H e a v y metal ions removal by biosorption on mycelial wastes
L. Stoica and G. Dima University "Politehnica" Bucharest, Dept. Inorganic Chemistry, 1 Polizu St. 7000, Bucharest, Romania
Mycelial wastes of Penicillium resulted in great quantities from pharmaceutical industry were studied regarding theft ability for heavy metal ions (Cd(II), Pb(II)) removal from synthetic aqueous solutions. Different preliminary treatments were applied to mycelial wastes in order to use them during biosorption experiments. The main parameters of the biosorption process were studied: pH solution, heavy metal ions and biomass concentrations and the type of metallic salt anion. Equilibrium sorption isotherms of Cd(II) and Pb(II) follow the typical Langmuir adsorption model. The loaded inactive biomass was separated from the aqueous solutions by dissolved aft flotation and consequently was submitted to elution for heavy metal ions recovery. The following parameters were studied for the biosorption-flotation process: pH, sorbentmetallic ion contact time and metallic ion concentration in order to obtain an effective separation of the loaded biomass from the aqueous solution. 1. INTRODUCTION Heavy metals can be removed from aqueous solutions by different physico-chemical processes according to the real pollution context. Some toxic metals (i.e. cadmium, lead) must be removed up to very low concentrations, less than 0.1 mg/1. These severe limits imposed for the effluents discharging require advanced purification processes for heavy metals removal. The last ten years researches demonstrated that heavy metals removal from aqueous systems could be successfully accomplished by using active or inactive biomasses, the process being already known as "biosorption". Active and inactive biomasses have similar biosorption capacities (1). Hence, the mycelial residues produced during many industrial fermentation processes like enzymes, flavors or antibiotics production, may be used as biosorbents at least for economically reasons. Fungal mycelial residues were found to accumulate metals and radionuclids by physicochemical and biological mechanisms including theft binding by metabolites and biopolymers or specific polypeptides (2). Biosorption on fungal cell walls was studied for many metallic species (3), the possibilities of subsequently metals desorption and biomasses reuse being also investigated. If the biosorption process is operated in stirred tanks using a suspended biomass (4), a subsequently solid/liquid separation stage is required. The specific characteristics of this kind of sorbat/sorbent system make difficult the separation by filtration (the process needs more time and may face filter blocking problems especially in the case of fine or ultrafine particles),
410 centrifuging (apparent more expensive) or sedimentation (relatively slow process inadequate to biological materials which are usually of low density. Some flotation techniques were applied for microorganisms separation (5) and the possibility of combining biosorption and flotation were also studied (6). Thus, flotation became of great interest among the bioseparation processes. Our work presents a study on heavy metal ions (Cd(II), Pb(II)) recovery from synthetic aqueous solutions by biosorption on mycelial Penicillium residues resulted from pharmaceutical industry and also on loaded biomasses separation by dissolved-air-flotation (DAF). The optimal values founded for the critical processes parameters allowed us to reduce the Cd(II) and Pb(II) concentrations under the limits of effluents discharging imposed by international legislation.
The biosorption experiments were carried out in batch system. The samples of Cd(II) or Pb(II) solutions were shaken together with corresponding quantities ofbiomass in 50 ml flasks on a Braun shaker at a low constant rate ( 150 rpm) for 24 hours (the time we considered for reaching the equilibrium). A mycelial Penicillium residue, resulted from the fermentation process in pharmaceutical industry, was used as biosorbent. The biomass was previously treated by repeated washing and drying up to constant weight; the size particles was less than 100~tm. Cadmium and lead solutions of different initial concentrations were prepared using p.a. reagents: Cd(NO3)2.H20, Pb(NO3)2; CdCI2 5/2H20, CdSO4. Chemical analyses by AAS of the remaining solutions were used in order to assay the unremoved cadmium or lead. For pH regulation we used a Philips pH-meter and 0.1 M HC1 or NaOH solutions. The separation by the flotation process was carried out in a dissolved-air-flotation apparatus (7). The stirring of synthetic mixtures (300 ml ), containing the cadmium or lead solution and the biomass was carried on by an electrical stirrer.
3. RESULTS AND DISCUSSIONS
Biosorption of metallic ions from aqueous solutions on inactive biomass occurs by sorption mechanisms. In figure 1, equilibrium isotherms for Cd(II) and Pb(II) biosorption on inactive Penicillium biomass, obtained without pH controlling, are presented. It is obviously that Langmuir type isotherms describe well the biosorption process. The respective equation can be written as:
bqmCeq qe = (1 +
where Ceqrepresents the free metallic ions concentration at equilibrium, and b, qm are empirical constants, characterizing the metal-biomass interactions.
~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8j _ j j ~ " "-
19 o" 1.2 o
~,- C d ( l l )
g= .g ~. 0.6 -
//// J-~e ~/e
400e(mg/IC ) 600
Figure 1. Sorption equilibrium isotherms solution pH = 5.5; biomass amount=lg/1; sorption time=24h Sorption capacities obtained for the two metallic ions were expressed in terms of mmol M(II)/g biosorbent for a good comparison of the results. The respective values increase along with the increasing of the metallic ions initials concentrations. In the range of low concentrations (under 100 mg/l), the biomass presents similar sorption capacities for the two metallic species (figure 1) but in the range of high initials concentrations (over 500 mg/1) we observed a significant lowering of Pb(II) retention on the Penicillium biomass. 3.1. Biosorption Parameters The solution pH is an important parameter of the process. Sorption experiments of Cd(II) on Penicillium biomass showed a great influence of pH solution on biosorption process (figure 2), our experimental results indicating a significant drop in biosorption capacity values for pH = 6. Cd(II) ions are present in aqueous solutions as stable aqueous complexes [Cd(OH2)4]2+ in the domain of low pH values. Up to pH=7 the aqueous system contains (even for very low concentrations) only positively charged hydroxospecies (CdOIT) and Cd 2+, after this pH value being also present negatively charged hydroxospecies ([Cd(OH)3]-and [Cd(On)4]2). Considering that the biosorption process involves positives species of Cd(II) having different sizes and charges we can explain why the effectiveness of the biosorption process increase continuously with the solution pH increasing. Starting with pH = 7, for an aqueous solution with initial concentration of 100 mg Cd(II)/1, the metallic ion precipitation begins so that Cd(II) recovery occurs not only by biosorption but also by precipitation. It is interesting to note that, at the end of the biosorption process, aqueous solution pH values are the same (pH = 5,5). The amount ofbiomass added to the aqueous solution of Cd(II) represent also an important parameter. The experimental results obtained (figure 3) for Cd(II) sorption on different amounts ofbiomass show, as we expected, that the recovery efficiency of Cd(II) from aqueous
412 solutions (Ci =100mg/1 and Ci =50mg/1) is greater when the amounts of biomass added increase (from 1g/1 to 4g/l). In the same time the sorption capacity decreases accordingly.
90 ~, 80 "~" 70
4g Cd(OH) 2
~ 40 E
_ _ j r ..............,
~ ...._ \
Figure 2. Influence of solution pH on biosorption process; correlation with the structure of Cd(II) species in pH range of 0-13. Ci = 100 mg Cd(II)/1, sorption time = 24h, biomass amount = 1g/1
45 ................................................................................................................... 100 90
r. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
n~ , -=- qe,Ci=50mg/I
~ 2O 0
20 10 0
2 3 Penicillium (g/I)
Figure 3. Influence ofbiomass addition solution pH = 5.5, biosorption time=24h
--A-Cd ! recovery,Ci=100mg/I -~-Cd i recovery Ci=50mg/I . . . . . . . . Ji
. . . . . . . . . . . . . . . . . .
413 A considerable increase of Cd(II) recoveries from the solutions it was observed for a biomass amount of 2g/1 (64% for the solution with Ci =100 mg/1 and 87% for the solution with Ci =50 mg/1), so that a supplementary addition ofbiomass to Cd(II) solutions does not seem to be justified from economic point of view. The time of biosorption also was varied during sorption experiments (figure 4). For two initials concentrations of Cd(II) solutions we observed similar behavior which means that the process is rapid, the first 30 minutes of contacting being enough to reach the equilibrium sorption capacity (considered for 24 hours of contacting ).
..... 0135 .....i i.i..i;;~ iii iii-;.ii./
;.i i i
i iiiii?...;i i ;.i.i ..-.;.V..III ;_ii i~ .i.?. i.;.. ........................................ A
0.2 . ____........_--a
! ~ C i-l-00mg (~d(I i))/" -~ Ci=50mgCd(ll)/I I-~- Ci=100rag Pb(ll) L.....................................................
O Q. L O
Figure 4. Influence ofbiosorption time solution pH = 5.5, biomass amount =1 g/1 The process is rapid for Pb(II) sorption, too (the results for an aqueous solution of Pb(NO3)2 of Ci=100 mg/1 are presented), being in accord with other values reported (8, 9).The study of process parameters was carried on simple dried Penicillium biomass. 3.2 The influence of metallic salt anion
The sorption isotherms for three metallic salts, with corresponding anions sulphate, nitrite and, chloride and the same cation Cd(II) are presented in figure 5. We observed no significant differences between the obtained sorption capacities, the values being closed along the whole range of initials concentrations of Cd(II) solutions.
J ,~ 15o
-=- C d ( N O 3 ) 2 m 0
Figure 5. The influence of metallic salt anion solution pH = 5.5, biosorption time=24h, biomass amount=l g/1 3.3 The influence of pretreatment on biomass sorption capacity The residues of Penicillium were previously treated for sorption experiments development. The biomasses obtained by two different drying ways present distinguishes sorption capacities and recovery efficiencies. We obtained better results (figures 6 and 7), in terms of sorption capacity or R (%), for the acetone dried biomass, especially in the range of high initials concentrations both for Cd(II) and Pb(II).
E E 2.5 - m - C d ( l l ) , s i m ply dried biom ass - ~ - - C d ( l l ) , a c e t o ne dried biom ass --A- P b ( l l ) , a c e t o n e dried biom ass
m 1.5 0 C 0 "2~
._o 0.5 a3
C e (m g/I)
Figure 6. Sorption experiments on Penicillium biomasses obtained by different pretreatments solution pH - 5.5, biomass amount=lg/l, sorption time=24h The explanation for these results may be the nondestructive effect of acetone drying process (occurring probable by osmotic pressure modification).
'~176 l 80
- * - Cd(ll),sim ply dried biomass
-=- C d ( l l ) , a c e t o n e dried biomass
--k- P b ( l l ) , a c e t o n e dried biomass
,~ t 0
Ce (m g/I)
Figure 7. Cd(II) and Pb(II) recovery by different pretreated Penicillium biomass solution pH = 5.5, biomass amount=l g/l, biosorption time=24h 3.4 The biosorption-flotation as a separation process As the organic sorbent is difficult to separate from the liquid phase, particularly by filtration, we experimented the separation of loaded biomass by flotation, the biosorbent presenting a natural flotation tendency. Cd(II) solutions were contacted with the biosorbent, under stirring at different pH values. After the biosorption stage the suspensions were transferred in a flotation cell and submitted to the DAF process. Water saturated with air in the saturator and kept under pressure of 4.105 N/rn2 was introduced to the base of the cell. When releasing water to the atmospheric pressure, fine air bubbles were generating, appropriated for solid/liquid separation. Table 1 %R = f(pH) dependence in Cd(II) recovery Sorption C~ sorption sorption Cf (mg/1) time pH (mg/1) (n~) 10 10 6 2.8 10 10 7 0.6 10 10 8 0.85 10 10 9 0.9 10 20 6 0.9 10 20 7 0.5 10 20 8 0.8 10 20 9 1
by sorption and biosorption-flotation processes; Sorption-flotation pH Cf Cd(II) Biomass Cd(II) R(%) Sorpt( m g / 1 ) R(%) R(%) riot 6 1.9 81 95 72 94 7 0.5 95 95 91.5 8 0.65 93.5 95 91 9 0.8 92 95 6 0.85 91.5 95 91 7 0.4 96 95 95 92 8 0.6 94 95 90 9 0.8 92 95
416 Table 2 %R = f(sorption time) dependence in Cd(II) recovery by sorption and biosorption-flotation processes ;Ci=l 0mg/1; pH=7. Sorption Sorption-flotation Sorption Cf(mg/1) R(%) Cf(mg/1) R(%) Biomass time(min) R(%) 10 0.6 94 0.5 95 95 20 0.5 95 0.2 98 95 30 0.8 92 0.4 96 95 Table 3 %R = f(CiCd(II)) dependence in Cd(II) recovery by sorption and biosorption-flotation processes ;sorption time=20 min.;pH=7. Sorption Sorption-flotation C~(mg/1) Ct(mg/1) R(%) Cf(mg/1) R(%) Biomass R(%) 100 17 83 16 84 95 50 5 90 4.5 91 95 10 0.5 95 0.2 98 95 The dilution ratio (sample volume: water volume) was of 3" 1. Cd(II) concemration and pH value were determined for the effluents. The biosorbent was the acetone dried Penicillium biomass. A simple scheme for the whole process is presented in figure 8"
Cd (II) aq.solution NaOHaq
~l pH relulation I
v I Biosorption I'ql I-I+aq
~,1 Cd(II) loaded I I foam eh]tion I
,~] recovered v] biomass
417 We can assume that, in fact, the pH flotation correspond to the pH value resulted at the end ofbiosorption step. We must note that the efficiency of the flotation process is a promising one for this particular case: Cd(II) loaded biomass separation and pH = 5.5. 4. CONCLUSIONS Biosorption may be applied for Cd(II) recovery from aqueous solutions, the efficiency being dependent on process optimal parameters. Even for more concentrated solution, the acetone pretreated biomass may ensure, in optimal work conditions, high values for Cd(II) recovery. The difficulties of separation the Cd(II) loaded sorbent from the liquid phase may be avoided if DAF technique is applied. This separation process offer the possibility of higher Cd(II) removals (comparing with the simple biosorption) and also high loaded biomass removals (over 95%, the remaining being mechanical losses). What appear of great importance from our laboratory experiments is the fact that flotation does not need the subsequently change of optimal sorption parameters for the loaded biomass separation to occur very well.
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