Kinetics and equilibrium of desorption removal of copper from magnetic polymer adsorbent
Introduction
Printed circuit board (PCB) is an important production item in electronic manufacturing industry. However, due to its complicated manufacturing with the use of a lot of chemicals and special substances, the wastewater produced during process leads to heavy pollution. The characteristics of pollution tend to become more complex with more requirements of products. The contents of the aged pickling solution contain primarily Cu(II) ion and some organics. At present, the heavy metals-containing wastewater of the PCB plant is commonly treated by chemical coagulation and precipitation [1]. The treatment uses alkaline compounds, such as sodium hydroxide or calcium hydroxide, to assist the formation of insoluble hydroxides with heavy metals. The heavy metal-containing sludge after stabilization or solidification to meet the regulation standards is then disposed in the landfill of hazardous materials. The organics in wastewater can be handled by the traditionally biological processes of the secondary treatment and/or the activated carbon adsorption [2], [3], [4] and advanced oxidation processes [5], [6], [7] of the tertiary treatment so as to meet the effluent standards. Nevertheless, either chemical precipitation or biological treatment can cause the sludge problem. Moreover, the solidification results in a large volume of treated sludge which may also have potential risks of leaching of heavy metals. Therefore, better techniques are needed for treating the heavy metals of aged pickling solution in the PCB plant.
Recently, there have been growing interests in using adsorption materials with large specific external surface area, easy recycling, and high reusability [1], [8], [9], [10], [11], [12], [13], [14], [15]. Among them the synthesized magnetic polymer adsorbent (MPA) of micro-size has been used to recover heavy metal Cu(II) from the aged pickling solution in the PCB plant [1]. The MPA is essentially non-porous, thus preventing the clog problem and having good regeneration ability. Furthermore, it is easy to apply the chemical modification to change the chemical characteristics of the surface of polymer adsorbent. Therefore, the surface modified absorbents can have high affinity to some specific substances. However, for the beneficial application of such tiny adsorbents, one should consider the recovery of absorbents from the treated water after use. The tiny particles are too small to have good recovery efficiency via the conventional processes, such as gravity sedimentation, centrifugal separation, and filtration. Thus, the magnetic adsorbents were synthesized for the use so that a high gradient magnetic (HGM) force field can be applied to separate and recover them from the treated water after the adsorption process. Another advantage of the HGM separation is that the non-magnetic impurities can be excluded during the recovery of magnetic adsorbents [16], [17], [18], [19], [20], [21].
Previous work had described the syntheses of MPA and magnetite-polyvinyl acetate-iminodiacetic acid (M-PVAC-IDA), and the chemical modification to enhance the affinity of adsorbent to adsorb Cu(II) ion of the aged pickling solution [1]. Meanwhile, the adsorption isotherms were set up for three different pH values of 1, 2, and 4.5. Due to the super-paramagnetic property of M-PVAC-IDA, it is convenient to recover the magnetic adsorbent from the treated solution via the HGM separator. The magnetically separated adsorbents can be regenerated via the desorption and then recycled for the reuse of adsorption again [1]. Thus the objective of this study is to examine the desorption of Cu(II) from the exhausted or Cu(II) adsorbed M-PVAC-IDA (denoted as A-M-PVAC-IDA) adsorbents, including the corresponding desorption isotherm, the prediction of the desorption equilibrium isotherm, and the kinetic models of adsorption and desorption of the system of Cu(II) ion and M-PVAC-IDA, which have not been reported. Ethylenediaminetetraacetic acid (EDTA) solution was used to desorb Cu(II) from the A-M-PVAC-IDA by stage-wise desorption operation. Furthermore, cyclic adsorption and desorption operations (CADOs) were performed. For the adsorption in CADOs, the regenerated or Cu desorbed M-PVAC-IDA (noted as D-M-PVAC-IDA) was used to adsorb the Cu(II) ion from the Cu(II) ion-containing solution. As for the desorption in CADOs, the A-M-PVAC-IDA adsorbent was desorbed using EDTA. The obtained desorption isotherm of EDTA/A-M-PVAC-IDA system was compared with the adsorption isotherm of D-M-PVAC-IDA adsorbent and Cu(II) ion loaded solution so as to assess the ability of EDTA for the removal of Cu from the A-M-PVAC-IDA. The information obtained in this study is useful for the rational operation and design of the system for the recovery of Cu from the Cu(II) ion-containing solution and the regeneration and reuse of M-PVAC-IDA adsorbent.
Section snippets
M-PVAC-IDA adsorbent
The M-PVAC-IDA adsorbent used was synthesized via suspension polymerization using super-paramagnetic Fe3O4 gel and other organics. The chemicals used for the synthesis included the follows. Ammonia, ferric chloride, ferrous chloride, and methanol were supplied by Merck (Merck KGaA Co., Darmstadt, Germany). Oleic acid, epichlorohydrin, divinylbenzene, and vinyl acetate (VAC) were purchased from Aldrich. (Sigma–Aldrich Inc., St. Louis, MO, USA). Poly-vinyl alcohol (PVA) with molecular weight (MW)
Kinetic models and equilibrium isotherms
Many adsorption kinetic models had been reported [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39] with the pseudo-first-order and pseudo-second-order equations having been widely applied. Additionally, the Freundlich and Langmuir adsorption isotherms were often used to investigate adsorption mechanism and capacity [40], [41], [42], [43]. These four models were elaborated in the following sections.
Stage-wise desorption of Cu(II) from A-M-PVAC-IDA for the regeneration
Fig. 2 shows the relationship between the equilibrium concentration Ce and dosage of desorbent liquid (in terms of the ratio of accumulated desorbent liquid used LT to mass of adsorbent S0) after each desorption run. The Ce after each desorption stage decreases with increasing LT/S0 as expected. The decrease of Ce is rapid up to the third desorption stage while slow after the forth desorption stage. Consequently, further desorption of Cu(II) after six desorption runs is restrained by the low Ce
Conclusions
In this study, EDTA was employed as a regenerant solution to desorb Cu(II) from the exhausted or Cu(II) loaded magnetic polymer adsorbent M-PVAC-IDA noted as A-M-PVAC-IDA. The regenerated or Cu desorbed M-PVAC-IDA denoted as D-MPVAC-IDA regaining its adsorption ability was reused to adsorb Cu(II) ion from the Cu(II) ion-containing solution. Some conclusions may be drawn as follows:
- (1)
Stage-wise desorptions using EDTA for a certain runs, say 7 runs, satisfactorily remove the Cu(II) from the
Acknowledgement
This study was supported by the National Science Council of Taiwan.
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