Biosorption of heavy metals by marine algae Ulva rigida, Cystoseira barbata and Cystoseira crinita

 

Anna Simeonovaa, Sevdalina Petkovaa

a Department of Ecology and Environmental Protection, Technical University-Varna,

1 Studentska str., 9010 Varna, Bulgaria, e- mail: annsim@abv.bg

 

 

Abstract

In this study, the adsorption properties of three different marine algae (Ulva rigida ( green algae); Cystoseira barbata(brown algae) and Cystoseira crinita (brown algae)) were investigated. They were collected from the Black Sea coastal area in Varna region, Bulgaria. Kinetics were studied to evaluate the ability of the three algae to sequester Cu(II), Zn(II), Pb(II), Ni(II), Cd(II) from aqueous solution. The maximum biosorption capacity obtained was 2,84 mgeq Ni2+/g for  Cystoseira crinita and 2,28 mgeq Cu2+/g for Cystoseira barbata at a solution pH of 5± 0,5.The influence of pH of the solution and  algae mass on the heavy metal sorption was investigated either. Desorption using 0,05 M HNO3 was carried out and was determined that regeneration of biomass for use in multiple cycles of Cd(II) biosorption –desorption should be feasible.

 

Keywords: biosorption, desorption , heavy metals, marine algae, water treatment

 

1. Introduction

 

Heavy metal contamination exists in aqueous waste streams of many industries, such as metal plating facilities, mining operations, and tanneries. Heavy metals are not biodegradable and tend to accumulate in living organisms. To curtail heavy metal pollution problems, many processes have been developed for the treatment of metal containing waste waters.

The need for economical, effective and safe methods for removing heavy metals from waste waters has resulted in the search for unconventional materials that may be useful in reducing the levels of heavy metals in the environment. In this light, biological materials have emerged as an ecofriendly and economic option. Biosorption, which uses the ability of biological materials to remove and accumulate heavy metals  from aqueous solutions, has received considerable attention in recent years because of few advantages compared to traditional methods. Biosorption uses cheaper materials such as naturally abundant algae or byproducts of fermentation industries as biosorbents 1-5

Among the biological materials investigated for heavy metal removal, the biomass of marine algae otherwise known as seaweeds has been reported to have high uptake capacities for a number of heavy metal ions. 6-7 Numerous studies of metal biosorption by brown algae such as Sargassum have been reported in literature.8-11 The role of the cellular structure, storage polysaccharides, cell wall has been evaluated in terms of their potential for metal sequestration.

The brown seaweeds Sargassum hemiphyllum, Petalinia fascia and Colpomenia sinusa has been characterized by their number of binding sites.12 Due to their high number of binding sites Sargassum and Petalinia has been determined to be the most promising for biosorption application.

Many investigations have been carried out for biosorption of heavy metals by the other important divisions of algae Green algae (Chlorophyta) and Red algae (Rhodophyta).13-15 Ulva reticulate was the focus of the study of Kuppusamy Vijayaraghavan et al.The influence of several factors such as pH, initial metal concentration and contact time were analyzed.16 Green algae Cladophora fascicularis was determined by Liping Dend et.al. to be an effective and economical biosorbent material for the removal of heavy metal ions.17

In the present study, the biosorption of heavy metal ions such as Cu(II), Zn(II), Pb(II), Ni(II), Cd(II) by three different marine algae Ulva rigida( green algae), Cystoseira barbata (brown algae) and Cystoseira crinita (brown algae) was investigated. The effect of contact time, concentration of algal biomass, pH and regeneration were studied.

 

2. Materials and methods

2.1. Materials

Green algae Ulva rigida and brown algae Cystoseira barbata and Cystoseira crinita are used for investigation of heavy metal ion sorption. They were collected from the Black Sea coastal area in Varna region, Bulgaria.

Heavy metal solutions ware prepared by solving of Pb(NO3)2, Cu(NO3)2.3H2O, Ni(NO3)2.6H2O, Zn(NO3)2 and Cd(NO3)2 in deionized water. All reagents were analytical grade.

 

2.2. Methods

2.2.1.Preparation of biomass

Ulva rigida , Cystoseira barbata and Cystoseira crinita were harvested from Black Sea and were washed several times using deionized water to remove the sand particles and salts. They were then dried at room temperature to constant weight. Dry biomass was chopped to 1,5cm particles which were used for biosorption experiments.

 

2.2.2. Batch biosorption studies

 

All batch biosorption experiments were carried out by adding 1g of dried biomass to 50 cm3 of heavy metal ion solutions in Erlenmeyer flasks. The flasks were agitated at 250 rpm for 60 min. The experiments were conducted at room temperature (20oC) at pH 5± 0.5. Samples were taken at regular intervals till equilibrium was reached. The residual metal concentration in the solution was determined by EDTA titration. The metal uptake was calculated by the following equation:

where: V - volume of the solution in the contact batch flask(cm3); C0 - initial concentration of the metal ions, mg/dm3; Cf- equilibrium concentration , mg/ dm3; m - weight of the sorbent, g.

Algal mass quantity was varied from 0,1g to 1g  to study its influence upon sorption. For studying the influence of pH on the biosorption, experiments were conducted at various initial metal solution pH values of 2-8.  To avoid hydroxide precipitation at high pH, ammonium acetate was added.9 All experiments were carried out in triplicate to check the reproducibility of results.

 

2.2.3. Batch desorption studies

The desorption of heavy metal ions was carried out using 0,05Ì HNO3 The biosorbents were eluted for 60 min. After acid elution, deionized water was used to rinse the biomass until the pH of the wash effluent exceeded 6.0. The regenerated biosorbents then were used for next sorption. The effect of regeneration was determined  by the following equation-

Where: a1 – metal uptake of the fresh biosorbent; àn – metal uptake of the regenerated biosorbent

 

3. Results and discussion

3.1. Kinetic studies

 

In the present study the sorption characteristics of three biosorbents were examined- Ulva rigida (green algae), Cystoseira barbata(brown algae) and Cystoseira crinita(brown algae) with regard to Pb(II), Cu(II), Ni(II), Cd(II) and Zn(II) ions. The biosorbents  were harvested from the Black Sea coastal area in Varna region, Bulgaria during October. For kinetic experiments, 1g biomass was added in 50cm3 metal solution at  initial pH 5± 0.5 and initial concentration of the solution-1g/dm3. Fig.1, 2 and 3 shows the kinetics of heavy metal ions uptake onto Ulva rigida , Cystoseira barbata and Cystoseira crinita. The plots represent the heavy metal uptake a (mgeq/g) versus time. It was observed that equilibrium was reached for a period of 15-30 min. Significant proportion of metal ions  was removed from solution within the first 10 min of agitation and no further significant adsorption  was observed beyond that period.

Fig.1. Kinetics of heavy metal ions uptake onto Ulva rigida

 

Fig.2. Kinetics of heavy metal ions uptake onto Cystoseira barbata

Fig.3. Kinetics of heavy metal ions uptake onto Cystoseira crinita

 

Each of the biosorbents showed different affinity toward heavy metals.This could be explained with the difference in cell wall composition and the intra group differences which cause significant differences in the type and amount of metal ion binding to them. The cell wall consists of variety of polysaccharides and proteins which offers a number of active sites capable of bindind metal ions 4.The highest uptake of the investigated biosorbents is shown in table 1. Ulva rigida exhibited highest uptake with regard to Zn(II)  -1,25mgeq/g; Cystoseira barbata to Cu(II)-2,28mgeq/g and Cystoseira crinita to Ni(II)-2,84mg/g. According to  the results  for  biomass batch sorption the following metal order  was obtained for each biosorbent: Ulva rigida: Zn > Cd > Ni > Cu > Pb;. Cystoseira barbata: Cu > Zn > Cd >Ni ~ Pb; Cystoseira crinita: Ni > Cu > Zn > Cd > Pb.

 

Table 1. . Heavy metals maximum uptake(mgeq/g) by Ulva rigida, Cystoseira barbata,  Cystoseira crinita; initial concentration of metal  ions-1g/dm3, pH 5

Metal ions

sorbents

Cu(II)

Cd(II)

Ni(II)

Pb(II)

Zn(II)

Ulva rigida

0,79

1,05

0,95

0,67

1,25

Cystoseira barbata

2,28

1,20

0,80

0,80

1,25

Cystoseira crinita

1,56

1,25

2,84

0,69

1,50

 

The brown algae Cystoseira barbata and Cystoseira crinita exhibited highest uptake with regard to most of the heavy metal ions. In brown algae biomass, alginate in the cell wall is the main component responsible for the metal sorption. It is present in a gel form which appears very porous and easily permeable.18 This could explain the highest uptake of metal ions.

 

3.2. Heavy metals desorption

The exploitation of biosorbents for removal of heavy metals depends on the efficiency of the  regeneration after metal sorption. The desorption of metals in this study was investigated using dilute mineral acid 0,05 M HNO3 as desorbing agent. The effect of regeneration (Ep) was calculated and presented in fig.4, 5 and 6. The three sorbents exhibited effect of regeneration regarding Pb(II), Cu(II), Zn(II) è Ni(II) under 50% after the II cycle. Good regeneration and high sorption capacity after multiple cycles showed the three biosorbents with regard to Cd(II).The most effective regeneration with regard to Cd(II) was reached by Ulva rigida (82% after the IV cycle). During regeneration Cystoseira barbata and Cystoseira crinita showed higher mechanical stability than Ulva rigida where the eluation resulted in some changes in the cell structure of the green algae. Thus, regeneration of the biosorbents for use in multiple cycles of Cd(II) biosorption- desorption should be feasible.

 

 

 

 

 

 

 

 

 

Fig.4. Effect of regeneration of Ulva rigida

 

Fig.5. Effect of regeneration of Cystoseira barbata

 

Fig.6. Effect of regeneration of Cystoseira crinita

 

3.3.Effect of algae mass

 

The effect of Cystoseira barbata and Cystoseira crinita  mass for the removal of Cu(II) and Ni(II) was studied using algal mass in the range of 100-1000mg. Batch sorption is carried out till equilibrium was reached and sorption effect was calculated by the following equation :

E= ( C0- Cf )/ C0 .100, where: C0 - initial concentration of the metal ions, mg/dm3, Cf- equilibrium concentration , mg/ dm3. Results showed an increased uptake of Cu(II) and Ni(II) with the increase of algae quantity( table.2,3). Cystoseira barbata exhibited 90% effect of Cu(II) sorption at about 1000mg mass.

Table 2. Effect of sorption of Cu (II) by Cystoseira barbata at different algae mass

mass of the sorbent

Co

Cf

Å %

100 mg

1,9

1,59

16%

200 mg

1,9

1,27

33%

500 mg

1,9

1,27

33%

800 mg

1,9

0,95

50%

1000 mg

1,9

0,19

90%

Òable 3. Effect of sorption of Ni (II) by Cystoseira crinita at different algae mass

mass of the sorbent

Co

Cð

Å %

100 mg

2,55

2,43

5%

200 mg

2,55

2,32

9%

500 mg

2,55

2,08

18%

800 mg

2,55

1,90

25%

1000 mg

2,55

0,54

79%

 

3.4.Effect of initial solution pH

Marine algae contain high content of ionizable groups on the cell polysaccharides, which suggests that the biosorption process could be affected by changes in the solution pH.3,19 In the present study was investigated the influence of pH on the sorption uptake of Cystoseira crinita  with regard to Ni(II) and Cystoseira barbata to Cu (II).The experiments were carried out at pH interval 2-8 and the results are presented in fig.7.

Fig.7. Effect of initial pH on heavy metal sorption

The uptake for both sorbents showed a sharp increase with an increase in pH from 2.0 to 5.0.This could be explained with the extend of protonation which is determined by the pH. The amino and carboxyl groups, and nitrogen and oxygen of the peptide bonds are available for coordination bonding with metal ions. Such bond formation could be accompanied by displacement of protons4.At lower pH values (pH=2,3) the concentration of H+ protons was increased and showed  preponderance over that of heavy metal ions which could explain the lower sorption uptake of the algae in this pH range. Both sorbents reached  highest uptake at pH 5(Cystoseira crinita-2,84 mgeq Ni2+/g; Cystoseira barbata-2,28 mgeq Cu2+/g).

 

4. Conclusion

The results obtained showed that Cystoseira crinita , Cystoseira barbata and Ulva rigida could be used as efficient biosorbent material for treatment of heavy metal ions. Highest uptake was reached by Cystoseira crinita for Ni (II) and Cystoseira barbata  for Cu(II).The desorption experiments suggested that the regeneration of the biosorbents was possible for repeated use especially with regard to Cd(II).The good sorption characteristics with respect to heavy metals and  the availability of the algae in great quantities in nature, makes these biosorbents very perspective for successful purification of industrial waters.

 

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