Artículo original
Bioadsorption of Cu2+, Ni2+, Zn2+ and Pb2+ using an activated sludge process
Bioadsorción de Cu2+, Ni2+, Zn2+y Pb2+ empleando un proceso de lodo activado
Bioadsorption of Cu2+, Ni2+, Zn2+ and Pb2+ using an activated sludge process
Tecnología Química, vol. 38, no. 3, 2018
Universidad de Oriente
Received: 19 March 2018
Accepted: 20 July 2018
ABSTRACT: Heavy metals contents can be reduced from wastewater using microorganisms through different mechanisms included under the general denomination ofbioadsorption. Bioadsorption of heavy metals consists in a passive, fast and reversible catchment of the metal ions present in an aqueous solution by physic and chemical interactions on aviableornon-aviablebiological materials. In the present paper, a mixed culture of microorganisms from an activated sludge processwas used to reduce fourdifferent heavy metals contents from a synthetic solution. Copper, nickel, zinc and lead were prepared at a range of 5 - 30 mg/L. The bioadsorption intensity was determined at two values of mean cell retention time (sludge age).The results demonstrated that Freundlich’sisotherm adjusted well(R2> 0.95), with a relatively strong bioadsorption due to intensity parameter “n”.Copper, nickel, zinc and lead final contents were 74,60-77,68 %; 75,40-40,26 %; 74,54-78,38 % and 25,0-31,6 %for 5 and 10 days of sludge age respectively with non-significant differences.
Keywords: heavy metals, bioadsorption, Freundlich’s isotherm, activated sludge.
RESUMEN: El contenido de metales pesados puede ser reducido de las aguas residuales empleando microorganismos a través de diferentes mecanismos, incluyendo la bioadsorción. La bioadsorción de metales pesados consiste en una captación pasiva, rápida y reversible de iones metálicos presentes en una solución acuosa mediante interacciones físicas y químicas sobre materiales biológicos viables o no viables. En el presente trabajo se utilizó un cultivo mixto de microorganismos procedentes de procesos de lodos activados para remover metales pesados de un residual sintético. Las soluciones de cobre, de níquel, de zinc y plomo se prepararon en el intervalo de concentración entre5 - 30 mg/L. La intensidad de adsorción se determinó para dos tiempos de retención medio celular (edad del lodo). Los resultados demostraron que la isoterma de Freundlich ajustó bien (R2> 0,95), con una relativa fuerte bioadsorción de acuerdo con la intensidad del parámetro “n”. La eliminación de cobre, níquel, zinc y plomo fue 74,60-77,68 %; 75,40-40,26 %; 74,54-78,38 % y 25,0-31,6 %, para las edades de lodo de 5 y 10 días respectivamente, sin existir diferencias significativas.
Palabras clave: metales pesados, bioadsorción, Isoterma de Freundlich, lodo activado.
Introduction
The increase concentration of heavy metals in wastewater is mainly due to industrial origin. Growing efforts have been done to diminish such metal concentrations using adequate technologies.1,2)
Are well known diverse processes to remove heavy metal from wastewater. The adsorption at different media like activated carbon, zeolite, and others compounds of inorganic originare one of the most studied.3,4,5
Conventional methods to treat effluents containing Cu2+, Ni2+, Zn2+ and Pb2+ ions include traditional physical and physical-chemical treatment.6,7,8,9 They are costly as a consequence of its high reactive and energy requirements, toxic sludge generation and the secondary contamination that can be produced.10,11,12
The use of methods for heavy metals removal has incomplete results and is inefficient at relatively low metal concentrations (1-100 mg/L).13,14
There are reports of the use of microorganisms or bioadsorbent (algae, fungi and bacteria) for removing heavy metals from wastewater. 8,15,16,17
Generally, the bioadsorption process can reduce the capital costs by 20 %, the operational costs by 36 %, and the total treatment costs by 28 % compared with conventional systems. 18
The bioadsorption mechanism is complex and still not well understood. It depends on whether the organism is aviable or non-aviable, as well as the type and species of microorganisms. 15 Demonstrated that metal ions in a solution are adsorbed on the biomass through Van der Waals interactions. The adsorption isotherms are normally used to study the adsorption processes. Freundlich´s empirical isotherm is one of the most common model that can be applied.19
Thepresent work is a study of the removal of four of the most common heavy metals present in wastewater using a mixed culture of microorganisms from an activated sludge process.
Materials and methods
Start up and reactor operation withthe mixed culture of microorganisms from activated sludge
To obtain a proper amount of biomass, a batch,cylindrical, completely stirredaerobic reactor with 8 L of effective volume, was used. The Hydraulic Retention Time (HRT) was 1 day with5 and 10 days of sludge age. The biomass used as seed was collected from an activated sludge wastewater treatment plant located atJibacoa beach near the Havana. The inoculum was adapted to the new conditions during three weeks until constant Volatile Suspended Solids (VSS) and Chemical Oxygen Demand (COD) concentrations were achieved. The reactor was fed daily with synthetic wastewater concentration using asimilar composition as referred by 20, as is shown in table 1.
The synthetic wastewater had a COD value of 1 200 ± 14 mg/L. The pH was 7, 0 ± 0, 5. Addition of 0, 5 M NaO Hand 0, 2 M H2SO4 was necessary. The reactor was maintained at room temperature (30 ± 2ºC).
Analytical tests were performed based on APHA, AWWA, WPCF Standard Methods for Water and Wastewater Analyses.(21
Bioadsorptiontests
Stock solutions containing Cu2+, Ni2+, Zn2+, and Pb2+ions were prepared dissolving copper, nickel, zinc and lead usingCuSO4·5H2O, NiSO4·6H2O, ZnSO4·7H2O and Pb(NO3)2respectively in distilled water.
Kinetics study
Tests were conducted to determine the contact time to reach equilibrium conditions, adding the respective metal salt solution to 25 mL of activated sludge (mixed liquor). Metal concentration in the mixed liquor was 5 mg/L and pH 7, 0 ± 0, 5. It was agitated in anIKA-WERKE shaker at 150 rpm, 30 ± 2 °C, at time intervals of: 5, 10, 15, 20, 30, 40, 60 min at room temperature. Each heavy metal and sludge age was tested with 21 samples. Whatman No. 42 filters were used to separate the suspended biomass.
Information on the interaction between bioadsorbent and heavy metals (Ho et al. 1996) can be obtained from kinetics models. In that sense the pseudo-first order kinetics model or Lagergrenʼs model22 and the pseudo-second order model, can be tested.23,24,25
The pseudo-first order model can be represented through equation 1.
where
qe and qt: (g metal ion/g bioadsorbent): are the metal adsorbed at time t and equilibrium, respectively, k1 (min-1): is the pseudo-firs torder reaction rate equilibrium constant.
The pseudo-second order modelis shown in equation 2:
where
k2 (g/(mg min): is the pseudo-se condorder reaction rate equilibrium constant
The model parameters were calculated by non-linear regression using Statgraphics Centurion V15 program.
To determine the best model adjustment to the kinetic data, two statistics methods for error analysis were used: correlation coefficient (R2) and residual root mean square error (RMSE). Higher values of R2 and smaller values of RMSE indicate a better fit of the model.24
Isotherm experiments
Biomass specific bioadsorption capacity was estimated adding metal salt ion solutions in a range interval concentration from 5 to 30 mg/Lto 25 mL of sludge from the activated sludge process.
The specific metal ion adsorbed q(mg metalion/g bioadsorbent) in each case was calculated according to equation 3.
where
q (mg metal ion/g bioadsorbent): amount of heavy metal ion adsorbed, Co and Ce(mg/L): are the initial and the equilibrium metal ion concentrations respectively; V (L) is the suspension volume and m (g) is the mass of biomass used as bioadsorbent, measured as volatile suspended solids (VSS).
All experiments were carried out by triplicates.
Removal efficiency
Removal efficiency (% Removal) was calculated according to equation 4.
where
Co and Ce (mg/L): are the initial and the equilibrium metal ion concentrations respectively
Freundlich’s Isotherm
In order to determine the bioadsorption isotherm, the experiments were carried out as explained before with the same metal ions concentrations range.
The data obtained was adjusted to Freundlich’s isotherm model (19) for both sludge ages, according to equation 5.
where
qads: Specific bioadsorption capacity (mg metal ion/g bioadsorbent); Ce (mg/L): equilibrium concentration of metal ions in the solution; kf (L/mg): Freundlich constant related to the bioadsorption capacity and n: Freundlich constant related to the bioadsorption intensity.
The Isotherm’s Freundlich parameters were calculated by non-linear regression.
Results and discussion
Seed characterization
The seed characteristics are shown in table 2. As it can be seen, the VSS represents the 84 % of the Total Suspended Solid (TSS), denoting the excellent sludge quality used as inoculums.27
The reactors were considered in steady state conditions when the effluent concentrations in terms of VSS and COD were constantand all tests were carried out. The values of these parameters at steady-state conditions were VSS (mg/L):
1 100 ± 142 and 1 700 ± 128; COD (mg/L): 160 ± 19 and 250 ± 20for both sludge ages respectively.
Kinetic study
Kinetic study is a very important test in abioadsorption process, because it describes the rate of bioadsorption. Therefore, the effects of contact time of copper, nickel, zinc and lead solutions onto activated sludge is shown in figure 1 (a and b) at two values of sludge age (5 and 10 days)at the initial metal concentration of 5 mg/L and 30 ± 2 °C at 150 rpm with a contact time from 5 to 60 min
Fig. 1.
Figura 1.
Fig. 2.
Figura 2.
According to figure 1, the bioadsorption capacity increases with the increase of the contact time. Initially, all active sites on the bioadsorbentsur face are supposed not occupied and the metal solution concentration is high. After that, the surface active sites are increasingly occupied with the time until attaining an equilibrium between the solid and liquid phase. According to this, a contact time of 15 min was selected.
In comparison with other kinetic studies using biological adsorbent reported in table 3, this contact time is lower and may be indicating that the bioadsorption mechanism was superficial.28
Bioadsorption kinetics of cooper, nickel, zinc and lead solutions ions were analysed with two different adsorption models (pseudo-first and pseudo-second order). Pseudo first and second order models were adjusted to the results as graphically are shown in figure 2 (a, b). The model parameters were calculated using non-linear regression.
Fig. 2. (a)
Pseudo-first and pseudo-second order kinetic models adjusted to bioadsorption kinetics for 5 days of sludge age
Fig. 2 (b)
Pseudo-first and pseudo-second order kinetic models adjusted to bioadsorption kinetics for 10 days of sludge age
In table 4, the correlation parameter values for bothmodels (pseudo-first order and pseudo-second order) is shown for each method used to evaluate the metals. According to the results, a good adjustment was obtained for all cases with similar values ofR2and RMSE. As a consequence, both models can be used to predict the kinetics of the bioadsorption process. When considering the short time obtained to reach the equilibrium, it can be assumed that physical bioadsorption occurred. This result is similar to the one obtained. 22,33,34
Removal percentage of Cu2+, Ni2+, Zn2+ and Pb2+
The percentage of metal ions removedonce the equilibrium condition was reached, is shown in table 5, for 5 and 10 days of sludge age. The higher removal percentage were obtained for Cu2+, Ni2+, Zn2+ at 5 days of activated sludge; and Cu2+ and Zn2+ at 10 days of activated sludge. In all experiments, Pb2+ was more resistant to bioadsorption.
Similar result was obtained by 35 and 36). However, in these papers, the removal percentages of lead solution ion werehigher. Considering thatbioadsorption is influenced by different parameters like: surface properties and affinity of bioadsorbent for different heavy metal ions 37), different orders can be obtained in different bioadsorption studies. In table 6, is shows the removal percentages obtained by other authors in similar cases. Nearly all removal percentage are lower than removal percentage obtained in this paper. In general, no significant difference in the removal percentage wasfound for 5 and 10 days of sludge age.
Isotherm of bioadsorption
Figure 3 (a, b) showsthe experimental bioadsorption results and the corresponding Freundlich’s isotherm for the data in the range of metal ions concentration from 5 to 30 mg/L. Theisotherm is a type II shaped. It may indicate a physic multilayer bioadsorption. This suggeststhat bioadsorption of Cu2+, Ni2+, Zn2+ and Pb2+was more likely a heterogeneous surfaced adsorption, instead of a monolayer sorption.(14)
Fig. 3 (a)
Bioadsorption isotherm and Freundlich bioadsorption model adjustment to 5 days of sludge age
Fig. 3 (b)
Bioadsorption isotherm and Freundlich bioadsorption model adjustment to 10 days of sludge age
The parameters of theFreundlichmodel, adjusted by a none linear regression, areshownin table 7. The bioadsorption intensity, “n”values wherebetween 0 and 10, suggesting relatively strong bioadsorption 15 of the seions on to the surface of the activated sludge for each sludge ages. According to the magnitudes ofn value, the bioadsorption intensity orders were: Ni2+>Zn2+>Cu2+>Pb2+ and Zn2+>Cu2+>Ni2+>Pb2+ for 5 and 10 sludge age respectively.
An ANOVA analysis of the obtained results showsthat the age of sludge had not significant effect with 95 % of confidence level (P-value > 0, 05) (table 8).
All F-ratios are based on the mean square residual error
Conclusions
The sludge obtained from the activated sludge process at 5 and 10 days of sludge age demonstratedbe a good bioadsorbentfor Cu2+, Ni2+, Zn2+ removal at a concentration interval of the studied metal ion solution. The time to attainthe adsorption equilibrium conditions was considered 15 minutes in all cases. This short time obtained to reach the equilibrium suggests that physical bioadsorption occurred.
Both models, pseudo-first order and pseudo-second order can be used to predict the kineticsof the bioadsorption process.The higher removal were obtained for Cu2+, Ni2+, Zn2+ at 5 days of activated sludge; and Cu2+ and Zn2+ at 10 days of activated sludge. In all experiments, Pb2+ was more resistant to bioadsorption.The bioadsorption intensity orderswere: Ni2+>Zn2+>Cu2+> Pb2+ and Zn2+>Cu2+>Ni2+> Pb2+ for 5 and 10 sludge age respectively;the age of sludge had not significant influence onto biosorption intensity.
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Author notes
*Autor para la correspondencia. Correo electrónico: jaimedm89@gmail.com
Conflict of interest declaration