Bioadsorption of Cd (II) from Contaminated Water on Treated Sawdust: Adsorption Mechanism and Optimization ()
1. Introduction
The removal of toxic and polluting metal ions from industrial effluents, water supplies, as well as mine water is an important challenge to avoid one of the major causes of water and soil pollution. Cadmium is attracting wide attention of environmentalist as one of the most toxic heavy metals. Ionic cadmium, an exceedingly toxic metal, is released into the environment by wastewater from electroplating, pigments, plastic, battery and zinc refining industries [1,2]. Cadmium accumulates readily in living systems [3]. In humans it has been implicated as the cause of rental disturbances, lung insufficiency, bone lesions, cancer and hypertension [4]. Various methods [5] are proposed to remove heavy metal ions from industrial effluents using ion exchange, reverse osmosis and electro dialysis technique, which are efficient but expensive. Chemical precipitation can be envisaged, but the generation of precipitated bulky hydroxides and colloidal particles is often a major disadvantage. Adsorption provides one of the most effective methods for removing heavy metal ions from aqueous solution [6]. Activated carbon is a very efficient solid sorbent in many different applications.
However, it is expensive and the need for an alternative low-cost sorbent has encouraged the search for new sorption processes. Lignocellulosic materials have been used in order to obtain a cheaper adsorbent. Agricultural byproducts [7-11] have received attention for these types of applications. Some chemical modifications can improve the adsorbent behavior of these materials. Thus, modification reactions, including cross-linking, etherification, esterification and graft copolymerization, have been attempted as a means of enhancing the stability of the adsorbent and increasing its adsorption capacity. The previous work focused on preparation and application of MMTU-sawdust in water treatment [10]. The present work aims at improving the ability of sawdust for removal of Cd (II) ions from aqueous solutions. To achieve the goal, the work comprises the following studies: 1) Preparation of MMU; 2) Treatment of SD with MMU at high temperature to form MMU-SD; 3) Ability of the MMU-SD to adsorb Cd (II) from aqueous solutions and to what extent the experimental data obey some of adsorption theories; 4) a study of the adsorption kinetics of Cd (II) ions onto MMU-SD in order to establish the adsorption mechanism; 5) fitting the experimental data to the Freundlich, Langmuir and Redlich-Peterson isotherm models to determine the best-fit isotherm model; and 7) comparison of different error functions in minimizing the error distribution between the experimental and predicted isotherms.
2. Experimental
2.1. Materials
Adsorbent: Sawdust (SD) was kindly supplied from ElEkhlas Company for Wood Manufacturing Company, Sebha, Libya, and passed through 150 - 200 µm and used directly without any treatment. The adsorbent was characterized by FT-IR spectroscopy and by estimation of its nitrogen content.
Reagents: Urea, formaldehyde, triethanolamine, zinc chloride, cadmium acetate, EDTA, sodium carbonate, nitric acid and ethanol were of pure analytical grade and supplied by Merck Company, Germany.
2.2. Methods
2.2.1. Preparation of Mono Methylol Urea (MMU)
The MMU was prepared by mixing urea with formaldehyde using the ratios 1:1.1 (urea: formaldehyde) as follows
MMU (1)
The pH was adjusted to pH 9 using triethanolamine, and the mixture was kept at room temperature for 24 h.
2.2.2. Preparation of MMU-SD
Sawdust (4 g) was added to different amounts of MMU and zinc chloride and mixed well for 10 min using a mechanical stirrer. The mixture was kept in an electric oven at 150˚C for 2 h. The reaction product was Soxhlet extracted for 12 h using a 80:20 mixture of EtOH and water. The crude material was then dried and analyzed for nitrogen content.
2.2.3. Adsorption Studies
The adsorbate solutions of 100 - 1000 mg/l were prepared by dissolving certain weights of cadmium acetate in certain volumes of distill water. Different pH values of the solutions were used, ranging from (2 - 6) and were adjusted with nitric acid or sodium carbonate solutions. Batch experiments were carried out by adding 100 ml of metal ion solution to 0.05 g of the adsorbent and the whole flasks were shaken at 30˚C in a thermostatic waterbath at 150 rpm for 2 h. At the end of agitation time, the metal ion solutions were separated by filtration. The blank experiments were simultaneously carried out without the adsorbent. The extent of metal ion adsorption onto adsorbent was calculated mathematically by measuring the metal ion concentration before and after the adsorption through direct titration against the standard EDTA solution. The amount of Cd (II) adsorbed on MMU-SD at equilibrium, qe (mg/g) and percent removal of cadmium were calculated according to the following equations:
(2)
(3)
where Co and Ce are the initial and final concentrations of metal ion, mg/l. V is the volume of metal ion (l), W is the weight of MMU-SD (g).
2.2.4. Determination of Nitrogen Content (%)
The nitrogen content of MMU-SD was determined using the micro-Kjeldahl method [12].
2.2.5. Error Analysis
In the single-component isotherm studies, the optimization procedure requires an error function to be defined to evaluate the fit of the isotherm to the experimental equilibrium data. The common error functions for determineing the optimum isotherm parameters were, average relative error (ARE), the sum of the squares of the errors (ERRSQ), hybrid fractional error function (HYBRID), Marquardt’s percent standard deviation (MPSD) and sum of absolute errors (EABS) [13]. In the present study, the average relative error (ARE) was used to determine the best fit in isotherm model as:
(4)
3. Results and Discussion
MMU-SD was prepared by reacting SD with MMU. Different factors affecting this reaction were studied. These factors include, MMU:SD molar ratio, catalyst concentration and curing time and temperature.
The MMU reacts with the cellulose SD at high temperature in the presence of catalyst catalyst according to Equation (5):
(5)
3.1. Factors Affecting Modification of SD
3.1.1. Effect of Resin (MMU):SD Molar Ratio
The dependence of the degree of modification of SD, expressed as percent nitrogen on MMU:SD molar ratio was shown in Figure 1. SD was reacted with MMU using different MMU:SD molar ratios (0.5 - 5.0) in the presence of zinc chloride at 150˚C for 2 h. The data in Figure 1 show that the percent nitrogen of the MMU-SD depends on MMU:SD molar ratio. The nitrogen percent increases from 1.83 to 7.96 by increasing the molar ratio from 0.5 - 4.0 and then remained at approximately the same level for higher MMU:SD ratios. The increase in the nitrogen % by increasing MMU:SD molar ratio shows the favorable effect of resin on the modification reaction of SD within this range.
3.1.2. Effect of Catalyst Concentration
Figure 2 shows the effect of catalyst concentration on the nitrogen percent of MMU-SD. Sawdust was reacted with MMU at 150˚C for 2 h using different zinc chloride concentrations (0 - 4.41 mmole/l). The data show that the nitrogen content of prepared samples increases with increasing catalyst concentration to reach a maximum value and decreases on using higher catalyst concentration.
This maximum nitrogen occurs upon using 2.94 mmole/l. This can be explained on the basis of the liberated acidity, i.e., on using zinc chloride up to 2.94 mmole/l. The acidity thus generated is quite enough to catalyze the reaction between SD and MMU, while using higher concentrations of catalyst produces higher amounts of acidity that favor the hydrolytic reaction, which leads to lower nitrogen content.
Figure 1. Effect of molar ratio on %N of MMU-SD at 150˚C.
Figure 2. Effect of catalyst concentration on %N of MMUSD at 150˚C.
3.1.3. Effect of Reaction Time
Figure 3 shows the effect of reaction time on the nitrogen percent of MMU-SD. The data show that the percent nitrogen of MMU-SD increases by increasing the reaction time from 15 to 120 min. A further increase in the reaction time led to decrease the nitrogen percent. The enhancement in nitrogen percent during the first stage by increasing the reaction time is a direct consequence of the favorable effect of reaction time on swelling and accessibility of the cellulose component of the SD and mobility of the MMU molecules and their collision with the cellulose hydroxyls. The decrement in the nitrogen % by increasing the reaction time above 120 min could be attributed to the catalytic effect of reaction time above 120 min on the modification reaction.