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0South African Journal of Botany 2002, 68: 333-341 Printed in South Africa — All rights reserved
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SOUTH AFRICAN JOURNAL OF BOTANY
ISSN 0254-6299
Optimising heavy metal adsorbance by dried seaweeds
SA Nigro, WA Stirk* and J van Staden
Research Centre for Plant Growth and Development, University of Natal, Pietermaritzburg, P/Bag X01, Scottsville 3209, South Africa
* Corresponding author, e-mail: stirk@nu.ac.za
Received: 15 March 2001, accepted in revised form 14 January 2002
Industrialisation has resulted in large-scale production of anthropogenic pollutants, particularly heavy metals. Existing industrial techniques for the purification of waste water are expensive. A cheaper alternative may be 'bioremoval', that is, the accumulation and concentration of pollutants from aqueous solutions using biological material.
The adsorption of copper, zinc and cadmium using two dried seaweeds Ecklonia maxima and Laminaria pallida (order Laminariales) and Kelpak waste (also made from Ecklonia maxima), a byproduct from the manufacture of the seaweed concentrate Kelpak, were investigated under laboratory conditions, to determine some factors affecting heavy metal adsorption. Ion adsorption from single and mixed metal ion solutions of 10mgl-1 and 100mgl-1 containing copper, zinc and cad-
mium were tested at various temperatures and pH. Optimum adsorption occurred at pH 3 and pH 7 and Kelpak waste had equal or superior adsorption ability to dried Ecklonia maxima and Laminaria pallida, particularly for copper. Optimum adsorption occurred at temperatures of 20°C and 30°C. Heavy metal adsorption trends by the individual seaweed biosorbent remained constant regardless of the species of anion present. Drying of the experimental material (fan air and oven drying at 85°C) prior to adsorption cycles resulted in more efficient ion uptake, particularly after additional rehydration. Heavy metal uptake was monitored over a number of semi-continuous adsorption cycles, using the same biomasses. Ion uptake was the most efficient after 2-4 adsorption cycles.
Introduction
Due to their recalcitrant nature heavy metals tend to pervade the environment, minute quantities becoming biomagnified to greater concentrations. They accumulate throughout the food chain thereby becoming a health hazard (Volesky and Holan 1995, Bux and Kasan 1996). Their toxicity is compounded because once protein-denaturing heavy metals are absorbed by bodily tissues, they are not easily excreted. Heavy metals have been prioritised as leading contaminants in South Africa due to indiscriminate disposal and their carcinogenic and toxic nature (Bux et al. 1994).
Persistent metal contamination prompts two-fold concern. The supply of good potable water is essential for the establishment and maintenance of any community (Bux et al. 1997), and problems are exacerbated by low annual rainfall and the pollution of existing water supplies in South Africa. In 1986, the Department of Water Affairs reported South Africa's annual rainfall average to be 497mm, well below the world's average of 860mm (Bux and Kasan 1996). Secondly, Oliver and Cosgrove (1974) in Bux and Kasan (1996) reported that metal species present in sewage treatment systems reduce the efficiency of aerobic sewage treatment and
anaerobic digestion of sludges. Metal ion toxicity and the commercial value of depleted and non-renewable reserves therefore warrant metal recovery from metal-contaminated aqueous solutions.
Conventional treatment methods employ chemicals, utilising precipitation or neutralisation, or physical techniques to remove contaminating metals from effluents. These industrial processes include hydroxide and sulphide precipitation, addition of natural and synthetic polymers, ion exchange resins, activated carbon sorption, dialysis and membrane technology (Bux et al. 1997, Atkinson et al. 1998). These conventional treatment methods are only viable for large industrial units that can justify the high cost involved, as expensive active agents cannot be used in successive treatment cycles. In addition, the end product is a low-volume, high metal concentration sludge that is difficult to dispose of and to dewater, and is not suitable for final purification. The use of low-cost alternative treatment technologies and materials is critical for the purification of waste water.
Global interest has prompted wide-scale research into the use of biological systems for the remediation of heavy met-
als from industrial discharges (Bux et al. 1997). 'Biosorption' is the passive or physico-chemical attachment of the sorbate to a biomass of animal or plant origin (Bux et al. 1994). The biomass may be living or dead (Volesky 1990 in Aderhold et al. 1996). A number of mechanisms such as adsorption, complexation, ion exchange and intracellular transport are involved in metal-biomass interactions (Bux et al. 1997). Adsorption refers to the ability of the adsorbent to selectively concentrate the targeted adsorbate such as metal ions to its surface, while absorption implies the entry of a material through to an inner matrix.
Bioremoval has advantages over precipitation methods in terms of the ability to adjust to changes in pH and heavy metal concentrations. It is also more sensitive than reverse osmosis to the presence of suspended solids, organics, and other heavy metals. Further advantages lie in the use of abundant, renewable biosources which have high selectivity for the removal and recovery of specific metals. Also, large volumes of waste water containing multiple heavy metals and or mixed waste can effectively be treated due to rapid kinetics. The operation of bioremoval systems is cost-effective, without the need for expensive process reagents, and is still effective over a wide range of physio-chemical conditions (pH, temperature, presence of other ions such as Ca2+ and Mg2+). Finally, there is a vast improvement in the recovery of bound heavy metals and a reduction in the volume of hazardous waste produced (Wilde and Benemann 1993).
Both living and dead biomasses can be obtained at negligible cost: yeast and fungi for example are the waste products of several major industrial fermentation processes (Bux et al. 1997). Low cost materials investigated for their ability to sequester ions include wool, rice, straw, waste linseed straw, coconut husks, peat moss and sphagnum peat moss, walnut expeller meal, waste tea leaves, peanut skins, sugar cane bagasse and even waste rubber tyres (Randall et al. 1975, Knocke and Hempill 1981, Macchi et al. 1986, Ferro-Gargia et al. 1988, Tee and Khan 1988, Ho et al. 1994 in Williams et al. 1996). Microbial and microalgal biomasses have effective biosorptive potential for heavy metals and include bacteria, yeasts, filamentous fungi, and marine and freshwater algae (Atkinson et al. 1998). Seaweeds, with their high biomass, are another inexpensive source of biomass of cellulosic nature.
The distribution of the kelps, Ecklonia maxima and Laminaria pallida, are restricted to the western Cape coast. Ecklonia maxima forms extensive beds which are seen as a surface canopy from the initial surf-zone between Cape Agulhas to northern Namibia. Bulk dry mass of Ecklonia maxima is exported for alginate extraction. Other uses are the inclusion of the milled product in animal feeds, fertiliser and as a soil-binder/fertiliser used for the stabilisation and artificial revegetation of embankments. The negatively-buoyant Laminaria pallida is distributed from Cape Columbine to Cape Agulhas. The alginate extract for these brown seaweeds has wide commercial uses such as gelling, emulsifying and stabilising agents; food, paper, textiles, welding rods and pharmaceuticals (Anderson et al. 1989).
Metal-complexing capacity is a function of surface ionic charge, resulting from carboxylic and hydroxylic groups
(Kaplan 1988). The Phaeophyceae thallus comprises of alginate (polyuronic acid) and fucoidan (sulfonated glu-curono-xylo-fucane) embedded in a fibrous skeleton. It is reported that the alginate section of the biomass is responsible for the greater part of metal uptake (Fourest and Volesky 1996). At least two chemical groups in the cell wall polysaccharides are responsible for complex algal ion exchange: carboxyls of uronic acids and sulphates of car-rageenans, xylans, and galactans. The net negatively charged algal thalli have a high affinity for metal ions due to amino and phosphate groups in nucleic acids, amino, amido, sulfhydryl and carboxyl groups of proteins, hydrox-yls, carboxyls and sulfates in polysaccharidesites (Knauer et al. 1997). Different parts of the seaweed show varying biosorbent abilities (Aderhold et al. 1996). The biosorption equilibrium of activated sludge is reached quicker in mixed metal ion solutions than in single ion solutions due to partial non-specificity and their competitive nature (Bux et al. 1997).
More than four hundred tonnes of Kelpak waste is produced annually during the manufacture of Kelpak, a seaweed concentrate extracted from the brown alga Ecklonia maxima. The Kelpak waste rapidly adsorbs heavy metals over a wide concentration range, suggesting that it is suitable for the bioremediation of waste water (Stirk and Van Staden 2000). The aims of this study were to determine those factors which play a role in heavy metal adsorbance by the Kelpak waste and to compare it (the Kelpak waste) with seaweed biomass derived from dried brown seaweeds.
Materials and Methods
Two brown seaweeds and a seaweed waste product were tested for their ability to sequester metal ions under laboratory conditions. Two members of the Phaeophyta: Ecklonia maxima (Osbeck) Papenfuss and Laminaria pallida Greville ex J. Agardh were collected from Kommetjie, on the Western Cape Coast of South Africa. The harvested blades are spread out and left to dry slowly. The seaweed was ground and sieved to obtain a homogenous particle size of 0.5-1.0mm in diameter, a similar size to that of other commercial adsorbents (Williams et al. 1996). The Kelpak waste was obtained from the production of Kelpak, an agricultural concentrate. This seaweed concentrate is prepared from the stipes of the brown alga Ecklonia maxima through the unique process of 'cell burst' where no heat or chemicals are used (Featonby-Smith 1984). Freshly cut and washed Ecklonia maxima stipes are processed through a series of cutters producing particles. A degree of potential energy is induced in the seaweed particles by high pressure before being passed at high velocity through a low pressure area. The energy released causes the cells to expand, exceeding their elastic potential and burst, releasing cell contents which are used in Kelpak (Stirk and Van Staden 1997). The Kelpak waste essentially consists of cell walls. All three types of material were stored at 10°C until needed.
Single and multiple metal ion solutions were prepared to give copper, zinc and cadmium concentrations of 10mgl-1 and 100mgl-1 using CuSO4.5H2O, ZnSO4.7H2O and
3CdSO4.8H2O (unless otherwise stated). The chemicals used were of Analar grade and deionised distilled water was employed throughout all experimental preparation.
The technique used was modified from Aderhold et al. (1996), Williams and Edyvean (1997) and Stirk and Van Staden (2000). For each experiment, 50ml of metal ion solution was added to an Erlenmeyer flask containing 0.5g ground dry (excluding Kelpak waste) seaweed under investigation. The Kelpak waste wet:dry mass ratio was determined and the equivalent of 0.5g dry mass was added to each flask. The flasks were agitated on an orbital shaker at 100rpm for 24h at 20°C. When the experiment was terminated, 10ml aliquots of solution were pipetted from each flask, avoiding any particulate matter. The concentrations of the metal ions left in solution after the adsorption process were then determined using an Atomic Absorption Spectrophotometer (Varian AA-1275 series) in an air/acetylene flame. The 100mgl-1 samples were diluted to 10mgl-1 before readings were taken. To ensure consistency, all experiments were carried out in triplicate. The mean and standard error of the triplicate readings were calculated. Results were analysed statistically by analysis of variance and the Student-Newman-Keuls test using the Statistical Analysis System (SAS) computer package (SAS 1987) at a 95% confidence interval. All glassware was pretreated by soaking at ambient temperature in 3-5% Contrad, an industrial strength detergent, for 24h and then rinsed in running water for 2h. This ensured that all ions were removed from the glassware.
Effect of anions on copper and zinc adsorption
It was necessary to determine the effect of various anions on biosorption as industrial wastewater contains a mixture of different ions and anions. Copper and zinc anions tested included: CuSO4.5H2O, Cu(NO3)2.3H2O, CuCl2.2H2O, (CH3.COO)2Cu.H2O, ZnSO4.7H2O, ZnCl2 and (cH3COO)2Zn.2H2O made up at 10mgl"1 and 100mgl"1. The experiment ran for 24h and the pH was measured at the start of the experiment.
Effect of temperature on ion adsorption
This experiment was performed at three different temperatures: 10°C, 20°C and 30°C for 24h. The mixed ion solution of CuSO4.5H2O, ZnSO4.7H2O and 3CdSO4.8H2O was made up to 10mgM concentrations. The pH was again monitored.
Effect of pH on biosorption in single metal ion solutions
As most biosorption occurs between pH 4 and pH 6 (Bux et al. 1997), a range of pH's from pH 3 to pH 7 were tested in single metal ion experiments. The pH of the 10mgl-1 solutions was adjusted using KOH and HCl. After 24h, the solutions were analysed by Atomic Absorption Spectrophoto-metry to determine if any trend could be observed in the rate of biosorption in different pH conditions.
Effect of drying and rehydration on ion adsorption
The Kelpak waste is a moist sludge. If it is developed for bioremediation, transport costs would be an important consideration. It was also observed that the seaweed biomasses swelled during an adsorption cycle as a result of rehydration. This experiment was designed to determine the effect of drying and rehydration on the seaweed biomass's ability to sequester heavy metal ions.
The treatments were:
i) CONTROL: 0.5g dry weight of the biosorbent (or equivalent for the Kelpak waste) was added to 50ml single ion solutions of copper, zinc and cadmium and left on the shaker for 24h.
ii) REHYDRATED: the biomasses (0.5g dry weight) were soaked in deionised distilled water for 24h on a shaker after which the water was filtered off using Whatman No. 1 filter paper. Ion solutions were added as for the control (treatment i).
iii) AIR DRIED: Erlenmeyer flasks containing 0.5g biomass each were placed directly under a fan. The mass of each sample was measured every 24h until a stable weight was obtained. Ion solutions were added as for the control.
iv) AIR DRIED AND REHYDRATED: the biomass was dried as described above (treatment iii). The biomasses were then rehydrated as described in treatment ii after which ion solutions were added for 24h.
v) OVEN DRIED: 0.5g biomass was dried at 85°C. The mass of each sample was weighed every hour until a stable weight was obtained. Ion solutions were added as for the control (treatment i).
vi) OVEN DRIED AND REHYDRATED: the biomass was dried as described above (treatment v) and then rehydrated as in treatment ii after which ion solutions were added for 24h.
After a 24h adsorption cycle, 10ml aliquots were taken and the final ion concentration determined using the Atomic Absorption Spectrophotometer.
Semi-continuous adsorption of seaweed residues
This continous batch adsorber system, using the same biomass throughout, has similar application in industrial biore-actors. A modified method described by Williams et al. (1997) was adopted. Fifty ml of mixed metal ion solution was added to Erlenmeyer flasks containing 0.5g (or equivalent for Kelpak waste) seaweed. After each 24h period of adsorption, the solution and the metal loaded seaweed residues were filtered using Whatman No. 1 filter paper. Aliquots of solutions were collected and analysed for residual metal ion concentration using the Atomic Absorption Spectrophotometer. The seaweed residues were returned to the flasks and 50ml of new mixed metal ion solution was added. These flasks were agitated on an orbital shaker for 24h. This process can be described as an adsorption cycle.
A 10mgl-1 metal ion concentration was used during these experiments. The selective uptake of copper, cadmium and zinc in 10mgl-1 solutions was measured over 14 adsorption cycles, the equivalent of 14 days.
A) Copper 10mg M ^Kelpak w
I I Ecklonia maxima Ijijijíjíl Laminaria pallida vw
B) Zinc 10mg I1
C) Copper 100mg I"
D) Zinc 100mg I" x
CuS0..5H.0 Cu(NO,)..3H.O CuCI..2H.O (CHCOO).
(CHCOO)2
ANION TYPE
Figure 1: Copper (Kelpak: P=0.0134, Ecklonia maxima: P=0.0001, Laminaria pallida: P=0.0006) and zinc uptake (Kelpak: P=0.0001, Ecklonia maxima: P=0.0001, Laminaria pallida: P=0.0001) by different seaweed biomasses in the presence of different anions at 10mgl-1 and 100mgM metal ion solutions. Error bar represents the standard error. Significant difference is denoted by the alphabetical sequence a, b, c, d for Kelpak, m, n, o, p, q for Ecklonia maxima and u, v, w, x, y, z for Laminaria pallida
Results
Effect of anions on copper and zinc adsorption
The effect of different anions of copper and zinc on adsorption were determined (Figure 1).
At low initial copper concentrations (10mgl-1), the Kelpak waste was able to lower the final ion concentration to below 0.5mgl-1 in the presence of all the anion species. The E. maxima and L. pallida material did not adsorb the copper ions as well as the Kelpak waste (Figure 1A). At high initial copper concentrations (100mgl-1), the Kelpak waste gave best adsorption, lowering the final ion concentration to below 10mgl-1. The E. maxima performed better than the L. pallida biomass. At high ion concentrations, the anion species had no effect on copper adsorption (Figure 1C).
Similar adsorption trends were evident for 10mgl-1 zinc anion solutions (Figure 1B). The Kelpak waste did not show as efficient adsorption as with the copper anions. However, zinc chloride and zinc acetate at 100mgl-1 solutions were better adsorbed by the seaweed biomasses than zinc sulphide (Figure 1D).
Effect of temperature on ion adsorption
Overall, results indicate that optimum adsorption occurs between the temperatures 20-30°C. The Kelpak waste had superior copper uptake to E. maxima and L. pallida (Figure 2A), reducing final concentrations of ions left by Kelpak waste to 1.5-2.5mgl-1. Best copper adsorption was attained at 20-30°C (Figure 2A). Uptake of zinc was unaffected by temperature, with a general residual concentration of 2.0-2.5mgl-1 at all temperatures tested by all three biosor-bents (Figure 2B). Figure 2C indicates that cadmium adsorption was most efficient by E. maxima leaving a residual concentration of 0.51 mgl-1 and 0.53mgl-1 at 20°C and 30°C respectively. Cadmium adsorption by all three biomasses was most effective at 20-30°C with final ion concentrations below 2mgl-1.
Effect of pH on ion adsorption in single ion solutions
Initial pH values of the single ion solutions before adjusting were as follows: cadmium pH 4.6, zinc pH 4.2, copper pH 4.3. The Kelpak waste exhibited the highest uptake of copper (pH 3) and zinc (pH 7), (Figure 3A and B) compared to
the two seaweed biomasses, lowering the 10mgl-1 ion solution to below 1mgl-1. The adsoprtion efficiency of the Kelpak waste was not significantly affected by the range of pH's tested. The E. maxima and L. pallida biomasses tended to sequester copper and zinc ions more efficiently at pH 3 compared to pH 5 and 7 (Figure 3A and B). Lower pH (pH 3) caused a significant increase in adsorption for the Kelpak waste and Laminaria pallida (Figure 3C).
Effect of drying and rehydration on ion adsorption
Rehydration greatly improved ion adsorption compared to
the control treatments in both E. maxima and L. pallida, leaving a final concentration ranging from 0.2-0.8mgl-1. Rehydration also improved ion adsorption by the Kelpak waste but not to the same extent as the other two seaweeds (Figure 4A and C). Air drying and oven drying alone did not improve dsorption of copper, zinc and cadmium compared to the control treatment for Ecklonia maxima and Laminaria pallida. There was a slight increase in adsorption by the Kelpak waste.
However, air and oven drying followed by a rehydration step greatly improved ion adsorption by E. maxima and L. pallida lowering the final ion concentration to below 0.25mgl-1,
A) CUS04.5H20 H Ke|pak
I I Ecklonia maxima l=======3 Laminaria pallida
B) ZnS04.7H20
C) 3CdS04.8H20
10 20 30
TEMPERATURE (°C)
3.5 3.0 2.5 2.0 1.5 1.0 0.5
¿ 3.5
% 2.5 z
_, 0.5 A U
(ü 3 5 E
R3.0 2.5 2.0 1.5 1.0 0.5
- B)ZnS04.7H20
A) CUS04.5H20
i!:!:!: I---:!:!•!:
- C) 3CdS04.8H20
^M Kelpak
I I Ecklonia maxima
|i;i;i;ii Laminaria pallida
Figure 2: Effect of temperature on adsorption of (A) copper (Kelpak: P=0.1902, Ecklonia maxima: P=0.0001, Laminaria pallida P=0.0004), (B) zinc (Kelpak: P=0.9118, Ecklonia maxima P=0.0040, Laminaria pallida: P=0.0006), and (C) cadmium (Kelpak P=0.0356, Ecklonia maxima: P=0.0720, Laminaria pallida P=0.0176) 10mgl-1 metal ion solution by seaweed biomasses. Error bar represents the standard error. Significant difference is denoted by the alphabetical sequence a, b for Kelpak, m, n, o for Ecklonia maxima and x, y, z for Laminaria pallida
Figure 3: Effect of pH on (A) copper (Kelpak: P=0.2559, Ecklonia maxima: P=0.001, Laminaria pallida: P=0.0051), (B) zinc (Kelpak: P=0.1084, Ecklonia maxima: P=0.1605, Laminaria pallida P=0.0065), and (C) cadmium (Kelpak: P=0.0069, Ecklonia maxima P=0.0930, Laminaria pallida: P=0.0171) ion adsorption in single ion solutions of 10mgl-1 concentrations. Error bar represents the standard error. Significant difference is denoted by the sequence a, b for Kelpak, m, n, o for Ecklonia maxima and x, y for Laminaria pallida
even for the zinc ions which consistently showed the poorest adsorption in the other experiments. Ion adsorption by the Kelpak waste was also improved by drying and rehydra-tion with oven dried giving better results than air dried Kelpak waste.
Semi-continuous adsorption of seaweed residues
The selective uptake of copper, cadmium and zinc from a 10mgl-1 mixed ion solution by the seaweed biomasses over 14 successive cycles, is shown in Figure 5. In all the sea-
weeds, a general trend can be observed where ion uptake was most efficient after 2-4 (Kelpak) and 2-6 in the other seaweed biomasses. With each successive cycle, there was an increase in the residual ion concentration of all the metals. However, a more rapid increase in residual ion concentration, over fewer adsorption cycles, is reached with Kelpak residue than with E. maxima and L. pallida. A trend of preference for metal-ion uptake (copper, then cadmium, followed by zinc) with the same degree of metallic bonding was observed.
Discussion
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
A) CuSO4.5H2O x
a b l°W I
B) ZnSO4.7H2O
3.5 3.0 2.5
2.0 h am 1.5 1.0
llnW II llnw I
C) 3CdSO4.8H2O
Kelpak I I Ecklonia maxima ¡1:1:1:11 Laminaria pallida
PRETREATMENT
Figure 4: Effect of various drying and rehydration regimes of seaweed biomasses on ion adsorption (P=0.0001 for all seaweed biomasses and ions) of (A) copper, (B) zinc, and (C) cadmium ion adsorption. Error bar represents the standard error. Significant difference is denoted by the alphabetical sequence a, b, c, d, e for Kelpak, m, n, o, p, q for Ecklonia maxima and v, w, x, y, z for Laminaria pallida
Ion uptake by living algae occurs in two phases: the first reversible phase is fast where the ions only bind to inert lig-
2 4 6 8 10 12 14 NUMBER OF ADSORPTION CYCLES
Figure 5: The selective uptake of copper (0), cadmium (A) and zinc (□) by seaweed residues in 10mgl-1 mixed ion solutions, over 14 successive adsorption cycles. Final ion concentration for each cation is represented on the figure
ands on the cell surface. The second, slower phase results in the ions being bound to the cell wall and then actively taken up and bound intracellularly (Wilde and Benemann 1993, Knauer et al. 1997). As non-living samples were used in this study, only the first, reversible passive phase uptake would be expected (Stirk and Van Staden 2000).
The results indicated that the two seaweeds, E. maxima and L. pallida and the E. maxima-derived Kelpak waste, exhibited a variation in their ability to sequester different metal ions. L. pallida consistently adsorbed the least compared to the other two biosorbents, indicating species-specificity. Different ions exhibited different affinities for different functional groups (Stirk and Van Staden 2000).
Different salts of copper and zinc cations (Figure 1), showed trends similar to those reported by Stirk and Van Staden (2000), in that the Kelpak waste exhibited superior adsorption efficiency, particularly for copper (irrespective of the associated anion) than the other two seaweeds. Also little competition was observed for uptake sites in the presence of other anions associated with either zinc or copper cation. This is promising for application to industrial effluents which comprise a mixture of ions (and cations) in solution. The Kelpak waste was able to lower the final concentration of cationic copper in solution to well within the range that South African legislation requires for effluent to be released into natural water systems.
Results confirmed that experiments conducted at 20-30°C were at the optimal temperature. However, Kelpak's adsorption efficiency was not significantly affected by differences in temperature, suggesting that temperature does not have a significant effect on the kinetics between adsorption binding sites and metal ions present in solution. This could be expected as the experimental material was non-living. The material surface structure and chemistry of the groups involved in sequestration is also not affected by temperature fluctuations (Sanyahumbi et al. 1998).
Availability of metals for biosorption is influenced by the pH and is optimal in the range of pH 4-6 (Bux et al. 1997) as was illustrated at pH 3 and 7 in the present study (Figure 3). The pH values before adjustment were approximately 4 in 10mgl-1 solutions, confirming that biosorption naturally occurs within the optimal pH range. Lead removal using a water fern indicated that pH affected ion adsorption, exhibiting optimal adsorption at pH 3.5-6.5. The pH of a solution has a significant effect on the interaction between metal-ions and biomass as it determines the association of metal cations with functional groups (Bux et al. 1997). However, metal ions precipitate out of solution in a hydroxide complex at an elevated pH (Cheng et al. 1975 in Bux et al. 1997). Sanyahumbi et al. (1998) showed that at pH 7 metal ions precipitated out of solution. The present study showed similar trends with a significantly gradual decrease in adsorption efficiency for all ions by Laminaria pallida, copper adsorption by Ecklonia maxima and cadmium adsorption by Kelpak waste, as the pH became more neutral.
McHale and McHale (1994) in Bux et al. (1997) postulated that any pretreatment which affected cell wall character will ultimately affect the nature of uptake. Results reported by Williams and Edyvean (1997) showed that an additional
rehydration step improved ion uptake in seaweed biomasses. They attributed this to an increased surface:volume ratio, allowing greater uptake of ions over available binding sites. The swelling process is also associated with the release of alginates and possibly other material previously bound to the biomasses (Williams and Edyvean 1997). The solutions from the dried Ecklonia maxima and Laminaria pallida were also of a viscous nature after an adsorption cycle. This could be due to the release of alginates, which may have ion binding capacity, and other surface polymers. Whereas it would be expected that the Kelpak waste would have fewer alginates due to the washing of E. maxima stipes in the preparation of Kelpak.
Williams and Edyvean (1997) postulated that the pre-soaking of material activates the available binding sites, allowing more metal to be removed from solution. In the present study, rehydration of the milled seaweeds prior to an adsorption cycle significantly improved adsorption efficiency. Rehydration had less effect on the Kelpak waste. This decrease in adsorption was similar to that of dealginate fibre, made from algae, used by Williams and Edyvean (1997). Kelpak waste is already a moist sludge of 57.2% moisture, as was the dealginate fibre used by Williams and Edyvean (1997), therefore it is unlikely that pre-soaking of the Kelpak waste would have highly significant positive effects on adsorption efficiency. Other chemical pre-treat-ment methods have been reported to enhance ion adsorption ability. These methods include heat shock, salt shock and acid treatment which expose and activate the cellular binding sites and increase surface area, perhaps due to exposure of cellular binding sites and increased surface area for the binding of ions, or the activation of binding sites (Wilde and Benemann 1993). The use of freeze-dried algal powder of Ulva lactuca (Link) indicated greater uptake than intact thalli due to a greater surface to volume ratio with more binding sites being available (Webster et al. 1997). However, the use of water as cheap pretreatment method offers greater economic benefits as well as the production of a less toxic waste (Williams and Edyvean 1997).
The effect of drying the experimental material was directed at reducing transport costs and increasing shelf life of potential biosorbents. Cost analysis conducted by Atkinson et al. (1998) showed that transport contributed 55% of the final cost involved using a waste activated sludge as a biosorbent. Therefore if these exorbitant transport costs could be minimised by reduced weight of the potential biosorbent, such as Kelpak waste, overall costs would make the biosorbent cheaper and therefore more readily available for the remediation of waste water.
Air and oven drying of the material followed by a rehydration step greatly enhanced adsorption efficiency which was highly significant. Thus, the Kelpak waste could be dried from its moist sludge state prior to transportation, and on reaching its destination, be rehydrated before its application in a waste water bioreactor. The findings that drying prior to sorption significantly improve ion adsorption in the seaweed biomasses has encouraging implications for its use on an industrial scale.
The low-selective uptake of copper, cadmium and zinc in
a semi-continuous batch adsorber system, mimicked conditions of continual sorption cycles in industrial biosorbents. Again the cost-effectiveness of the reuse of the seaweed biomasses and Kelpak waste were an underlying motive for the experiments. It would be labour intensive and subsequently uneconomical if the biosorbent to be used is only effective for one adsoprtion cycle. The general trend observed in all experimental material was improved ion adsorption after the first adsorption cycle. The first adsorption cycle possibly serves as a rehydration step, effectively improving removal of metal ions from solution on subsequent adsorption cycles. A similar trend was shown by linseed fibre for copper and nickel removal over successive adsorption cycles (Williams and Edyvean 1997).
All seaweeds exhibited the same preference for metal-ion uptake resulting in the greatest degree of metallic bonding for copper, then cadmium, and finally zinc as shown by the number of cycles taken to reach saturation. This suggests that each seaweed or Kelpak waste selectively adsorbs specific metal ions when other ions are present (Aderhold et al. 1996). No competition for uptake sites between ions were apparent.
After termination of the semi-continuous cycles, the concentrations of zinc at 10mgl-1 remaining in solution exceeded the added concentration prior to a new cycle, indicating that the experimental material was releasing ions adsorbed in previous cycles. This concurs with results obtained by Williams et al. (1996) with waste linseed straw and nickel. However, the two seaweeds and Kelpak residue continued to adsorb copper and cadmium ions for the number of cycles tested.
After the semi-continuous cycles were terminated, it was observed that the capacity of the material tested did not appear to be exhausted as there were still low concentrations of residual copper left in solution. This suggests that the material tested could be used for further sorption in single copper solutions. Results indicate that continual use of the seaweeds and Kelpak waste does not adversely affect ion adsorption ability, thereby reducing operational costs. However, similarity of results with other research indicates that metal biosorption from mixed metal solutions is a complicated process as certain biosorbents have an affinity for certain ions, selectively adsorbed over others (Williams et al. 1996).
These results hold promise for Kelpak waste as a cheap alternative for bioremediation of industrial effluent. The waste material comprises mainly of cell walls, produced during the processing of E. maxima to make Kelpak concentrate. It exhibits equal and in many cases better uptake efficiency than its 'mother' seaweed E. maxima, probably due to the presence of exposed ligands which occur during the 'cell burst' process. It is of particular interest that reduction of transport costs could be implemented as drying of the residue does not have any adverse effects, but rather increases adsorption efficiency. The Kelpak waste can also be used for successive adsorption cycles, a necessary characteristic for an industrial biosorbent.
Acknowledgements — The NRF, Pretoria and the Natal University Research Fund are thanked for their financial support.
References
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Edited by RN Pienaar
