Removal of Cu(II) from Aqueous Solutions using Dried Activated Sludge and Dried Activated Nano-Sludge: Adsorption on a Fixed-Bed Column

Introduction

Heavy metals pollution are present in industrial wastewater and are discharged in the enviroment. The presence of toxic heavy metals in aqueous source is one of the most important problems that effectiveness in to enviroment is very important^1,2.Copper is widely used in various industrial activities such as metal plating, mining, tannering and vehicle industry³.

These applications have introduced Cu into aquatic ecosystems, which cause serious environmental pollution problems and bring harmful effect to living organisms. Therefore, it is necessary to reduce Cu (II) from surface waters.

Chemical methods for the removal of heavy metals from wastewater include electrochemical removal, chemical precipitation, ion exchange and memberance. Some of the toxic sludge or biological sludge and can^,t  be regenerated^4,5 .

Today, biosorption of heavy metals from industrial wastewater can be considered as an alternative method, due to high removal efficiency and without yielding harmful by-products⁶.

Adsorbent materials obtained from suitable can be used effectively in the removal heavy metal ions⁷.Biological  removal of  heavy metal ions with high efficiency, high adsorption capacity, easy fixation and fast adsorption-desorption kinetics. Biosorption is a property of certain types of inactive, dead  microbial biomass to bind and concentrate heavy metals from very dilute aqueous solutions⁸.

Activated sludge from wastewater treatment processes is probably the most abundant source of mixed biomass. The ability of activated sludge biomass to remove and accumulate heavy metals and organics has been recognized in different studies⁹. Excess sludge from such wastewater systems can be separated and utilized for removal of heavy metal ions as an abundant and cheaper biosorbent¹⁰. Due to the adsorptive capacity of the microorganisms for heavy metal ions, the biomass can be also be successfully used as a sorbing^11,12. Capability of microorganisms to bind heavy metals in aqueous solutions has long been of scientific interest¹³. Homaidan et al. (2014) studied the removal of Copper ions from aqueous solutions by Spirulina platensis biomass¹⁴. Abdel Aty et al(2013) evaluated the potential of removing cadmium and lead from aqueous solution using fresh water alga Anabaena Sphaerica biomas¹⁵.

In this study, removal of Copper by dried activated sludge (biomass) and activated  nano sludge (nano-biomass) has been studied in fixed bed column reactor. For heavy metal biosorption in packed bed reactors, cells are generally immobilized to improve their structural strength, rigidity and porosity ¹⁶. In order to predict the breakthrough curve of an adsorption process in a fixed-bed the Thomas model has been often used¹⁷.

Important design parameters such as column bed height and flow rate of metal solution in to the column have been evaluated. The efficiency of removal and recovery Cu(II) as a desorption process from adsorption column was evaluated at different operation condition.

Material and Methods

Preparation of Biosorbent and Copper Solution

Activated sludge was obtained from Shazand Petrological Co. (Arak, Iran). The activated sludge was refluxed for 60 min with HNO3 1 M and sieved through a Watman paper, and it was washed with distilled water. Then, the activated sludge was refluxed for 60 min with NaOH 1 M.

The product was washed with the distilled water and then dried at room temperature (25°C).

The dried biomass was grinding to yield a powder. Activated sludge was characterized with scanning electron microscopy (model EM 3200 made KYKY). Figure 1a, b shows SEM pictures of the adsorbents (DAS and DANS) that particles size distribution of the activated nano sludge in uniform and homogeneous shapes. The experiment solutions were prepared by diluting 1000 mg/L of Cu (NO₃)₃⋅3H₂O standard solution with deionized water to the desired concentrations (10-100mg/L) of Cu(II). All chemicals used in this work were of analytical reagent grade.

Figure1: SEM images of (a) DAS (b) DANS   Click here to View figure

Immobilization of Activated Sludge

Activated sludge was immobilized in sodium alginate polymer beads. Immobilization of activated sludge biomass was carried out by the entrapment method. Na-alginate solution of 3% was prepared by dissolving 0.4 g of Na-alginate in 20 mL of hot distilled water with constant stirring to avoid aggregation formation. The mixture was cooled to 40⁰C and activated sludge added in weigh ratio of 1:2 under stirring condition to have a uniform mixture. The slurry was extruded by syringe into 0.2 mol/L calcium chloride solution under shaking by using a peristaltic pump. After 35 min, the bead were amassed from solution and washed with deionized water. Then beads were placed into a solution of calcium chloride (0.2mol/L) for 10 min, through Ca²⁺ diffused into the beads and produced the gelification on the bead surface. After that, the beads were washed with deionized water and kept in a solution of calcium chloride (0.2 M) for 60 min at 4⁰C and finally washed with NaCl 0.7 %^18,19.

Column Set Up

The fixed-bed column were performed in a glass tube with internal diameter of 2/5 cm and length of 50cm.About 20 cm of its height has been packed with  immobilized active sludge,

And a valve has been fitted at the bottom of the column. The pH of the solutions was maintained constant at 3.0. Biosorption experiments conducted in a fixed-bed column at 25⁰C and effluent concentration(25mg/L). The Cu(II) concentration at the outlet of the column at various time periods was determined using atomic absorption spectrometer (model AA 680 made of SHIMADZU) at 324/7 nm.

Mathematical Model

Several models illustrate the dynamic behavior of biosorption in a fixed-bed system that based on concentration-time curves²⁰.In this study, the Thomas model has been applied to biosorption columns. The Thomas model is one of the most general and widely applied to demonstrate the operation and predicted of breakthrough curves of the biosorption process in fixed bed column. The linearized form of the Thomas model can be expressed by the following equation:

 

Where K_Th (mL/min mg) is the Thomas model constant, q₀(mg/g) is the amount of metal ion concentration adsorbed by biomass, C₀(mg/L) is the influent heavy metal ion, C_t(mg/L) is the effluent metal ion concentration, W(g) is the mass of adsorbent in the fixed- bed column and Q(mL/ min) is the flow rate. The parameters K_Th and q₀ may be determined from a plot of 1n(C₀/C_t-1) versus time.

Results and Discussion

Effect of Flow Rate on The Biosorption of Copper

The investigation on the effect of flow rate on the biosorption of Cu(II) on immobilized dried activated sludge and dried activated nano sludge, the influent metal concentration was held constant at 25 mg/L for Cu (II) , bed height (10 cm) constant and changing flow rate 2 to6 mL/min.The breakthrough curve shown in Fig.2 was a plot of dimensionless concentration (C_t/C₀) versus time(t). It was shown that breakthrough curve becomes steeper when the flow rate is increased with which the break point time and exhaustion time decreased.

At higher flow rates, metal ions had less time to contact with microorganism that resulted in lower removal efficiency of metal ions in column. While increasing the flow rate, the result shown that the biosorption capacity could reach the equilibrium value faster, which may cause an increase in mass transfer rate and biosorbent biomass acquires saturated in the early hours²¹.Table 1 show, the parameters Thomas model at the various flow rates.

Figure2: Breakthrough curves: the effect of flow rate on the adsorption of Copper onto (a) DAS and (b) DANS   Click here to View figure

 

Table1: Column parameters with various flow rate and constant bed height (10cm) for adsorption of Cu (II) onto DAS and DANS

 

Effect of Bed Height on the Biosorption of Copper

The effect of bed height on the biosorption of Cu(II) using various bed heights 10,15 and 20 cm, as shown in Figure 3. Accumulation of Copper(II) ion (25mg/L) in the packed bed column is mainly dependent on the quantity of immobilized biosorbents (biomass alginate) inside the fixed bed column. As bed height increases, the breakthrough point time increases and the amount of copper adsorbed on biosorbent surface also increases. This is mainly due to the higher contact time between metal ion solution and DAS and DANS surface²².

Increasing in bed height from 10 to 20 cm has signification effect on removal efficiency and metal uptake capacity²³. Table 2 show, the parameters Thomas model at the various bed heights.

Figure3: Breakthrough curves: the effect of bed height on the adsorption of Copper onto (a) DAS and (b) DANS  Click here to View figure

 

Table2: Column parameters with various bed height and constant flow rate (2ml/min) for adsorption of Cu(II) onto DAS and DANS

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