SO(l/h Afncan Journal of Barany 200 1. 57 61 5-6/9

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SOUTH AFRICAN JOURN AL OF BOT AN Y ISSN 02~5299

Some physical factors affecting adsorption of heavy metals from solution by dried brown seaweed material

WA Stirk* and J van Staden

Research Centre for Plant Growth and Development. School of Botany and Zoology. University of Natal Pietermaritzburg.

Private Bag X01. Scottsville 3209. South Africa

* Corresponding author, e-mail: stirk@nu.ac.za

Received 3 August 2000 . accepted in revised form 27 February 200 1

The use of derived seaweed products is gaining support as an alternative cheap method for the remediation of industrial wastewater. Adsorption properties of dried Ecklonia maxima (Osbeck) Papenf., Laminaria pallida ex. J. Ag . and Ketpak waste, the waste product f rom the manufacture of a commercial seaweed concentrate made from E. maxima, were investigated. The ratio of algal biomass to the initial ion concentration affects ion adsorption . At low ion concentrations , small amounts of

Introduction

Industries and mines are major sources of water pollu tion. One group of pollutants are the heavy metals which accu­mulate in natural water systems. They are part icularly haz­ardous to al l fo rms of life especially when they accumulate above critical levels. The average annual rainfall in South Africa is 497mm, well below the world average of 860mm (Bux and Kasan 1996) . As the popula tion and industrialisa­tion increase, there is an increased demand for clean water. This makes it essential for water to be purified and recycled.

The most common method used to remove metal ions from wastewater is precipitation but this is not effective at low ion concentrations. Other processes employed include filtration , ion exchange, electrolysis, membrane processes and evapora ti ve recovery. These are extremely costly, espe­cia lly for small commercial concerns and have operational problems such as limited flow-rates, sensitivity to acid and salt condi tions and fouling (Aderhold et al. 1996). Alterna tive low cost methods which can remove trace amounts of met­ats and are easy to apply are currently being sought.

One such method is adsorption. Adsorption tech niques are relatively simple where the sorbent concentra tes the tar­get particle onto its surface. Industrial sorbents include acti­vated carbon and silica gel (Aderhotd et a/. 1996). 'Biosorption' is where both pJant- and animal-derived mate­rial are used to sequester metal ions from aqueous solution (Will iams and Edyvean 1997). Meta l sequestration involves a number of mechanisms including ion exchange, chelation, adsorption by physical forces and ion entrapment. Chemical

sorbent gave the best results. At high ion concentra­tions, more sorbent was required for maximum uptake. The size of the sorbent particles affects the initial rapid ion sequestration. Smaller sized sorbent particles showed more rapid adsorbance. However, the fina l ion concentration taken up after 24 hours was not affected by particle size. It is necessary to agitate the sorbent for maximum adsorbance to occu r but the rate and method (stirring versus shaking) of agitation had no effect.

groups that are best suited for sequestration of metal ions are polysaccharides, proteins and phosphate groups with carboxyls, hydroxyls and sulphates. Thus the biosorbent potential of any biological biosorbent depends on its mor­phological structure and chemical make-up (Volesky and Holan 1995) . Physiochemicat parameters of the solution such as temperatu re, pH , initial ion concentration and sor­bent concentration are also important factors affecting metal ion sequestration (Aksu and Acikel 1999).

Most commonly, microbial, fungal and microalgal biomass­es have been investigated (see Volesky and Holan 1995 for a list). Marine micro- and macro-algae are gaining increasing attention as they are rich sources of biological material which can accumula te metal ions as well as being relatively cheap to process (Zhou et a/. 1998). Living cultures of Cladophora have a high removal efficiency, being able to concentrate cadmium and remove between 86-96% of the added cadmi­um over 48h (Sobhan and Sternberg 1999).

Recent research has shown that non-viable biological bio­mass and materia l derived from them are also effective as sorben ts of heavy metals. Non-viable biomass has the advantage over living biomass of increased tolerance to harsher environmental condi tions and no nutrient req uire­ments (Will iams and Edyvean 1997). The potential use of non-viable brown seaweeds and their derived products as sorben ts has also been investigated and appears well suit­ed for binding meta l ions owing to their polysaccharide con­tent. Ascophyllum nodosum and Sargassum natans have

616

higher metal biosorbent uptake of cadmium than a commer­cial ion-exchange resin (Volesky and Holan 1995). Alginate , extracted from brown seaweeds performed well as a biosor­bent but would be costly to use commercially for the reme­diation of wastewater. Alternatively, the alginate-free sea­weed waste product from the commercial manufacture of alginate was also shown to be an effective biosorbent (Williams and Edyvean 1997).

The waste from the producti on of the seaweed concen­tra te Kelpak, which is used as an agricultural growth stimu­lant, has been investigated as a possible metal ion sorbent. Kelpak is made from the southern African kelp E. maxima. During its production, no chemicals or heat are used. The waste consists mainly of ruptured cell waH debris. Over 4 tonnes of Kelpak waste is discarded month ly, making it an inexpensive and readily available source of biological 50r­bent. Initial investigations comparing copper, zinc and cad­mium adsorption of the Kelpak waste to three dried and milled kelp species , showed the Kelpak waste had similar or superior adsorbance capacity. The Kelpak waste displayed rapid adsorption kinetics with over 50% uptake occurring within the first hour and it was effective over a wide range of ion concentrations (1-200mg I"). Best results were obtained for copper adsorption , fo llowed by cadmium and then zinc (Stirk and Van Staden 2000).

The objectives of this study were to investigate some of the physical parameters which affect metal ion sequestration by the Kelpak waste. Two dried brown kelp species were included for comparative purposes.

Material and Methods

The waste from the production of the seaweed concentrate Kelpak was obtained from the Kelpak factory in Simonstown , Cape Province, South Africa. This waste was a moist sludge. The wet dry weight was determined so that the equivalent of 0.5g dry weig ht could be used in the adsorp­tion experiments. Two common southern African kelp species , E. maxima and L. pa/lida were also collected from the west coast of South Africa and air dried. This material was then ground and sieved to 0.5-1 .0mm size.

Both single and multiple-ion solutions were prepared using CuSO,.5H,Q, Zn SO •. 7H,O and 3CdS04.8H,O. The basic method for the adsorption experiments was modified from Aderhold et a/. (1996) and Williams and Edyvean (1997). For each replicate, 50ml of the metal-ion solution was added to an Erlenmeyer flask containing 0.5g of the dried seaweed material. The flasks were placed on an orb ital shaker at 100rpm and left for 24h at 20°C. A sample was pipetted off and the final ion concentration of the metal solution measured using an Atomic Absorption Spectrophotometer (Varian AA-1275 series). All treatments were carried out in triplicate.

Experiment 1: Effect of biomass on adsorption

To determine the optimal quantity of biomass needed to achieve maximum adsorption, the amount of sorbent added to each flask was varied to 0.1, 0.3, 0.5 and 1.0g. Multiple ion solutions of 1, 10 and 100mg 1-1 were used and fi nal ion concentrations measured after 24h.

Stlrk and Van Sladen

Experiment 2: Effect of sorbent particle size

The two dried seaweeds were ground and sieved into four size classes i.e. 0-0.5 ; 0.5-1 .0; 1.0-2.B and 2.B- 4.0mm. Adsorption of copper applied at 10mg I" was monitored and samples taken after 30min and 24h. The Kelpak residue was not used in this experiment as it is only one size.

Experiment 3: Effect of agitation on adsorption

To assess the effect of agitation on adsorption , mixed ion solutions at concentrations of 10 and 100mg I~ ' were added to 0.5g dry weight of the three seaweed derived sorbents. These were shaken on the orbital shaker at various rates i.e. 50, 100, 150 and 200rpm for 24h. Another set of sorbents was placed on a magnetic stirrer for 24h prior to final ion concentrations being measured.

BIOMASS (9 DRY WEIGHT)

Figure 1: Effect of sorbent biomass on adsorption of copper from solu tion at an initial ion concentration or A) 1mg ml·1, 8) 10mg ml-1

and C) 100mg mi·'. The error bar represents the standard error

South African Journal or Botany 2001. 67. 61 5--6 19

Results

Experiment 1: Effect of biomass on adsorption

At initial low copper concentrations (1 mg I" ), the amount of sorbent had little effect on the fina l ion concentration with the Kelpak waste giving the best results (Figure 1A). At 10mg I" copper, an increase in biomass resulted in a reduction of adsorbance for both E. maxima and L. pallida (Figure 1 S). The opposite trend was observed at initial high copper con­centrations (1 OOmg I" ) where the best adsorbance was achieved with 0.5 and 1.0 g dry weigh I sorbent (Figure 1 C). Similar results were obta ined for zinc and cadmium uptake (Figures 2-3) where at low ini tial ion concentrations, the best results were achieved using little sorbenl. At high initial ion concentrations, more sorbent was required for maximum sequestration.

Cl

.s z o

~ r­z w u z o u z Q ..J < Z u:

0.4 A) 1 mg (1 Zn ... Kelpak waste • Ecklonia maxima • Lamlnaria pallida

' .J

'.2

.. ,

0.0 -:====;:===========i B) 10 mg I" Zn J.'

2.'

, .•

.. -:====;::==========i C) 100 mg 1'1 Zn '"

". .. 60

.. 20

BIOMASS (g DRY WEIGHT)

Figure 2: Effect of sorbent biomass on adsorption or zinc from solu­tion at initial ion concentration or A) 1mg m!·l, 8) 10mg ml-1 and C) 100mg mi". The error bar represents the standard error

617

Experiment 2: Effect of sorbent particle size

Initially, the sorbent particle size did affect the rate of copper adsorbance with the smaller particles showing a faster response after 30min for both. E. maxima and L. pallida (Figure 4). However, after 24h exposure to copper, there was very little difference in the final ion concentration , regardless of the sorbent size for L. pallida and provided the size was larger than O.5mm for E. maxima. The E. maxima material smaller than 0.5mm gave poor results (Figure 4A).

Experiment 3: Effect of agitation on adsorption

The results clearly indicate that some form of agitation is required for effective adsorption to occur when compared to the control which was not agitated . However, the rate and manner of agitation had no significant effect on adsorption with similar final ion concentrations being recorded in all agi­ta ted samples (Figures 5- 6). Generally, the Kelpak waste

0.15 A) 1 mg r1 Cd ... Kelpak waste • Ecklonia maxima • Laminaria pallida

0.10

0.05

2.' g 0.00 ~====;=:=:::':=::'::======::'::=:::':;

B) 10 mg r' Cd z o

~ r­z w u z o u z Q ..J < Z u:

, .•

' .5

, .. ... ~:::::::=:::::;;::==:::==~====~ C) 100 mg r' Cd

80

" .. "

BIOMASS (g DRY WEIGHT)

Figure 3: Effect of sorbent biomass on adsorption of cadmium from solution at an initial ion concen tration of A) 1mg mI'" B) 10mg ml·1

and C) 100mg ml- l. The error bar represents the standard error

618

showed equal or superior adsorption rates, especially for copper and zinc, Ihan the dried seaweeds (Figures. 5A- B).

Discussion

The ra tio of sorbent to concentration of metal ions can be important in determining the final equilibrium concentration. The results suggest that at high initial ion concentrations (1 OOmg I" ), if 100 litlle sorbenl is used, Ihe uplake siles become saturated and so poor results are achieved. At low initial ion concentrations. the amount of Kelpak waste did not affect the final ion concentration. However, at low initial ion concentra­tions, if too much of the dried seaweed was used, there was reduced ion adsorption. Similar results were reported by Williams and Edyvean (1997) using brown algae and Iheir derived products fo r nickel uptake. Their explanation was that overcrowding caused the active uptake sites to be obscured.

Another explanation may be the rise in pH caused by the release of alginale by E. maxima and L pallida (Slirk and Van Staden 2000). The greater the volume of sorbenl, the larger Ihe change in pH which would ultimalely affecl melal ion sequestration . The Kelpak waste, which does not release alginales , did not show such a greal pH fluclualion (Stirk and Van Staden 2000) which may accounl for Ihe sor­bent volume not affecting the final ion concentration in solu­tions of low initial ion concentrations.

Dried and sieved Laminaria japonica and Sargassum kjell-

10

6

'" .s z 4 0 ;::

" '" 2 >-Z W U

0 Z 0 u 10 Z Q -'

" 8 Z u::

,

4

2

0

A) Ecklonia maxima

B) Laminaria pal/ida

30 minutes

EEl 0.0-0.5 mm

.0.5-1.0 mm

fZ] 1.0-2.8 mrn

D 2.8-4.0 rnm

L ,

24 hours

TIME EXPOSED TO ell SOLUTION

Figure 4: Effect of sorbent particle size on adsorption of copper applied at 10mg m1 1• The error bar represen ts the standard error

Sllrk and Van Staden

manianum showed high sorption capacity for copper and cadmium in single ion solutions and their sorption was not dependant on the initial ion concentration. This makes them effective sorbents at both high and low metal concentrations (Zhou et al. 1998). These results indicale Ihal Ihe Kelpak waste is an effective sorbent at both high and low ion con­centrations, increasing its usefulness for application in waste water remediat ion .

Alkinson et al. (1996) were working wilh wasle-activaled sludge from sewage works which consisted of a diverse mix­ture of micro- and macro-organisms. They found that milled sludge performed only slighlly better Ihan unmilled sludge although Ihe initial uplake in Ihe mil led sludge was more rapid . They altributed this 10 a larger surface area: volume ral io. Similarly, freeze-dried and Iyophilised ChIarella cells showed superior cadmium adsorption capacity and kinetics

8

6

2

0 -'" .s z 0 , ;::

" 0: >-Z W 4 U Z 0 U Z 2

Q ...J

" Z 0

u: ,

AICu

BIZn

CICd

o rpm

[ZJ Kelpak waste

• Ecklonia maxima

~ Laminaria pa/lida

50 rpm 100 rpm 150 rpm 200 rpm Mag . stir.

RATE OF AGITATION

Figure 5: Effect of rate and method of agitation on adsorption of A) copper. 8) ZinC and C) cadmium applied as sing le-ion solutions at 10mg mil, The error bar represents the standard error

South African Journal of BOlany 2001. 67 615-619

'00 A)Cu o Kelpak waste

• Ecklonia maxima 80 t:Z3 Laminaria pal/ida

60

40

20

- 0 Cl

'00 .§. B)Zn z 0

'0 i= <{

'" I- 60 Z W () Z 0 40 ()

z Q 20

...J <( Z u: 0

100 C)Cd

'0

60

40

20

o rpm 50 rpm 100 rpm 150 rpm 200 rpm Ma g. s tir.

RATE OF AGITATION

Figure 6: Effect of rate and melhod of agitation on adsorption of A) copper, 8) zinc and C) cadmium applied as a multiple-ion solution with each ion at 10mg ml·1• The error bar represents the standard error

to living Chiarella cells (Malsunaga ef al. 1999). The same trend was observed with E. maxima and L. pa/lida tested in these experiments. This could also explain, in part, the superior adsorption rates of the Kelpak waste (Stirk and Van Sladen 2000) which, consists mainly of ruptured cell wall debris and thus has a large surface area to volume ratio.

Williams and Edyvean (1997) found that a magnetic stirrer caused a reduction in nickel adsorption compared to using an orbital shaker for E. maxima and Lessonia flavicans. They attributed this to the vigorous action of the stirrer caus­ing fragmentation of the seaweed particles which were ini­tially 1-10mm in size. The method of agitation had no effect on the seaweed used in this experiment. This may be due to the smaller particle size used (0.5-1.0mm) or the fact that

Edited by RN Pienaar

619

the stirring was less vigorous. Although the rate of agitation had little effect on the final ion concentration, it may infiu­ence the initial uptake rates. This bears further investigation if the Kelpak waste is to be developed for industria l use.

These results indicate the potential fo r using brown sea­weed products for the remediation of industrial wastewater. These seaweed products, especially the Kelpak waste, have a high affinity for heavy metal ions, being effective at both high and low ion concentra tions (Sti rk and Van Staden 2000) . The Kelpak waste's effectiveness may, in part, be due to its surface area to volume ratio. The results suggest that it would lend itself well to water treatment on a larger scale. Another advantage wi th lhe Kelpak waste is that it does not release copious amounts of alginate which would make it difficult to filter and could cause operational prob­lems (Williams and Edyvean 1997). This makes the use of Kelpak waste more attractive than the dried seaweeds.

As there is no existing application for this waste product, it will also be relatively cheap to use. It offers an economical­ly attractive alternative treatment in waste water manage­ment which is envi ronmentally friendly as it makes use of a waste product to treat industrial effluent and eliminates the use of expensive chemicals. Further work is currently underway investigating the most effective method to recov­er the heavy metal ions from the Kelpak waste so that both the metal ions and seaweed biomass can be reused.

Acknowledgments - Kelp Products is thanked for supplying the Ketpak waste. The National Research Foundation, Pretoria and the University of Natal Research Fund financially supported this study.

References

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Aksu Z. Acikel U (1999) A single-staged bioseparation process For simultaneous removal of copper (II) and chromium (VI) by using C. vulgaris. Process Biochemistry 34: 589-599

Atkinson BW. Bux F, Kasan HC (1996) Bioremediation of metal-con­taminated industrial efftuents using waste sludges. Waler Science and Technology 34: 9-15

Bux F, Kasan HC (1996) Assessment of wastewater treatment sludges as metal biosorbents. Resource and Environmental Biotechnology 1: 163-177

Matsunaga T, Takeyama H, Nakoo T, Yamazawa A (1999) Screening of marine microalgae for bioremediation of cadmium­polluted seawater. Journal of Biotechnology 70: 33-38

Sobhan R, Sternberg SPK (1999) Cadmium removal using Cladophora. Journal of Environmental Science and Health 34: 53-72

Stirk WA, Van Siaden J (2000) Removal of heavy metals from solu­tion using dried brown seaweed malerial. Botanica Marina 43: 467-473

Williams CJ , Edyvean RGJ (1997) Optimization of metal adsorption by seaweeds and seaweed derivatives. Transactions. Institute of Chemical Engineers 75(8): 19-26

Volesky B. Holan ZR (1995) Biosorption of heavy metals. Biotechnology Progress 11 : 235-250

Zhou JL, Huang PL, lin RG (1998) Sorption and desorption of Cu and Cd by macroalgae and microalgae. Environmental Pollution 101 : 67- 75