- Email: [email protected]
Biosorptive flotation for metal ions removal: the influence of surface tension
Desalination 248 (2009) 740–752
Biosorptive flotation for metal ions removal: the influence of surface tension A.I. Zouboulis*, K.A. Matis Division of Chemical Technology, Department of Chemistry, Aristotle University, GR-54124 Thessaloniki, Greece. email: [email protected] Received 29 October 2008; accepted 16 December 2008
Abstract The removal of divalent metal ions mixture, such as zinc, copper and nickel, from dilute aqueous solutions or effluents was investigated by applying effectively the intergraded ‘‘biosorptive flotation’’ treatment method. The presence (as alternative cheap sorbent or biosorbent) of nonliving fungi biomass, that is, a waste by-product from industrial fermentation, in a stirred tank contactor was found to assist the removal of toxic metals by adsorption and their subsequent separation by flotation. Parallel laboratory measurements, including surface tension, electrokinetic and contact angle measurements, which are among the major physicochemical parameters influencing this system, were found to correlate quite satisfactorily with the process effectiveness. Under the optimized experimental conditions, the removal of metals by sorption and the recoveries of biomass by flotation were found to be in the order of 95% (or even more) at pH around 7, whereas the surface tension measurements were lowered and the contact angles were increased in this system, hence improving the biomass floatability. On the contrary, although zeta-potential measurements showed a small decrease toward more negative values, this was not found to influence substantially the overall treatment process. Keyword: Penicillium chrysogenum; Wastewater; Copper; Zinc; Nickel; Cadmium; Biosorption; Biomass; Flotation
1. Introduction The sustainable development linked to chemical technology is one of the topic areas and key themes of the technical roadmap for the chemical engineering of 21st century [1]. The removal of *Corresponding author. Presented at the Conference on Protection and Restoration of the Environment IX, Kefalonia Greece, June 30–July 3, 2008
toxic substances from wastewaters, or from the contaminated water sources, by the application of novel, and considered as cleaner (less polluting), treatment methods, is of primary importance that is reflected also in the tightening and enforcement of environmental controlling regulations. Among the potential treatment processes, when applied especially for the removal of heavy metals from industrial effluents, the flotation process can play an important role.
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.12.041
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
An extension of the older ‘‘carrier flotation’’ process led to the development of the new process termed ‘‘sorptive flotation,’’ which generally is a technique capable of scavenging metal ions from dilute aqueous solutions, by using appropriate sorbent materials [2]. Further on, the ability of microorganisms to abstract metal ions from water (usually termed as ‘‘bioaccumulation’’ or ‘‘biosorption,’’ depending on whether the microorganisms are living or not, respectively) is an already well-known and extensively studied treatment process. The potential use of the specific ability of microorganisms in cleaning metal-polluted wastewaters has been reviewed extensively [3–5]. The role of microorganisms in affecting the metal mobility in soil matrices is also worth attention because relevant phenomena might occur as well in this case. The need for effective and low-cost treatment methods in metal removal from dilute aqueous solutions has resulted in the development of new separation technologies, and among these, biosorption in combination with flotation, that is, ‘‘biosorptive flotation,’’ may play an important role [6–8]. Rhodococcus opacus was recently examined, including microflotation studies, to remove trivalent chromium species from contaminated liquid streams [9]. Surface chemistry issues, such as surface tension, zeta-potential etc., are incorporated in several treatment methods, such as the adsorption of specific chemical agents (collectors) to enhance process efficiency, the change of hydrophobicity of a solid particle for the purpose of separation, that is, by the application of flotation etc. Especially, the concept of critical surface tension for the examination of wetting effects proved to be important to improve selective flotation [10]. However, these surface phenomena, such as surface tension, as well as zeta-potential and contact angle, have been rather seldom examined for the case of effective removal of toxic metals from aqueous solutions by the application
741
of biosorptive flotation, and for this reason, specific emphasis has been given in the present paper on the examination of these surface properties and for their correlation with the removal efficiency of heavy metals.
2. Materials and methods 2.1. Fungal biomass The examined biosorbent was Penicillium chrysogenum (with trade name Mycan), an industrial solid waste of filamentous fungal biomass, kindly supplied by the company Synpac Ltd. (UK). More information, concerning the used biomass type, was elsewhere presented [6]. The biomass samples were thermally inactivated and repeatedly washed to remove any soluble substances. Nonliving biomass material was generally used throughout this investigation at the concentration of 1.14 g/L (as dry solids); in this work, a batch at 3 g/L (as wet solids) was also tested (denoted here as Mycan-3), while relevant results, denoted as Mycan-1 and Mycan-2, were earlier reported [6]. The contact and mixing time for biosorption was kept constant at 15 min. The used biomass samples were initially unmodified, although in subsequent experiments pretreatment (modification) was performed, either by the addition of Na2SO4 solution or by the cationic polyelectrolyte solution, Zetag-64 (kindly supplied by the ex-Allied Colloids, UK). Following biosorption experiments, elution of metal-loaded biomass was also subsequently examined, aiming at the regeneration and reuse of biosorbent in subsequent treatment cycles. The effective elution of metals from biomass was carried out by the addition of a solution comprising sodium sulphate (1 M) and sodium citrate (0.1 M). It is worth noting also that although the application of fungal technology (termed sometimes as ‘‘myco-remediation’’) in the cleanup of polluted soils had shown
742
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
promising results since 1985, its wider application has been retarded by limited knowledge in basic research [11]. 2.2. Methods The examined aqueous mixture of heavy metals for the laboratory-performed biosorption experiments contained the following concentrations (unless stated otherwise): Zn: 50, Cu: 10, Ni: 2, Ca: 100 and Na: 100 (as mg/L), aiming at the simulation of commonly found toxic metals in industrial wastewaters. The initial volume of the aqueous (deionized water) solution to be treated was 500 mL. The solution pH was adjusted during the tests by the addition of nitric acid, or sodium hydroxide solutions, as required. The pH was usually set at 7, unless otherwise stated. A conventional flotation technique for bubble generation, that is, the dispersed-air flotation, was selected as a suitable separation method to harvest the metal-loaded biomass. It was found that dispersed-air flotation, when compared to the dissolved-air flotation technique commonly applied in wastewater treatment, can perform better, regarding separation, under specific conditions [6]. The flotation column had an appropriate volume of 500 mL. The retention time was 10 min and the airflow rate 200 cm3/min, that is, a superficial velocity of 0.27 cm/s was employed. The dispersed-air flotation cell and the technique were previously discussed in a relevant review on sorptive flotation [2]. Repeated biosorption–elution cycles were also conducted by applying biomass separation by flotation and in the presence (or not) of an appropriate surfactant. Dodecylamine (denoted hereafter as DA), a common cationic surfactant (being a primary amine), was applied in most experiments as the collector reagent in flotation experiments to enhance biomass floatability. DA was used at a concentration of 3 10 4 M, dissolved in
0.6% v/v ethanol, which acts as a convenient frother. The conditioning (preliminary mixing) time before flotation (but after the addition of flotation reagents) was 15 min. For comparison reasons, in certain experiments another cationic surfactant was also examined, that is, CTMABr (cetyl-trimethyl-ammonium bromide). 2.3. Experimental measurements From the middle of the flotation column, a liquid sample was received at the end of the flotation experiments for the determination of residual metal concentrations, carried out by Atomic Absorption Spectrophotometry in the usual way. The quantity of heavy metals (zinc, copper, nickel) sorbed onto the biosorbent was expressed as percentage removal from the aqueous solution. The floated concentrate (as foam) was, as carefully as possible, collected by sucking and calculated gravimetrically after drying, as percentage of the initial volume of solution, keeping in mind that it should be kept as low as possible (usually it was in the order of 5% or less). The flotation results were expressed as recovery (Re, %) of loaded biomass particles. The surface tension measurements were carried out by an interfacial tensiometer, obtained from Kr€uss/Germany (type K-8) and applying the ring method. The contact angle measurements at the three phases involved, that is, air (bubble), solid (biomass) and liquid (aqueous solution), were conducted in a special meter/apparatus, obtained also from Kr€uss (G1). The contact angle recorded in this case was formed between an air bubble and the specific bacterial (solid) surface (previously prepared on a glass slide), held in contact with an aqueous solution of specified composition. The Rank Brothers (U.K.) experimental device was used for the electrokinetic measurements of biomass, expressed as zetapotential (mV).
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
3. Results and discussion 3.1. Flotation of metal-loaded biomass Fungal biomass, when modified appropriately, or when initially used, was quite hydrophobic, despite the precautions taken, as the biomass was kept in the refrigerator in the form of a thick paste. This biomass was capable of floating even without the presence of a collector (flotation reagent), but for a relatively narrow pH range (about 7), where flotation recovery around 80% was obtained. However, this recovery was further improved after the addition of collector (appropriate surfactant). Nevertheless, it is interesting to note that the examined biomass, being an industrial by-product, has no stable characteristics, being dependent on the sort of ‘‘pretreatment,’’ it had received. Therefore, two different samples (batches), obtained during different operational periods, although from the same industrial source, may produce different flotation results. In these experiments, biomass was modified only with the addition of a (cationic) polyelectrolyte. This modification was better understood, when the residual turbidity of solution, that is, following flotation, was measured; the existence of turbidity in the effluent was due to the slight disintegration of used biomass. Despite the fact that the (initial) concentration of fungi biomass was increased up to 6 g/L and correspondingly, that of surfactant up to 1 10 3 M in order to be able to recover by flotation the increased particles concentration, following the addition of cationic polyelectrolyte, the respective residual turbidity was reduced and the flotation efficiency was improved. The flocculation process of various finely dispersed sorbents, when utilized for water purification, has been recently discussed [12]. A typical example of the results obtained, for the pH values between 6.5 and 9, showed that the flotation of biomass was almost quantitative.
743
It was further observed that the optimum pH value for biosorption is not necessarily the same as that for flotation; however, a sufficient selection seems to be the pH range 8–9. Noting that the initial concentrations of the three examined toxic metals were different, specific attention is required, when the results are presented as metals removal (%). Copper was efficiently removed (around 80%) for pH values higher than 7, zinc for pH values higher than 8 and nickel for pH values higher than 9.5. The removal of metals by biosorption in absence of the flotation reagent (surfactant) was comparable with the aforementioned data. In essence, the metal removal should be, more or less, completed during the initial stage of biosorption and, hence, before the insertion of flotation collector. However, the sampling (and the analytical determinations of residual metals) was commonly performed after flotation. As far as the flotation recovery is concerned, this reached an optimum value with the increase of dodecylamine addition (i.e., at 3 10 4 M). As the initial biomass concentration was increased (up to 6 g/L, at pH 7), the removal of metals was also increased (particularly that of zinc and nickel), but the biomass recovery by flotation seriously dropped to less than 40%, showing that higher surfactant addition was needed, as discussed earlier. Tables 1 and 2 show the optimum experimental conditions found during the initial biosorption and the following flotation. Other types of biosorbents were also evaluated [6–8]. The influence of different physicochemical properties on flotation efficiency is known from the relevant previously published work in the minerals enrichment field; similar effects (positive or negative) can be expected also on metal biosorption [13]. Among the major parameters affecting flotation is the surface tension, being largely affected by the surfactant concentration; the respective results are presented in
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752 100
R% Cu(II)
80 60 40 BACTERIA FUNGI YEAST STALKS
20 0 2
4
6
8
10
12
pH 100 80 R% Zn(II)
Fig. 2. It has to be noted, however, that this is a combined effect, also due to the simultaneous presence of frother, because the (flotation) collector (in this case: dodecylamine) was added as ethanolic solution to improve solubility. Surface tension is also a measure of the wetting ability of the medium. Additionally, the addition of ethanol results in the decrease of air bubble size, and hence can stabilize the produced froth [2]. Surface tension measurements for another biosorption system (Streptomyces clavuligerus) were previously reported and discussed [14]; cadmium was the heavy metal under investigation in this case (see also [15]). The values of surface tension for the unmodified biomass were roughly constant in all the examined pH range, whereas the biomass modification was found to decrease the surface tension. The presence of dodecylamine as flotation collector resulted also to significant reduction of surface tension of the biomass dispersion and particularly, at pH values 4.0–7.5. The simultaneous presence of metals was again found to decrease the surface tension (Fig. 2c), fact that can be connected with the improved biomass recovery by flotation. Zeta-potential values of fungal biomass were found to be more negative in the examined pH range (Fig. 3). However, the presence of metals, as well as of surfactant, was found to produce a positive effect for separation. Electrokinetic measurements on this system were elsewhere published [6]. In aquatic environments, it is known that the surface charge of biomass is counterbalanced by the oppositely charged ions, some of which can be bound on its surface, whereas the rest are distributed in a diffuse layer around the biosorbent [16], whose thickness depends mainly on the ionic strength of the solution, as well as on the valency (charge) of counterions. Taking into account the aqueous speciation of dodecylamine (DA), this compound mainly exists as RNH3+ ions at pH values up to (around) 10.5. Therefore, considering also the surface
60 40 BACTERIA FUNGI YEAST STALKS
20 0 2
4
6
8
10
12
pH 100 80 R% Ni(II)
744
60 40 BACTERIA FUNGI YEAST STALKS
20 0
2
4
6
8
10
12
pH
Fig. 1. pH influence on copper, zinc, and nickel biosorption: comparable evaluation between different biomass types; modified from [6, 8].
charge of biomass particles, the noticed behavior of this surfactant can be expected. The solubility
745
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752 Table 1 Optimum experimental conditions for the removal of metals from dilute aqueous solutions by biosorption
Penicillium MYCAN-1 Unmodified Zn 1 2 3 4 Cu 1 Ni 1 Modified (Na2SO4) Zn 1 2 3 Cu 1 2 3 Ni 1 2 Modified (ZETAG-64) Zn 1 2 Cu 1 2 Ni 1 2
Concentration of Biomass (g/L)
pH
Airflow rate Q (cc/min)
Surfactant or polyelectolyte
Contact Time, t (min)
Re % Metal
2.5–3 1.14 1.14 1.14
7.5 8–11 9–10 9–11
200 200 400 200
DA: 3 10 4 M DA: 3 10 4 M DA: 3 10 4 M CTMA-Br: 3 10
30 30 30 30
75–85 80–100 90–100 >95
1.14
11
200
—
15
70
1.14
11–11.5
200
DA: 3 10
M
30
70–75
1.14 1.14 0.5–3
8–11.5 9–11.5 9
200 200 200
— DA: 3 10 DA: 3 10
M 4 M
15 30 30
>95 >95 >95
1.14 0.5–1.5 1.14
6.5–11 9 6.5–11
200 200 200
— DA: 3 10 DA: 3 10
15 30 30
>95 >95 >95
1.14 1.14
9–11.5 8.5–11.5
200 200
— DA: 3 10
4
M
15 30
90–100 >90
1.14 1.14
7.7–10.7 8 10.6
200 200
— DA: 3 10
4
M
15 30
92–100 90–100
1.14 1.14
5.1–10.7 6–10.6
200 200
— DA: 3 10
15 30
82–99 87–99
1.14 1.14
9.7–10.7 8.4–10.6
200 200
— DA: 3 10
15 30
92–96 80–97
4
4
4
M 4 M
4
M
4
M
4
M
(Continued)
746
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
Table 1 (Continued) Penicillium MYCAN-2 Modified (HCl) Zn 1 2 Cu 1 2 Ni 1 2 Penicillium MYCAN-3 Modified (ZETAG-64) Zn 1 2 3 Cu 1 2 Ni 1
1.14 1.14
8–11 8.5–11
200 200
— DA: 3 10
4
M
15 30
90–100 90–100
1.14 1.14
7–11 7–11
200 200
— DA: 3 10
4
M
15 30
95–100 90–100
1.14 1.14
9–11 9.5–11.5
200 200
— DA: 3 10
4
M
15 30
85–90 >95
1.14 1.14 4–6
8–11 7.9–11.2 7
200 200 200
— DA: 3 10 DA: 3 10
M 4 M
15 30 30
85–99 88–100 67–80
1.14 1.14
6–11 5.9–11.2
200 200
— DA: 3 10
1.14
9–11.2
200
DA: 3 10
of long-chain amine flotation collectors was previously discussed [17]; depending on the concentration and the length of hydrocarbon chain, surfactant species may be present on the solid surface either as individual ions, or as aggregates of ions (i.e., as hemi-micelles), or even as precipitates. Flotation requires the formation of a stable bubble–particle aggregate, which enables the particle to be carried out from the suspension. The stability of this aggregate depends on the interfacial free energies, or on the corresponding interfacial tensions of the solid/liquid, solid/ vapor (gas) and liquid/gas interfaces, which are involved in the overall attachment process [10]. The tendency of the particle to replace its solid/liquid by the solid/gas interface, as it happens during the bubble–particle attachment, is commonly termed as ‘‘hydrophobicity.’’
4
4
M
15 30
70–97 73–98
4
M
30
96–100
Hydrophobicity in a solid/liquid/gas system, as flotation is, is certainly a complex phenomenon and a result of different interactions; in this way, the measurement of contact angle becomes significant. The major factors that affect the hydrophobicity of microorganisms are the following: the type of microorganisms, their growth rate, the growth conditions (composition of substrates, type of used carbon source, temperature etc.), the pH and ionic strength of medium, the growth phase of the microorganisms (e.g., static or logarithmic), the specific time of taking the measurement (aging effect), the presence of other polycations, the aeration and the presence of oxygen. Figure 4 presents the respective results for the examined system. The contact angle measurements correlate quite well with the floatability of biomass, as both curves at pH
747
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
Table 2 Optimum experimental conditions for the recovery of biomass by flotation (concentration of metals: Zn: 50/Cu: 10/Ni: 2/Ca: 100/Na: 100 (mg/L); t(flotation): 10 min) Concentration of Biomass (g/L)
pH
Airflow rate (cc/min)
Surfactant or polyelectolyte
Contact time (Min)
Re % Biomass
Penicillium MYCAN–1 1. Unmodified A B
1.14 1.14
7–9.5 7–9
200 200
30 30
70–85 75–80
C
0.5–1.5
7.5
200
DA: 3 10 4 M CTMA-Br: 3 10 4 M DA: 3 10 4 M
30
70–80
2. Modified (Na2SO4) A B C
1.14 1.14 0.5–1.5
7.5–9.8 7–10 9
200 200 200
DA: 3 10 — DA: 3 10
30 15 30
80–96 70–100 90–100
3.Modified (ZETAG-64) A B C
1.14 1.14 1.14
7–9.7 8–10.6 7
200 200 200
— DA: 3 10 4 M DA: 3 10 10 4 M
15 30 30
75–90 84–100 78–100
Penicillium MYCAN–2 1. Modified (HCl) A B
1.14 1.14
7.5–9.3 7.5–9
200 200
DA: 3 10 —
4
M
30 15
80–100 75–80
Penicillium MYCAN–3 1. Modified (ZETAG-64) A B
1.14 0.5–1
3.1–9 7
200 200
DA: 3 10 DA: 3 10
4
M M
30 30
70–100 83–98
values around 11 started to decrease. The contact angles for the unmodified biomass (data not shown) were comparatively low (25–338). The addition of flotation surfactant to the modified biomass was found to increase the contact angle, that is, with 5% dodecylamine addition, higher contact angles (54–628) were obtained, fact connecting with the increase of biomass hydrophobicity. The hydrophobicity of bacteria cells is mainly due to the properties of cell wall (i.e., the biosorption sites), which strongly depend on the presence of various polysaccharides, pro-
4
M
4
M
4
teins and lipids that form a biopolymer surface layer, whose properties can be affected with the changing of growth conditions. In this paper, nonliving biomass was used, as it has been previously found that, for example, for the case of actinomycetes biomass (grampositive bacteria) greater binding capacities for cadmium were found, than the living one [18]. Another problem, not examined in the present, is to find the suitable type of biomass for the effective removal of toxic metals, leading to the screening of various biomass types [19]. By screening several microorganism strains, a
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
748 60 50
Mycan unmodified
σ mN/m
40 30
Mycan (unmod) + 3 x 10−4 M DA σmin = 29 mN/m pH = 8.5
20
Mycan (unmod) +1 x 10−4 M DA σmin = 32.2 mN/m pH = 8.5−9
10 0 2
4
6
8
10
12
pH 60
σ mN/m
50 40
Mycan modified 0.5% DA σmin = 41 mN/m pH = 5.5
30
Mycan modified 1% DA σmin = 36.5 mN/m pH = 5.5
20
Mycan modified (5% DA) σmin = 28 mN/m pH = 6
10 0 2
4
6
8
10
12
pH 60 50
MYCAN DRY MYCAN slurry (5 % DA) 1g/L + 5 ppm CdCl2 MYCAN slurry (5 % DA) 1g/L
σ mN/m
40 30 20 10 0 2
4
6
8
10
12
pH
Fig. 2. pH influence on the surface tension measurements of biomass dispersions; different experimental conditions were applied; (a) unmodified; (b, c) modified in the presence of dodecylamine; and (c) in the simultaneous presence of a metal ion (here Cd).
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752 BACTERIA FUNGI YEAST STALKS
z-potential (mV)
0
−5
−10
−15 in presence of metals mixture
−20 2
4
6
8
10
12
pH
Fig. 3. Electrokinetic measurements of different types of biomass as function of solution pH, in presence of metals mixture; modified from [6, 8].
comparative investigation, regarding the biosorption of lead ions by the addition of filamentous fungal biomass, was also performed [20]. Filamentous fungi are commonly used in fermentation industries to produce several commercial products/metabolites, such as enzymes,
749
flavorings, or antibiotics, as well as industrial by-products, which are able to remove (mostly by chelation) several heavy metals. The main chemical groups in biomass surface, taking part in biosorption, are the electronegative groups (e.g., hydroxyl or sulphydryl), the anionic groups (e.g., carboxyl or phosphate) and the nitrogen-containing groups (e.g., amino) [13]. A chemical speciation study was performed for the examined mixture of metals (by the application of Mineql+ software program), showing that in these conditions, and over the pH limit of 5.9, copper had a thermodynamic phase change to tenorite (CuO), whereas at pH around 7.3, zinc changed to zincite (ZnO) and at pH around 7.8, nickel to Ni(OH)2; on the contrary, calcium ion was stable for the studied pH range. Taking into consideration these results, and comparing them with the respective metal removal, it looks possibly that the main mechanism of metal biosorption is precipitation or coprecipitation onto the biomass cell wall
80
contact angle, °
70 60 50 40 30 3
4
5
6
7 pH
8
9
10
11
12
Mycan modified (ZETAG-64) Mycan modified (ZETAG-64) + 3 x 10−4 M DA + 0.6% ethanol Mycan modified (ZETAG-64) + 3 x 10−4 M DA + 0.6% ethanol + Metal mixture
Fig. 4. Contact angle measurements of modified biomass as influenced by the solution pH (in presence also of collector and metals’ mixture).
750
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
surface. Due to metal speciation, possibly selectivity can be also obtained, at least for the case of copper. 3.2. Multiple cycle operation The influence of coexisting cations (cadmium, cobalt and nickel) on the biosorption of lead and zinc by the addition of Streptoverticillium cinnamoncum and Penicillium chrysogenum was evaluated [21]. The order of metal biosoprtion in a multimetal system can be sufficiently predicted on the basis of Langmuir parameters, applied for binary metal systems; the same sorption model equation is often used for the interpretation of experimental data [18, 22]. In the latter publication, the enthalpy change of metal biosorption on Zoogloea ramigera and on Rhizopus arrhizus (a filamentous fungus) has been also calculated. An exothermic process was proposed for the case of divalent copper and nickel; however, for the cases of Fe(III), Cr(VI) and Pb(II) the process was better described as an endothermic one. Multiple cycles of biosorption/flotation/ elution were also examined, by involving appropriate elution between the treatment cycles of the flotation concentrate (i.e., the separated metals-loaded biomass). It was observed that supplementary addition of surfactant during the third cycle was necessary; otherwise, the biomass flotation was substantially dropped (to around 50%). Therefore, the surfactant was added during the first and the third cycles of operation. The removal of metals was rather unaffected, although after the aforementioned supplementary DA addition during the third cycle, a floatability reduction was observed during the fourth cycle, but that was further improved with a supplementary DA addition during the fifth cycle. In the odd cycles, where the flotation collector was added, biomass recoveries were over 95%; copper removal, in this case, was
over 90%, zinc removal was around 30% and nickel removal was 10–20% at this pH range, noting meanwhile the coexistence of calcium and sodium cations in the feed solution, which can be considered as responsible for the comparatively low elution figures obtained (apart from the pH 7). It seems that biomass could be recycled and floated for at least five cycles. Similar results were also reported for the case of Streptomyces rimosus [7, 8]. Fungi can accumulate metal (and radionuclide species) by the implication of several physicochemical and biological mechanisms, including the extracellular binding by metabolites and biopolymers, the binding to specific polypeptides, as well as the metabolism-dependent accumulation [23]. To date, the most promising approach to metal removal by fungi is biosorption. The fungal cell wall is thought to have two main components: interwoven skeletal framework microfibrils, usually of chitin, embedded in an amorphous layer of proteins and various polysaccharides [24]. 3.3. Concluding remarks The increasing awareness of heavy metals accumulation and contamination of the environment has led to a quest for new and improved, cleaner technologies for the removal and recovery of them, especially from aqueous solutions. An integrated approach (‘‘biosorptive flotation’’) has been examined in the present investigation. The metal-loaded biomass particles, being fine particulates and light enough, can be easily separated by the subsequent application of flotation, whereas the biosorbents may be efficiently recycled after the appropriate metal desorption/elution in further treatment steps. The conducted measurements of surface tension, contact angle and zeta-potential correlate quite reasonably with the observed
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
separation by flotation results. Bacteria that appear to have increased hydrophobicity may also play an important role for the formation and stabilization of froth or foam in various treatment processes, such as flotation, aeration, activated sludge treatment etc. Recently, the control of liquid/gas surface tension was also examined as an alternative means to regulate flotation efficiency. The role of electrophoretic mobility in bacterial adhesion and the surface characteristics of cells in general are considered of primary significance, noting that microbial walls are mostly anionic in nature, due to the presence of carboxyl, hydroxyl, phosphoryl and other negatively charged sites [13]; therefore, cationic metal ions can rapidly bind to these sites by an energy-independent reaction. The present applications of biosorption make use mostly of nonliving biomass. Filamentous fungi are used in fermentation industries to produce various metabolites, such as enzymes, flavorings, orantibiotics, resulting also in the production of large quantities of industrial by-products per year, which can eventually find further uses, for example, by recycling as biosorbents for wastewater treatment [2]. There are several parameters that can influence the hydrophobicity of microorganisms. Under the optimised experimental conditions, the removal of metals by sorption and the recoveries of biomass by flotation were found to be in the order of 95% (or even more) at pH around 7, whereas the surface tension measurements were lowered and the contact angles (between the solid/biomass, the air/bubble and the liquid/ solution to be treated) were increased in this system, hence leading to the improvement of flotation process. On the contrary, although zeta-potential measurements showed a small decrease toward more negative values under these experimental conditions, this was not found to influence substantially the hydrophobicity and the overall treatment process, as this parameter has an effect on the surface
751
charge primarily. Bacteria showing increased hydrophobicity are of particular importance, regarding their subsequent effective separation by flotation. Acknowledgments This work was partially funded by a European Union Environment and Climate programme and a Greek PENED research programme of the Gen. Secret. Res. Tech. Thanks are also due to Ms. M. Palaiomylitou for experimental collaboration, and Mr. A. Phillipson (Microbiology Department, University Newcastle-upon-Tyne, UK) for his help with the biomass preparation. References [1] I. Chem. E. (2007) A Roadmap for 21st Century Chemical Engineering, www.icheme.org/ TechnicalRoadmap [2] A.I. Zouboulis and K.A. Matis, Removal of metal ions from dilute solutions by sorptive flotation, Crit. Rev. Envir. Sci. Tech. 27 (1997) 195–235. [3] Z. Aksu, Application of biosorption for the removal of organic pollutants: a review, Proc. Biochem. 40 (2005) 997–1026. [4] A.I. Zouboulis and I.A. Katsoyiannis, Recent advances in the bioremediation of arsenic-contaminated groundwaters, Envir. Int. 31 (2005) 213–219. [5] A.I. Zouboulis, K.A. Matis and N.K. Lazaridis, The application of flotation for the downstream separation of microorganisms, Intl. J. Envir. & Pollut. 30(2) (2007) 287–295. [6] A.I. Zouboulis, E.G. Rousou, K.A. Matis and I.C. Hancock, Removal of toxic metals from aqueous mixtures: Part 1. Biosorption, J. Chem. Tech. Biotech. 74 (1999) 429–436. [7] A.I. Zouboulis, K.A. Matis, E.G. Rousou and D.A. Kyriakidis, Biosorptive flotation for metal ions recovery, Water Sci. & Tech. 43(8) (2001) 123–129. [8] A.I. Zouboulis, N.K. Lazaridis and K.A. Matis, Removal of toxic metal ions from aqueous systems by biosorptive flotation, J. Chem. Tech. Biotech. 77 (2002) 958–964. [9] B.A. Calfa and M.L. Torem, On the fundamentals of Cr(III) removal from liquid streams by a bacterial strain, Minerals Eng. 21 (2008) 48–54.
752
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740 –752
[10] J. Freund and B. Dobia´sˇ, The role of surface tension, in: K.A. Matis (ed.), Flotation Science and Engineering, M, Dekker, New York, 1995, p. 45. [11] V. Sˇasˇek, Why mycoremediation have not yet come into practice, in: V. Sˇasˇek (ed.), The Utilization of Bioremediation to Reduce Soil Contamination: Problems and Solutions, NATO Advanced Research Workshop, Liblice Castle, Czech Rep., 2000, 14–19 June, lecture notes. [12] N.N. Rulyov, A. Maes and V.J. Korolyov, Optimization of hydrodynamic treatment regime in the processes of sorption-flocculation water purification from organic contaminants, Coll. & Surfaces-Part A’ 175 (2000) 371–381. [13] A.I. Zouboulis, K.A. Matis and I.C. Hancock, Biosorption of metals from dilute aqueous solutions, Sepaqr. & Purif. Methods 26 (1997) 255–295. [14] K.A. Matis and A.I. Zouboulis, Flotation of cadmium-loaded biomass, Biotech. Bioeng. 44 (1994) 354–360. [15] K.A. Matis, A.I. Zouboulis, A.A. Grigoriadou, N.K. Lazaridis, L.V. Ekateriniadou, Metal biosorptionflotation. Application to cadmium removal, Appl. Microbiol. Biotech. 45 (1996) 569–573. [16] M.C.M. van Loosdrecht, J. Lyklema, W. Norde, G. Schraa, A.J.B. Zehnder, (a) The role of surface hydrophobicity in adhesion, and (b) Electrophoretic mobility and hydrophobicity as a measure to predict the initial steps of bacterial adhesion, Appl. Envir. Microbiol. 53 (1987) 1893–1897 & 1898–1901.
[17] M.C. Fuerstenau, Oxide and silicate flotation, in: K.A. Matis (ed.), Flotation Science and Engineering, Dekker, New York, 1995, p. 106. [18] M.I. Kefala, A.I. Zouboulis and K.A. Matis, Biosorption of cadmium ions by Actinomycetes and separation by flotation, Envir. Poll. 104 (1999) 283–293. [19] I. Bakkaloglu, T.J. Butter, L.M. Evison, F.S. Holland and I.C. Hancock, Screening of various types biomass for removal and recovery of heavy metals (Zn, Cu, Ni) by biosorption, sedimantation and desorption, Water Sci. & Tech. 38(6) (1998) 269–277. [20] W. Lo, H. Chua, K.H. Lam and S.P. Bi, A comparative investigation on the biosorption of lead by filamentous fungal biomass, Chemosphere 39 (1999) 2723–2736. [21] P.R. Puranik and K.M. Paknikar, Influence of co-cations on biosorption of lead and zinc – a comparative evaluation in binary and multimetal systems, Bioresource. Tech. 70 (1999) 269–276. [22] Y.T. Sag˘ and T. Kutsal, Determination of the biosorption heats of heavy metal ions on Zoogloea ramigera and Rhizopus arrhizus, Biochem. Eng. J. 6 (2000) 145–151. [23] J.M. Tobin, C. White and G.M. Gadd, Metal accumulation by fungi: applications in environmental biotechnology, J. Industr. Microbiol. 13 (1994) 126–130. [24] B. Volesky, Biosorbents for metal recovery, TIBTECH 5 (1987) 96–101.
Biosorptive flotation for metal ions removal: the influence of surface tension A.I. Zouboulis*, K.A. Matis Division of Chemical Technology, Department of Chemistry, Aristotle University, GR-54124 Thessaloniki, Greece. email: [email protected] Received 29 October 2008; accepted 16 December 2008
Abstract The removal of divalent metal ions mixture, such as zinc, copper and nickel, from dilute aqueous solutions or effluents was investigated by applying effectively the intergraded ‘‘biosorptive flotation’’ treatment method. The presence (as alternative cheap sorbent or biosorbent) of nonliving fungi biomass, that is, a waste by-product from industrial fermentation, in a stirred tank contactor was found to assist the removal of toxic metals by adsorption and their subsequent separation by flotation. Parallel laboratory measurements, including surface tension, electrokinetic and contact angle measurements, which are among the major physicochemical parameters influencing this system, were found to correlate quite satisfactorily with the process effectiveness. Under the optimized experimental conditions, the removal of metals by sorption and the recoveries of biomass by flotation were found to be in the order of 95% (or even more) at pH around 7, whereas the surface tension measurements were lowered and the contact angles were increased in this system, hence improving the biomass floatability. On the contrary, although zeta-potential measurements showed a small decrease toward more negative values, this was not found to influence substantially the overall treatment process. Keyword: Penicillium chrysogenum; Wastewater; Copper; Zinc; Nickel; Cadmium; Biosorption; Biomass; Flotation
1. Introduction The sustainable development linked to chemical technology is one of the topic areas and key themes of the technical roadmap for the chemical engineering of 21st century [1]. The removal of *Corresponding author. Presented at the Conference on Protection and Restoration of the Environment IX, Kefalonia Greece, June 30–July 3, 2008
toxic substances from wastewaters, or from the contaminated water sources, by the application of novel, and considered as cleaner (less polluting), treatment methods, is of primary importance that is reflected also in the tightening and enforcement of environmental controlling regulations. Among the potential treatment processes, when applied especially for the removal of heavy metals from industrial effluents, the flotation process can play an important role.
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.12.041
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
An extension of the older ‘‘carrier flotation’’ process led to the development of the new process termed ‘‘sorptive flotation,’’ which generally is a technique capable of scavenging metal ions from dilute aqueous solutions, by using appropriate sorbent materials [2]. Further on, the ability of microorganisms to abstract metal ions from water (usually termed as ‘‘bioaccumulation’’ or ‘‘biosorption,’’ depending on whether the microorganisms are living or not, respectively) is an already well-known and extensively studied treatment process. The potential use of the specific ability of microorganisms in cleaning metal-polluted wastewaters has been reviewed extensively [3–5]. The role of microorganisms in affecting the metal mobility in soil matrices is also worth attention because relevant phenomena might occur as well in this case. The need for effective and low-cost treatment methods in metal removal from dilute aqueous solutions has resulted in the development of new separation technologies, and among these, biosorption in combination with flotation, that is, ‘‘biosorptive flotation,’’ may play an important role [6–8]. Rhodococcus opacus was recently examined, including microflotation studies, to remove trivalent chromium species from contaminated liquid streams [9]. Surface chemistry issues, such as surface tension, zeta-potential etc., are incorporated in several treatment methods, such as the adsorption of specific chemical agents (collectors) to enhance process efficiency, the change of hydrophobicity of a solid particle for the purpose of separation, that is, by the application of flotation etc. Especially, the concept of critical surface tension for the examination of wetting effects proved to be important to improve selective flotation [10]. However, these surface phenomena, such as surface tension, as well as zeta-potential and contact angle, have been rather seldom examined for the case of effective removal of toxic metals from aqueous solutions by the application
741
of biosorptive flotation, and for this reason, specific emphasis has been given in the present paper on the examination of these surface properties and for their correlation with the removal efficiency of heavy metals.
2. Materials and methods 2.1. Fungal biomass The examined biosorbent was Penicillium chrysogenum (with trade name Mycan), an industrial solid waste of filamentous fungal biomass, kindly supplied by the company Synpac Ltd. (UK). More information, concerning the used biomass type, was elsewhere presented [6]. The biomass samples were thermally inactivated and repeatedly washed to remove any soluble substances. Nonliving biomass material was generally used throughout this investigation at the concentration of 1.14 g/L (as dry solids); in this work, a batch at 3 g/L (as wet solids) was also tested (denoted here as Mycan-3), while relevant results, denoted as Mycan-1 and Mycan-2, were earlier reported [6]. The contact and mixing time for biosorption was kept constant at 15 min. The used biomass samples were initially unmodified, although in subsequent experiments pretreatment (modification) was performed, either by the addition of Na2SO4 solution or by the cationic polyelectrolyte solution, Zetag-64 (kindly supplied by the ex-Allied Colloids, UK). Following biosorption experiments, elution of metal-loaded biomass was also subsequently examined, aiming at the regeneration and reuse of biosorbent in subsequent treatment cycles. The effective elution of metals from biomass was carried out by the addition of a solution comprising sodium sulphate (1 M) and sodium citrate (0.1 M). It is worth noting also that although the application of fungal technology (termed sometimes as ‘‘myco-remediation’’) in the cleanup of polluted soils had shown
742
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
promising results since 1985, its wider application has been retarded by limited knowledge in basic research [11]. 2.2. Methods The examined aqueous mixture of heavy metals for the laboratory-performed biosorption experiments contained the following concentrations (unless stated otherwise): Zn: 50, Cu: 10, Ni: 2, Ca: 100 and Na: 100 (as mg/L), aiming at the simulation of commonly found toxic metals in industrial wastewaters. The initial volume of the aqueous (deionized water) solution to be treated was 500 mL. The solution pH was adjusted during the tests by the addition of nitric acid, or sodium hydroxide solutions, as required. The pH was usually set at 7, unless otherwise stated. A conventional flotation technique for bubble generation, that is, the dispersed-air flotation, was selected as a suitable separation method to harvest the metal-loaded biomass. It was found that dispersed-air flotation, when compared to the dissolved-air flotation technique commonly applied in wastewater treatment, can perform better, regarding separation, under specific conditions [6]. The flotation column had an appropriate volume of 500 mL. The retention time was 10 min and the airflow rate 200 cm3/min, that is, a superficial velocity of 0.27 cm/s was employed. The dispersed-air flotation cell and the technique were previously discussed in a relevant review on sorptive flotation [2]. Repeated biosorption–elution cycles were also conducted by applying biomass separation by flotation and in the presence (or not) of an appropriate surfactant. Dodecylamine (denoted hereafter as DA), a common cationic surfactant (being a primary amine), was applied in most experiments as the collector reagent in flotation experiments to enhance biomass floatability. DA was used at a concentration of 3 10 4 M, dissolved in
0.6% v/v ethanol, which acts as a convenient frother. The conditioning (preliminary mixing) time before flotation (but after the addition of flotation reagents) was 15 min. For comparison reasons, in certain experiments another cationic surfactant was also examined, that is, CTMABr (cetyl-trimethyl-ammonium bromide). 2.3. Experimental measurements From the middle of the flotation column, a liquid sample was received at the end of the flotation experiments for the determination of residual metal concentrations, carried out by Atomic Absorption Spectrophotometry in the usual way. The quantity of heavy metals (zinc, copper, nickel) sorbed onto the biosorbent was expressed as percentage removal from the aqueous solution. The floated concentrate (as foam) was, as carefully as possible, collected by sucking and calculated gravimetrically after drying, as percentage of the initial volume of solution, keeping in mind that it should be kept as low as possible (usually it was in the order of 5% or less). The flotation results were expressed as recovery (Re, %) of loaded biomass particles. The surface tension measurements were carried out by an interfacial tensiometer, obtained from Kr€uss/Germany (type K-8) and applying the ring method. The contact angle measurements at the three phases involved, that is, air (bubble), solid (biomass) and liquid (aqueous solution), were conducted in a special meter/apparatus, obtained also from Kr€uss (G1). The contact angle recorded in this case was formed between an air bubble and the specific bacterial (solid) surface (previously prepared on a glass slide), held in contact with an aqueous solution of specified composition. The Rank Brothers (U.K.) experimental device was used for the electrokinetic measurements of biomass, expressed as zetapotential (mV).
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
3. Results and discussion 3.1. Flotation of metal-loaded biomass Fungal biomass, when modified appropriately, or when initially used, was quite hydrophobic, despite the precautions taken, as the biomass was kept in the refrigerator in the form of a thick paste. This biomass was capable of floating even without the presence of a collector (flotation reagent), but for a relatively narrow pH range (about 7), where flotation recovery around 80% was obtained. However, this recovery was further improved after the addition of collector (appropriate surfactant). Nevertheless, it is interesting to note that the examined biomass, being an industrial by-product, has no stable characteristics, being dependent on the sort of ‘‘pretreatment,’’ it had received. Therefore, two different samples (batches), obtained during different operational periods, although from the same industrial source, may produce different flotation results. In these experiments, biomass was modified only with the addition of a (cationic) polyelectrolyte. This modification was better understood, when the residual turbidity of solution, that is, following flotation, was measured; the existence of turbidity in the effluent was due to the slight disintegration of used biomass. Despite the fact that the (initial) concentration of fungi biomass was increased up to 6 g/L and correspondingly, that of surfactant up to 1 10 3 M in order to be able to recover by flotation the increased particles concentration, following the addition of cationic polyelectrolyte, the respective residual turbidity was reduced and the flotation efficiency was improved. The flocculation process of various finely dispersed sorbents, when utilized for water purification, has been recently discussed [12]. A typical example of the results obtained, for the pH values between 6.5 and 9, showed that the flotation of biomass was almost quantitative.
743
It was further observed that the optimum pH value for biosorption is not necessarily the same as that for flotation; however, a sufficient selection seems to be the pH range 8–9. Noting that the initial concentrations of the three examined toxic metals were different, specific attention is required, when the results are presented as metals removal (%). Copper was efficiently removed (around 80%) for pH values higher than 7, zinc for pH values higher than 8 and nickel for pH values higher than 9.5. The removal of metals by biosorption in absence of the flotation reagent (surfactant) was comparable with the aforementioned data. In essence, the metal removal should be, more or less, completed during the initial stage of biosorption and, hence, before the insertion of flotation collector. However, the sampling (and the analytical determinations of residual metals) was commonly performed after flotation. As far as the flotation recovery is concerned, this reached an optimum value with the increase of dodecylamine addition (i.e., at 3 10 4 M). As the initial biomass concentration was increased (up to 6 g/L, at pH 7), the removal of metals was also increased (particularly that of zinc and nickel), but the biomass recovery by flotation seriously dropped to less than 40%, showing that higher surfactant addition was needed, as discussed earlier. Tables 1 and 2 show the optimum experimental conditions found during the initial biosorption and the following flotation. Other types of biosorbents were also evaluated [6–8]. The influence of different physicochemical properties on flotation efficiency is known from the relevant previously published work in the minerals enrichment field; similar effects (positive or negative) can be expected also on metal biosorption [13]. Among the major parameters affecting flotation is the surface tension, being largely affected by the surfactant concentration; the respective results are presented in
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752 100
R% Cu(II)
80 60 40 BACTERIA FUNGI YEAST STALKS
20 0 2
4
6
8
10
12
pH 100 80 R% Zn(II)
Fig. 2. It has to be noted, however, that this is a combined effect, also due to the simultaneous presence of frother, because the (flotation) collector (in this case: dodecylamine) was added as ethanolic solution to improve solubility. Surface tension is also a measure of the wetting ability of the medium. Additionally, the addition of ethanol results in the decrease of air bubble size, and hence can stabilize the produced froth [2]. Surface tension measurements for another biosorption system (Streptomyces clavuligerus) were previously reported and discussed [14]; cadmium was the heavy metal under investigation in this case (see also [15]). The values of surface tension for the unmodified biomass were roughly constant in all the examined pH range, whereas the biomass modification was found to decrease the surface tension. The presence of dodecylamine as flotation collector resulted also to significant reduction of surface tension of the biomass dispersion and particularly, at pH values 4.0–7.5. The simultaneous presence of metals was again found to decrease the surface tension (Fig. 2c), fact that can be connected with the improved biomass recovery by flotation. Zeta-potential values of fungal biomass were found to be more negative in the examined pH range (Fig. 3). However, the presence of metals, as well as of surfactant, was found to produce a positive effect for separation. Electrokinetic measurements on this system were elsewhere published [6]. In aquatic environments, it is known that the surface charge of biomass is counterbalanced by the oppositely charged ions, some of which can be bound on its surface, whereas the rest are distributed in a diffuse layer around the biosorbent [16], whose thickness depends mainly on the ionic strength of the solution, as well as on the valency (charge) of counterions. Taking into account the aqueous speciation of dodecylamine (DA), this compound mainly exists as RNH3+ ions at pH values up to (around) 10.5. Therefore, considering also the surface
60 40 BACTERIA FUNGI YEAST STALKS
20 0 2
4
6
8
10
12
pH 100 80 R% Ni(II)
744
60 40 BACTERIA FUNGI YEAST STALKS
20 0
2
4
6
8
10
12
pH
Fig. 1. pH influence on copper, zinc, and nickel biosorption: comparable evaluation between different biomass types; modified from [6, 8].
charge of biomass particles, the noticed behavior of this surfactant can be expected. The solubility
745
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752 Table 1 Optimum experimental conditions for the removal of metals from dilute aqueous solutions by biosorption
Penicillium MYCAN-1 Unmodified Zn 1 2 3 4 Cu 1 Ni 1 Modified (Na2SO4) Zn 1 2 3 Cu 1 2 3 Ni 1 2 Modified (ZETAG-64) Zn 1 2 Cu 1 2 Ni 1 2
Concentration of Biomass (g/L)
pH
Airflow rate Q (cc/min)
Surfactant or polyelectolyte
Contact Time, t (min)
Re % Metal
2.5–3 1.14 1.14 1.14
7.5 8–11 9–10 9–11
200 200 400 200
DA: 3 10 4 M DA: 3 10 4 M DA: 3 10 4 M CTMA-Br: 3 10
30 30 30 30
75–85 80–100 90–100 >95
1.14
11
200
—
15
70
1.14
11–11.5
200
DA: 3 10
M
30
70–75
1.14 1.14 0.5–3
8–11.5 9–11.5 9
200 200 200
— DA: 3 10 DA: 3 10
M 4 M
15 30 30
>95 >95 >95
1.14 0.5–1.5 1.14
6.5–11 9 6.5–11
200 200 200
— DA: 3 10 DA: 3 10
15 30 30
>95 >95 >95
1.14 1.14
9–11.5 8.5–11.5
200 200
— DA: 3 10
4
M
15 30
90–100 >90
1.14 1.14
7.7–10.7 8 10.6
200 200
— DA: 3 10
4
M
15 30
92–100 90–100
1.14 1.14
5.1–10.7 6–10.6
200 200
— DA: 3 10
15 30
82–99 87–99
1.14 1.14
9.7–10.7 8.4–10.6
200 200
— DA: 3 10
15 30
92–96 80–97
4
4
4
M 4 M
4
M
4
M
4
M
(Continued)
746
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
Table 1 (Continued) Penicillium MYCAN-2 Modified (HCl) Zn 1 2 Cu 1 2 Ni 1 2 Penicillium MYCAN-3 Modified (ZETAG-64) Zn 1 2 3 Cu 1 2 Ni 1
1.14 1.14
8–11 8.5–11
200 200
— DA: 3 10
4
M
15 30
90–100 90–100
1.14 1.14
7–11 7–11
200 200
— DA: 3 10
4
M
15 30
95–100 90–100
1.14 1.14
9–11 9.5–11.5
200 200
— DA: 3 10
4
M
15 30
85–90 >95
1.14 1.14 4–6
8–11 7.9–11.2 7
200 200 200
— DA: 3 10 DA: 3 10
M 4 M
15 30 30
85–99 88–100 67–80
1.14 1.14
6–11 5.9–11.2
200 200
— DA: 3 10
1.14
9–11.2
200
DA: 3 10
of long-chain amine flotation collectors was previously discussed [17]; depending on the concentration and the length of hydrocarbon chain, surfactant species may be present on the solid surface either as individual ions, or as aggregates of ions (i.e., as hemi-micelles), or even as precipitates. Flotation requires the formation of a stable bubble–particle aggregate, which enables the particle to be carried out from the suspension. The stability of this aggregate depends on the interfacial free energies, or on the corresponding interfacial tensions of the solid/liquid, solid/ vapor (gas) and liquid/gas interfaces, which are involved in the overall attachment process [10]. The tendency of the particle to replace its solid/liquid by the solid/gas interface, as it happens during the bubble–particle attachment, is commonly termed as ‘‘hydrophobicity.’’
4
4
M
15 30
70–97 73–98
4
M
30
96–100
Hydrophobicity in a solid/liquid/gas system, as flotation is, is certainly a complex phenomenon and a result of different interactions; in this way, the measurement of contact angle becomes significant. The major factors that affect the hydrophobicity of microorganisms are the following: the type of microorganisms, their growth rate, the growth conditions (composition of substrates, type of used carbon source, temperature etc.), the pH and ionic strength of medium, the growth phase of the microorganisms (e.g., static or logarithmic), the specific time of taking the measurement (aging effect), the presence of other polycations, the aeration and the presence of oxygen. Figure 4 presents the respective results for the examined system. The contact angle measurements correlate quite well with the floatability of biomass, as both curves at pH
747
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
Table 2 Optimum experimental conditions for the recovery of biomass by flotation (concentration of metals: Zn: 50/Cu: 10/Ni: 2/Ca: 100/Na: 100 (mg/L); t(flotation): 10 min) Concentration of Biomass (g/L)
pH
Airflow rate (cc/min)
Surfactant or polyelectolyte
Contact time (Min)
Re % Biomass
Penicillium MYCAN–1 1. Unmodified A B
1.14 1.14
7–9.5 7–9
200 200
30 30
70–85 75–80
C
0.5–1.5
7.5
200
DA: 3 10 4 M CTMA-Br: 3 10 4 M DA: 3 10 4 M
30
70–80
2. Modified (Na2SO4) A B C
1.14 1.14 0.5–1.5
7.5–9.8 7–10 9
200 200 200
DA: 3 10 — DA: 3 10
30 15 30
80–96 70–100 90–100
3.Modified (ZETAG-64) A B C
1.14 1.14 1.14
7–9.7 8–10.6 7
200 200 200
— DA: 3 10 4 M DA: 3 10 10 4 M
15 30 30
75–90 84–100 78–100
Penicillium MYCAN–2 1. Modified (HCl) A B
1.14 1.14
7.5–9.3 7.5–9
200 200
DA: 3 10 —
4
M
30 15
80–100 75–80
Penicillium MYCAN–3 1. Modified (ZETAG-64) A B
1.14 0.5–1
3.1–9 7
200 200
DA: 3 10 DA: 3 10
4
M M
30 30
70–100 83–98
values around 11 started to decrease. The contact angles for the unmodified biomass (data not shown) were comparatively low (25–338). The addition of flotation surfactant to the modified biomass was found to increase the contact angle, that is, with 5% dodecylamine addition, higher contact angles (54–628) were obtained, fact connecting with the increase of biomass hydrophobicity. The hydrophobicity of bacteria cells is mainly due to the properties of cell wall (i.e., the biosorption sites), which strongly depend on the presence of various polysaccharides, pro-
4
M
4
M
4
teins and lipids that form a biopolymer surface layer, whose properties can be affected with the changing of growth conditions. In this paper, nonliving biomass was used, as it has been previously found that, for example, for the case of actinomycetes biomass (grampositive bacteria) greater binding capacities for cadmium were found, than the living one [18]. Another problem, not examined in the present, is to find the suitable type of biomass for the effective removal of toxic metals, leading to the screening of various biomass types [19]. By screening several microorganism strains, a
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
748 60 50
Mycan unmodified
σ mN/m
40 30
Mycan (unmod) + 3 x 10−4 M DA σmin = 29 mN/m pH = 8.5
20
Mycan (unmod) +1 x 10−4 M DA σmin = 32.2 mN/m pH = 8.5−9
10 0 2
4
6
8
10
12
pH 60
σ mN/m
50 40
Mycan modified 0.5% DA σmin = 41 mN/m pH = 5.5
30
Mycan modified 1% DA σmin = 36.5 mN/m pH = 5.5
20
Mycan modified (5% DA) σmin = 28 mN/m pH = 6
10 0 2
4
6
8
10
12
pH 60 50
MYCAN DRY MYCAN slurry (5 % DA) 1g/L + 5 ppm CdCl2 MYCAN slurry (5 % DA) 1g/L
σ mN/m
40 30 20 10 0 2
4
6
8
10
12
pH
Fig. 2. pH influence on the surface tension measurements of biomass dispersions; different experimental conditions were applied; (a) unmodified; (b, c) modified in the presence of dodecylamine; and (c) in the simultaneous presence of a metal ion (here Cd).
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752 BACTERIA FUNGI YEAST STALKS
z-potential (mV)
0
−5
−10
−15 in presence of metals mixture
−20 2
4
6
8
10
12
pH
Fig. 3. Electrokinetic measurements of different types of biomass as function of solution pH, in presence of metals mixture; modified from [6, 8].
comparative investigation, regarding the biosorption of lead ions by the addition of filamentous fungal biomass, was also performed [20]. Filamentous fungi are commonly used in fermentation industries to produce several commercial products/metabolites, such as enzymes,
749
flavorings, or antibiotics, as well as industrial by-products, which are able to remove (mostly by chelation) several heavy metals. The main chemical groups in biomass surface, taking part in biosorption, are the electronegative groups (e.g., hydroxyl or sulphydryl), the anionic groups (e.g., carboxyl or phosphate) and the nitrogen-containing groups (e.g., amino) [13]. A chemical speciation study was performed for the examined mixture of metals (by the application of Mineql+ software program), showing that in these conditions, and over the pH limit of 5.9, copper had a thermodynamic phase change to tenorite (CuO), whereas at pH around 7.3, zinc changed to zincite (ZnO) and at pH around 7.8, nickel to Ni(OH)2; on the contrary, calcium ion was stable for the studied pH range. Taking into consideration these results, and comparing them with the respective metal removal, it looks possibly that the main mechanism of metal biosorption is precipitation or coprecipitation onto the biomass cell wall
80
contact angle, °
70 60 50 40 30 3
4
5
6
7 pH
8
9
10
11
12
Mycan modified (ZETAG-64) Mycan modified (ZETAG-64) + 3 x 10−4 M DA + 0.6% ethanol Mycan modified (ZETAG-64) + 3 x 10−4 M DA + 0.6% ethanol + Metal mixture
Fig. 4. Contact angle measurements of modified biomass as influenced by the solution pH (in presence also of collector and metals’ mixture).
750
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
surface. Due to metal speciation, possibly selectivity can be also obtained, at least for the case of copper. 3.2. Multiple cycle operation The influence of coexisting cations (cadmium, cobalt and nickel) on the biosorption of lead and zinc by the addition of Streptoverticillium cinnamoncum and Penicillium chrysogenum was evaluated [21]. The order of metal biosoprtion in a multimetal system can be sufficiently predicted on the basis of Langmuir parameters, applied for binary metal systems; the same sorption model equation is often used for the interpretation of experimental data [18, 22]. In the latter publication, the enthalpy change of metal biosorption on Zoogloea ramigera and on Rhizopus arrhizus (a filamentous fungus) has been also calculated. An exothermic process was proposed for the case of divalent copper and nickel; however, for the cases of Fe(III), Cr(VI) and Pb(II) the process was better described as an endothermic one. Multiple cycles of biosorption/flotation/ elution were also examined, by involving appropriate elution between the treatment cycles of the flotation concentrate (i.e., the separated metals-loaded biomass). It was observed that supplementary addition of surfactant during the third cycle was necessary; otherwise, the biomass flotation was substantially dropped (to around 50%). Therefore, the surfactant was added during the first and the third cycles of operation. The removal of metals was rather unaffected, although after the aforementioned supplementary DA addition during the third cycle, a floatability reduction was observed during the fourth cycle, but that was further improved with a supplementary DA addition during the fifth cycle. In the odd cycles, where the flotation collector was added, biomass recoveries were over 95%; copper removal, in this case, was
over 90%, zinc removal was around 30% and nickel removal was 10–20% at this pH range, noting meanwhile the coexistence of calcium and sodium cations in the feed solution, which can be considered as responsible for the comparatively low elution figures obtained (apart from the pH 7). It seems that biomass could be recycled and floated for at least five cycles. Similar results were also reported for the case of Streptomyces rimosus [7, 8]. Fungi can accumulate metal (and radionuclide species) by the implication of several physicochemical and biological mechanisms, including the extracellular binding by metabolites and biopolymers, the binding to specific polypeptides, as well as the metabolism-dependent accumulation [23]. To date, the most promising approach to metal removal by fungi is biosorption. The fungal cell wall is thought to have two main components: interwoven skeletal framework microfibrils, usually of chitin, embedded in an amorphous layer of proteins and various polysaccharides [24]. 3.3. Concluding remarks The increasing awareness of heavy metals accumulation and contamination of the environment has led to a quest for new and improved, cleaner technologies for the removal and recovery of them, especially from aqueous solutions. An integrated approach (‘‘biosorptive flotation’’) has been examined in the present investigation. The metal-loaded biomass particles, being fine particulates and light enough, can be easily separated by the subsequent application of flotation, whereas the biosorbents may be efficiently recycled after the appropriate metal desorption/elution in further treatment steps. The conducted measurements of surface tension, contact angle and zeta-potential correlate quite reasonably with the observed
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740–752
separation by flotation results. Bacteria that appear to have increased hydrophobicity may also play an important role for the formation and stabilization of froth or foam in various treatment processes, such as flotation, aeration, activated sludge treatment etc. Recently, the control of liquid/gas surface tension was also examined as an alternative means to regulate flotation efficiency. The role of electrophoretic mobility in bacterial adhesion and the surface characteristics of cells in general are considered of primary significance, noting that microbial walls are mostly anionic in nature, due to the presence of carboxyl, hydroxyl, phosphoryl and other negatively charged sites [13]; therefore, cationic metal ions can rapidly bind to these sites by an energy-independent reaction. The present applications of biosorption make use mostly of nonliving biomass. Filamentous fungi are used in fermentation industries to produce various metabolites, such as enzymes, flavorings, orantibiotics, resulting also in the production of large quantities of industrial by-products per year, which can eventually find further uses, for example, by recycling as biosorbents for wastewater treatment [2]. There are several parameters that can influence the hydrophobicity of microorganisms. Under the optimised experimental conditions, the removal of metals by sorption and the recoveries of biomass by flotation were found to be in the order of 95% (or even more) at pH around 7, whereas the surface tension measurements were lowered and the contact angles (between the solid/biomass, the air/bubble and the liquid/ solution to be treated) were increased in this system, hence leading to the improvement of flotation process. On the contrary, although zeta-potential measurements showed a small decrease toward more negative values under these experimental conditions, this was not found to influence substantially the hydrophobicity and the overall treatment process, as this parameter has an effect on the surface
751
charge primarily. Bacteria showing increased hydrophobicity are of particular importance, regarding their subsequent effective separation by flotation. Acknowledgments This work was partially funded by a European Union Environment and Climate programme and a Greek PENED research programme of the Gen. Secret. Res. Tech. Thanks are also due to Ms. M. Palaiomylitou for experimental collaboration, and Mr. A. Phillipson (Microbiology Department, University Newcastle-upon-Tyne, UK) for his help with the biomass preparation. References [1] I. Chem. E. (2007) A Roadmap for 21st Century Chemical Engineering, www.icheme.org/ TechnicalRoadmap [2] A.I. Zouboulis and K.A. Matis, Removal of metal ions from dilute solutions by sorptive flotation, Crit. Rev. Envir. Sci. Tech. 27 (1997) 195–235. [3] Z. Aksu, Application of biosorption for the removal of organic pollutants: a review, Proc. Biochem. 40 (2005) 997–1026. [4] A.I. Zouboulis and I.A. Katsoyiannis, Recent advances in the bioremediation of arsenic-contaminated groundwaters, Envir. Int. 31 (2005) 213–219. [5] A.I. Zouboulis, K.A. Matis and N.K. Lazaridis, The application of flotation for the downstream separation of microorganisms, Intl. J. Envir. & Pollut. 30(2) (2007) 287–295. [6] A.I. Zouboulis, E.G. Rousou, K.A. Matis and I.C. Hancock, Removal of toxic metals from aqueous mixtures: Part 1. Biosorption, J. Chem. Tech. Biotech. 74 (1999) 429–436. [7] A.I. Zouboulis, K.A. Matis, E.G. Rousou and D.A. Kyriakidis, Biosorptive flotation for metal ions recovery, Water Sci. & Tech. 43(8) (2001) 123–129. [8] A.I. Zouboulis, N.K. Lazaridis and K.A. Matis, Removal of toxic metal ions from aqueous systems by biosorptive flotation, J. Chem. Tech. Biotech. 77 (2002) 958–964. [9] B.A. Calfa and M.L. Torem, On the fundamentals of Cr(III) removal from liquid streams by a bacterial strain, Minerals Eng. 21 (2008) 48–54.
752
A.I. Zouboulis, K.A. Matis / Desalination 248 (2009) 740 –752
[10] J. Freund and B. Dobia´sˇ, The role of surface tension, in: K.A. Matis (ed.), Flotation Science and Engineering, M, Dekker, New York, 1995, p. 45. [11] V. Sˇasˇek, Why mycoremediation have not yet come into practice, in: V. Sˇasˇek (ed.), The Utilization of Bioremediation to Reduce Soil Contamination: Problems and Solutions, NATO Advanced Research Workshop, Liblice Castle, Czech Rep., 2000, 14–19 June, lecture notes. [12] N.N. Rulyov, A. Maes and V.J. Korolyov, Optimization of hydrodynamic treatment regime in the processes of sorption-flocculation water purification from organic contaminants, Coll. & Surfaces-Part A’ 175 (2000) 371–381. [13] A.I. Zouboulis, K.A. Matis and I.C. Hancock, Biosorption of metals from dilute aqueous solutions, Sepaqr. & Purif. Methods 26 (1997) 255–295. [14] K.A. Matis and A.I. Zouboulis, Flotation of cadmium-loaded biomass, Biotech. Bioeng. 44 (1994) 354–360. [15] K.A. Matis, A.I. Zouboulis, A.A. Grigoriadou, N.K. Lazaridis, L.V. Ekateriniadou, Metal biosorptionflotation. Application to cadmium removal, Appl. Microbiol. Biotech. 45 (1996) 569–573. [16] M.C.M. van Loosdrecht, J. Lyklema, W. Norde, G. Schraa, A.J.B. Zehnder, (a) The role of surface hydrophobicity in adhesion, and (b) Electrophoretic mobility and hydrophobicity as a measure to predict the initial steps of bacterial adhesion, Appl. Envir. Microbiol. 53 (1987) 1893–1897 & 1898–1901.
[17] M.C. Fuerstenau, Oxide and silicate flotation, in: K.A. Matis (ed.), Flotation Science and Engineering, Dekker, New York, 1995, p. 106. [18] M.I. Kefala, A.I. Zouboulis and K.A. Matis, Biosorption of cadmium ions by Actinomycetes and separation by flotation, Envir. Poll. 104 (1999) 283–293. [19] I. Bakkaloglu, T.J. Butter, L.M. Evison, F.S. Holland and I.C. Hancock, Screening of various types biomass for removal and recovery of heavy metals (Zn, Cu, Ni) by biosorption, sedimantation and desorption, Water Sci. & Tech. 38(6) (1998) 269–277. [20] W. Lo, H. Chua, K.H. Lam and S.P. Bi, A comparative investigation on the biosorption of lead by filamentous fungal biomass, Chemosphere 39 (1999) 2723–2736. [21] P.R. Puranik and K.M. Paknikar, Influence of co-cations on biosorption of lead and zinc – a comparative evaluation in binary and multimetal systems, Bioresource. Tech. 70 (1999) 269–276. [22] Y.T. Sag˘ and T. Kutsal, Determination of the biosorption heats of heavy metal ions on Zoogloea ramigera and Rhizopus arrhizus, Biochem. Eng. J. 6 (2000) 145–151. [23] J.M. Tobin, C. White and G.M. Gadd, Metal accumulation by fungi: applications in environmental biotechnology, J. Industr. Microbiol. 13 (1994) 126–130. [24] B. Volesky, Biosorbents for metal recovery, TIBTECH 5 (1987) 96–101.