals ranging from 2 to 30 min using DCE biosorbent. The results in Fig. 2 show that biosorption increased rapidly at the beginning and became very slow at the end. Each metal ion reached equilibrium at different contact times. For example Zn2þ, Cd2þ and Fe2þ achieved equilibrium at 8e10 min, recording removal efficiencies of 64.7% for Zn2þ and 66.5% for Fe2þ, 78.7% for Cd2þ, while Ni2þ and Cr3þ reached equilibrium at 20 and 30 min, recording removal efficiencies of 69.2% and 60.9% (Fig. 2), respectively. These results are in line with Miretzky et al. (2008) which reported that within 7 min of contact time, the uptake efficiency of positively charged metal ions by Opuntia streptacantha increased to 86.5%. The increased rate of metal biosorption within 10e30 min for Cr3þ and 10e20 min for Ni2þ can be attributed to the high rate of accumulation of the metal ions in the biosorption sites within the mucilage (Tavenga et al., 2013). Saturation of active sites within the mucilage may be responsible for the slow sorption activities displayed by metal ions except Cr3þ towards 30 min of contact time (Pehlivan et al., 2009).

Recovery %

Biosorption studies results are presented in Figs. 2e6. It should be pointed out that unmodified mucilage (DCE) was used mostly in this biosorption study to optimise operating parameters. Modified mucilage (PCE and SCE) was only used for comparative study.

Zn

60

Ni 40

Fe

20

Cr Cd

0 7.5

12.5

25

IniƟal concentraƟon (mg/L)

80 70

Fig. 5. Effect of initial concentration of metal ions. [Parameters held constant: pH (6), biosorbent concentration (20% v/v) and contact time (10 min)].

60 Recovery %

4

50

Zn

40

Ni

30

Fe

20

Cr

10

Cd

0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 Contact Ɵme (mins)

Fig. 2. Effect of contact time on uptake of metal ions by DCE. [Parameters held constant: pH (6), initial metal ion concentration (7.5 mg/L) and biosorbent concentration (20%v/v)].

3.2.2. Effect of pH The change in pH of the aqueous solution influenced the biosorption efficiency of DCE mucilage (Fig. 3). Fig. 3 shows fluctuations in biosorption efficiency with a steady increase in the pH of the aqueous solution. The maximum uptake of metal ions by mucilage biosorbent occurred at pH 4e8. Optimum binding of mucilage with Zn2þ, Ni2þ, Fe2þ and Cd2þ occurred at pH 4. The optimum binding with Cr3þ occurred at pH 6. The minimum uptake of all the metal ions was observed at pH 2. The results of this study show similarity to those of Qi and Aldrich (2008), where biosorption of heavy metal ions by tobacco

Recovery %

B.O. Jones et al. / Journal of Environmental Management 177 (2016) 365e372

100 90 80 70 60 50 40 30 20 10 0

DCE SCE PCE

Zn

Ni

Fe

Cr

Cd

Fig. 6. Influence of biosorbent modification on the removal efficiency of mucilage. [Parameters held constant: pH (6), initial metal ion concentration (12.5 mg/L), biosorbent concentration (20% v/v) and contact time (10 min)].

dust started at pH 4e5 and decreased at pH 9e10. The results also agree with those of Bhatti et al. (2007) and Sharma et al. (2006). The uptake of zinc ion by Moringa oleifera (MO) increased with an increase in the pH (Bhatti et al., 2007). Biosorption of Cd (II) by MO seeds occurred at pH range of 4.5e7.5 with optimum binding with Cd ions at pH 6.5 (Sharma et al., 2006). The increase in the removal efficiency with an increase in pH from 2 to 8 could be explained from the fact that in extreme acidic conditions (pH 2), the carboxylic groups binding sites in the mucilage are less ionised and remain constant. Thus, the negatively charged carboxylic group acts like positively charged species due to protonation and cannot attract positively charged metal ions. With an increase in pH of the solution to less acidic or basic conditions; the carboxylic groups become ionised and are converted to negatively charged species due to deprotonation. They attract positively charged metal ions. Thus the multiple metal ions (Zn2þ, Cr3þ, Ni2þ, Cd2þ and Fe2þ) and the Hþ in the mucilage compete for the same sorption site resulting in high biosorption activity. The slight decrease in biosorption activity in the pH regions of 4e6 may be due to poor transfer while the pH is approaching or is the neutral region (Fox et al., 2012). Thus extremely low acidic conditions result in poor metal binding activity, while at optimum pH range between pH 4.0 to 8.0, the binding sites are unprotonated and metal binding is maximised (Pehlivan et al., 2009). From the pH analyses results, it can be noted that the initial pH of the metal ions solution can influence the binding affinity between mucilage molecules and metal ions.

3.2.3. Effect of biosorbent concentration The results (Fig 4) show that the removal efficiency of heavy metals ions increased with an increase in the concentration of mucilage. For example, Fig. 4 shows that an increase in mucilage concentration from 10 to 50%v/v generally resulted in an increase in the removal of each metal ions. Removal efficiency of Cd2þ increased from 77.4% to 89.7%, Ni2þ from 71.3% to 87.6%, Zn2þ from 66.1% to 87.8%, Fe2þ from 57.6% to 83.4% and Cr3þ from 46.6% to 77.4%. The increase in the biosorption efficiency with sorbent concentration is highly attributed to better availability of the sorption sites within the mucilage concentration to bind metal ions. The concentration range observed in this study differs from Mane et al. (2011). Mane et al. (2011) reported that 10% v/v of mucilage concentration gave the maximum removal efficiency more than 20% v/v using Opuntia species-mucilage. The variation in the results can be attributed to different experimental conditions such as: the percentage concentration of the mucilage, plant species (Opuntia species), concentration of the metal ions, variation in biochemical composition, contact time and pH. At higher concentration, binding activities between metal ions

369

and biosorbent tend to increase and this can result in an increase in removal efficiency. Medina-Torres et al. (2000) also reported that mucilage is likely to form macromolecular bonds at high concentration than at low concentration (Leon-Martinez et al., 2011). Therefore, increasing mucilage concentration could results in high biosorption efficiency compared to low concentration within the present conditions of the study. 3.2.4. Effect of initial concentration of heavy metals The effect of initial concentration of metal ions on DCE biosorption efficiency was investigated. The results (Fig. 5) show that uptake of heavy metals by DCE was very effective with increase in the initial concentration within the range of study. All the metal ions were adsorbed uniformly with increase in initial concentration of metal ions except for Cr3þ. The uptake of Cr3þ showed a slight decrease with increase in the initial concentration from 7.5 to 12.5 mg/L. It later increased significantly with an increase in initial metal ion concentration from 12.5 to 25 mg/L. The difference in the uptake of Cr3þ with increase in the initial concentration from 7.5 to 12.5 can be attributed to the influence of the presence of other metal ions in the solution. Tsezo and Volesky (1981) reported that the uptake of Thorium and Uranium was found to be influenced by the presence of Fe2þ and Zn2þ in solution. Veglio and Beolchini (1997) also confirm that the presence of multiple metal ions is a factor that could influence the removal of another metal ion. These can be attributed to several metal ions competing against each other for open sites available on the biosorbent thus; influencing the uptake of one metal ion compared to the other metal ions in solution. The results achieved in this study concur with literature whereby sorption capacity increase with increase in the initial concentrations of the heavy metals (Mishra et al., 1997; Van Wyk and Gericke, 2000; Chuah et al., 2005). Van Wyk and Gericke (2000) reported that a natural biosorbent such as rice husk also increased the adsorption capacity of Zn (II) with increment in the initial concentration of the heavy metals. 3.2.5. Influence of biosorbent modification Fig. 6 illustrates that metal ions removal efficiency of modified mucilage (PCE and SCE) was significantly higher than for the unmodified mucilage (DCE). The highest removal efficiency of metals ions was achieved with PCE sorbent followed by SCE sorbent. DCE sorbent gave the lowest biosorption efficiency. Although DCE and SCE displayed near similarity in the uptake of Cd2þ ion with sorption efficiency of 73.6% and 74.3%, respectively. It is important to mention that from the illustration in the FTIR spectra (Fig. 1) there were increases in broad peaks of the hydroxyl, carboxyl and carbonyl groups for PCE and SCE compared to DCE. SCE had broad large peak followed by PCE displaying satisfactory structural stability. Similar results were also obtained by Ghanem et al. (2009) indicating that levels of rhamnose and uronic acid in Kosteletzkya virginica plant increased when exposed to salt. These results also compare with results of Chubar et al. (2004) where cork oak powder was modified with salts such as NaCl and CaCl2 solely for the uptake of Cu, Zn and Ni from aqueous solution. The modified salt extract showed greater sorption capacity than the unmodified cork with NaCl displaying high sorption efficiency than CaCl2. The high biosorption activities of PCE and SCE are due to the conversion of the binding sites in the functional group from Hþ form to Naþ and Kþ forms, thus increasing the density of sorbent sites in the mucilage (Chubar et al., 2004). Increase in biosorption activity in PCE more than SCE can be attributed to Kþ exhibiting a stronger ionic binding affinity to the carboxylic anions groups in the mucilage more than Naþ. It also compares with the observation of Tang and Allen (2009) in the

B.O. Jones et al. / Journal of Environmental Management 177 (2016) 365e372

A

90

Ni

80

Cr

60

y = 2.6999x - 0.0026 R² = 1 for Ni

y = 2.7035x + 0.0122 R² = 1 y = 2.7103x - 0.011 for Fe R² = 1 for Cr

30

20 10

Zn 3500

50 40

B

Cd

y = 2.6969x + 0.0055 R² = 1 for Cd

70 t/qt (min g/mg)

Fe

t/qt (min g/mg)

370

y = 105.96x + 27.493 R² = 0.9974 for Zn

3000 2500 2000 1500 1000 500 0

0 0

10

20

30

0

40

10

20

30

Time (min)

Time (min)

Fig. 7. (A-B) Pseudo-second-order kinetic plot for Cd, Ni, Cr, Fe and Zn biosorption onto DE mucilage.

binding affinity of Naþ versus Kþ to the carboxylic acid group of fatty acid monolayer. From the quantitative point of view, it was observed that an increase in the ionic strength of the modified mucilage by addition of salt influences metal ions uptake. Thus, it improves the binding of the metals ions in the mucilage functional groups sites. Although it has been discussed that the ionic strength of the mucilage increases with increase in salt concentration. It should be noted that further increase in salts concentration decreases the viscosity of the mucilage, and thus affect the hydrodynamic and flow properties of mucilage molecules (Medina-Torres et al., 2000; Koocheki et al., 2013). The high uptake of metal ions by mucilage extracted from salt solution may be attributed to using very low concentration of the salt. This therefore does not affect the viscous

Fe

Cr

y = -1.4533x + 9.0305 9.5 R² = 0.8774 8.5 for Cd 7.5

Cd

3.3. Biosorption kinetics Kinetics studies were carried using pseudo-first-order and pseudo-second-order models to determine the biosorption mechanism (Qiu et al., 2009). The kinetic time interval was from 4 to 30 min for Zn2þ, Cd2þ, Ni2þ, Cr3þ and Fe2þ biosorption onto DE mucilage. The values of log (qe-qt) versus time (t) and t/gt versus time (t) was used to obtain linear plots for pseudo-first-order and pseudo-second-order as described in Kanu et al., 2015 and Moyo et al., 2015b. Kinetic models results displayed in Figs. 7e8 showed that the data fit well to pseudo-second-order more than pseudo-first-order reaction. The correlation coefficient values of

B

6.5 5.5

3.5

y = -8.0106x + 241.86 R² = 0.9925 for Ni

250 200 150 100

y = -98.588x + 559.04 R² = 0.9707 for Zn

50

2.5

0 2

4

6

8

2

Ce Fe

C

Cr

y = 1.9333x - 1.0885 R² = 0.937

Cd

4

6

Ce

D

y = 1.2606x - 1.0455 R² = 0.9621

0.2 0.1

-1.6

0 -0.1

y = 2.917x - 3.3267 R² = 0.9994 for Zn Zn

log qe

log qe

Ni

300

y = -0.204x + 7.4434 R² = 0.9473 for Fe

4.5

Zn 350

y = -0.419x + 9.607 R² = 0.6299 Cr Ce/qe

Ce/qe

A

nature of the mucilage, but increases its ionic strength.

-0.2 -0.3

Ni y = 1.1629x - 2.412 R² = 0.9987 for Ni

y = 0.733x - 0.7049 R² = 0.9554

-0.4 -0.5

0.3

0.5

0.7 log Ce

0.9

-2.1 0.3

0.5

log Ce

0.7

0.9

Fig. 8. (A-B): Langmuir isotherm plot and fig (C-D) Freundlich isotherm for Cd, Ni, Cr, Fe and Zn biosorption onto DE mucilage.

B.O. Jones et al. / Journal of Environmental Management 177 (2016) 365e372

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Table 1 Comparison of maximum biosorption capacity of DE mucilage with other biosorbents. Biosorbent

Metal ions

Adsorption capacity qmax (mg g1)

References

Neem bark

Cd(II) Zn(II) Cr(III) Cd(II) Ni(II) Cd(II) Ni(II) Zn(II) Cr(III) Cr(III) Zn(II) Ni(II) Zn(II) Cr(III) Fe(II) Cd(II)

13.29 25.57 85.1 22.42 30 1.5 3.37 3.55 8.69 58.3 0.18 0.125 0.01 2.39 4.90 0.69

Naiya et al. (2009)

Yellow passion fruit Rooibos shoots Ascophyllum nodosum Walnut shell Jute fibres Untreated Pinus Sylvestris bark Dunaliella sp Sugar beet pulp Dicerocaryum eriocarpum

R2 ¼ 1 for Cd, Ni, Cr, Fe and R2 ¼ 0.9974 for Zn strongly suggest that pseudo-second-order was the kinetic model for DE mucilage biosorbent. 3.4. Biosorption isotherms Langmuir and Freundlich models were used to interpret the equilibrium data for biosorption of Zn, Ni, Fe, Cr and Cd onto DE mucilage (Fig 8). Isotherms in biosorption are important to determine the nature of interaction taking place between the metal ions and biosorbent (Guyo et al., 2015b). The chi-square values (2) and correlation coefficients (R2) were calculated from the Langmuir isotherm plot of Ce/qe versus Ce and Freundlich isotherm plot of log qe versus log Ce. The coefficient values derived from the Freundlich isotherm plots recorded 0.9994, 0.9987, 0.9554, 0.9621 and 0.937 for Zn, Ni, Fe, Cr and Cd, respectively while the coefficient values for Langmuir model were 0.9707, 0.9925, 0.9473, 0.6299 and 0.8774 for Zn, Ni, Fe, Cr and Cd as displayed in Fig. 8. The above data showed that Freundlich model correlation coefficients values were higher than that of Langmuir model. The lower values derived from the linearised equation showed that Langmuir model did not fully pronounced the equilibrium relationships between the metals ions and their equilibrium concentration in solution unlike Freundlich model (Guyo et al., 2015b). This study concluded that the equilibrium data fitted well with Freundlich model and it is more favourable describing the biosorption of Zn, Ni, Fe, Cr and Cd onto DE mucilage. 4. Comparison of DE mucilage with other biosorbents Comparison studies indicating the biosorption capacity of DE mucilage and other natural plants biosorbent is summarised in Table 1. It was observed that the biosorption capacity of DE mucilage was lower than other plants biosorbent as indicated in Table 1. This may be attributed to the characteristics of DE plant and the experimental procedure playing a key role in its biosorption capacity. However, the major advantage of using DE mucilage is that it is cheaper and easily available for multiple uses. 5. Conclusion The study has shown that both modified and unmodified mucilage of DE plant can be successfully applied as biosorbent media in the uptake of multiple metal ions species from aqueous solution. DE mucilage successfully acts as a biosorbent by taking

Jacques et al. (2002) Kanu et al., 2015 Volesky and Holan (1995) Orhan and Bueyuekguengoer (1993) Shukla and Pai (2005) Alves et al. (1993) Donmez and Aksu (2002) Pehlivan et al. (2006) Present study

advantage of its high active biosorption sites present in functional groups of the mucilage. The high removal efficiency observed in the modified mucilage may not be highly dependent on the viscous nature of the mucilage, but rather the increase in the ionic strength of the mucilage. Generally, the introduction of charged ions (NaCl and KCl) into negatively charged polyelectrolytes mucilage did not decrease the uptake of metal ions but rather increased the removal efficiency of the metals ions. The biosorption isotherm was best described with Freudlich model more than Langmiur model. The kinetic study showed that biosorption followed Pseudo-secondorder model. The major setback of using mucilage as biosorbent is that it cannot be regenerated and re-used after being used in the first batch since it is in mucilage form. It is proposed that DE mucilage as biosorbent will be most suitable and applicable especially in households in rural community because it is less costly and is a relatively simple process to use, especially if locally available. Acknowledgements The authors would like to thank the Research and Publications Committee (RPC) of the University of Venda for providing financial support. We would also want to thank the University of the Witwatersrand and University of Venda for their assistance in the experimental analyses. References Ahluwalia, S., Goyal, D., 2007. Review: microbial and plant derived biomass for removal of heavy metals from wastewater. Bioresour. Technol. 98, 2243e2257. Al-Masri, M.S., Amin, Y., Al-Akel, B., Al-Naama, T., 2010. Biosorption of cadmium, lead, and uranium by powder of poplar leaves and branches. Appl. Biochem. Biotechnol. 160, 976e987. ^ Alves, M.M., Beca, GonzaAlez C.G., Guedes de Carvalho, R., Castanheira, J.M., Pereira, Sol M.C., Vasconcelos, L.A.T., 1993. Chromium removal in tannery wastewaters “polishing’’ by Pinus sylverstris bark. Water Res. 27, 1333e1338. Barone, G., Corsaro, M., Giannttasio, M., Lanzetta, R., Moscariello, M., Parrilli, M., 1996. Structural investigation of the polysaccharide fraction from the mucilage of Dicerocaryum zanguebaricum Merr. Carbohydr. Res. 280, 111e119. Bellmann, K., Khare, A., 2000. Economic issues in recycling end-of-life vehicles. Technovation 20, 677e690. Benhura, M.A.N., Marume, M., 1993. The mucilaginous polysaccharide material isolated from ruredzo (Dicerocaryum zangebarium). Food Chem. 46, 7e11. Bhatti, H., Mumtaz, B., Muhammad, A., Nadeem, R., 2007. Removal of Zn(II) ions from aqueous solution using Moringa oleifera Lam. (horseradish tree) biomass. Process Biochem. 42, 547e553. Chuah, J., Jurnasiah, A., AzniI, I., Katayon, S., Thomas Choong, Y., 2005. Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal; an overview. Desalination 175, 305e316. Chubar, N., Calvalho, R., Correia, M., 2004. Heavy metals biosorption on cork biomass: effect of the pre-treatment. Colloids Surf. 238, 51e58. Donmez, G.C., Aksu, Z., 2002. Removal of chromium(VI) from saline wastewaters by Dunaliella species. Process Biochem. 38, 751e762.

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