활성탄에서 P-Chlorophenoxyacetic acid의 흡착특성

김태영*, 김관중, 정승현, 박기민, 김승재, 조성용

전남대학교 환경공학과

Adsorption Characteristics of

P-Chlorophenoxyacetic acid onto Activated Carbon

Tae Young Kim*, Gwan Joong Kim, Seung Hyun Cheng,

Gi, Min, Park, Seung Jai Kim, Sung Yong Cho

Department of Environmental Engineering Chonnam Nat'l. University

Introduction

The widespread and intensive use of chlorinated phenoxyacetic acid herbicides in agriculture and forestry may give rise to a number of toxicological and environmental problems, even though, when used at recommended application rates and procedures, the concentrations reached in the environment are well below the levels generally considered to be of concern. When misused or as the result of accidental spillage, these herbicides have potential to injure non-target cultivars and microorganisms, in particular herbicide degraders and others contributing to soil quality or used as biocontrol agents, and to cause adverse side-effects in mammals, including humans.

There is no conclusive evidence on the effects of phenoxy acetic acids and their salts on crop plants. It is not easy to assess the environmental fates of phenoxy herbicides. Especially the fate of phenoxy herbicides in fields is interesting because use of phenoxy acetic acid derivatives in agriculture and disposal of unused commercial preparations may result in pollution of arable soil[1].

Various treatment techniques have been employed to treat the wastewater, including precipitation, adsorption, ion exchange, and reverse osmosis[2]. Among them, adsorption onto solid adsorbents has great environmental significance, since it can effectively remove pollutants from both aqueous and gaseous streams. In wastewater treatment, activated carbon is a powerful adsorbent because it has a large surface area and pore volume, which remove liquid-phase contaminants, including organic compounds, heavy metal ions and coloring matters. The characteristics, pH, and temperature of wastewater are likely to vary with time, so design of a suitable adsorption system is not that simple. In order to design effective activated carbon adsorption units and to develop mathematical models which can accurately describe their operation characteristics, sufficient information on both the adsorption and the desorption of individual pollutants under different operating conditions is required [3]. The main purpose of this work is to eliminate of P-chlorophenoxyacetic acde (P-CPA) from its aqueous solution, and to study its adsorption characteristics experimentally as well as theoretically.

Experimental

The adsorbent used in this study was granular activated carbon (GAC), F400, manufactured by Calgon Co..  GAC was sieved into a narrow range of particle sizes (0.055∼0.065 cm), and washed with distilled water several times to remove impurities. The GAC were then stored in a sealed bottles with silica gel to prevent the readsorption of moisture.

1. Isotherm Test

Adsorption experiments were conducted by contacting a volume of solution with a carefully weighed amount of GAC in a conical flask. The amounts of GAC were varied (0.001∼0.3g), and the volume of the solution was 200ml. The flaskes were shaked for 7 days to provide enough time to reach equilibrium between the solid and liquid phase. Concentrations of P-CPA in the solution were determined using UV spectrophotometer (Shimadzu 1601) at 279nm. In this study, three isotherm models ; Langmuir, Freundlich, and Sips, were used to correlate our experimental equilibrium data. Langmuir and Freundlich equations have two parameters and Sips equation have three parameters. To find the parameters for each adsorption isotherm, the linear least square method and the pattern search algorithm (NMEAD) were used.

2. Kinetic and Column Test

Adsorption experiments were conducted in a batch adsorber with four baffles. The rotor speed was kept at approximately 400 rpm, so that the external mass transfer resistance should be negligible. All runs were carried out at a temperature of 25oC. The column tests were carried out in a water-jacketed pyrex column with inside diameter and length of 1.5 and 44 cm, respectively. The column was packed with beads to distribute the solution uniformly. The activated carbon with known weight dry was put into the column. Distilled water was pumped into the column from the bottom to remove air bubbles and to rinse the carbon. In adsorption cycle, P-CPA solution was continuously fed to the top of the column until the column was saturated with the feed P-CPA solution. The saturated column was then regenerated with distilled water. The column temperature for all the column tests was maintained at 25oC. The effluent samples were collected intermittently by a fraction collector and the concentration was measured by the UV spectrophotometer (Shimadzu 1601).

Result

Fig. 1 shows that the experimental equilibrium adsorption isotherms obtained at the different initial pHs for the adsorption of P-CAP onto GAC. As shown in this figure, the equilibrium adsorption amounts of P-CAP onto GAC increased with decreasing solution pH. It is common observation that anions are favorably adsorbed on the surface of adsorbents at low pH because the presence of hydrogen ions renders the surface active for the adsorption of cations at high pH values. The experimental data are well represented by the Sips equation. The calculated parameters of isotherm  equation are listed in Table 1. Fig. 2 shows the experimental data and model prediction for the adsorption of P-CPA in a batch adsorber. In this study, the pore diffusion coefficient, Dp, and surface diffusion coefficient, Ds, are estimated by pore diffusion model (PDM) and surface diffusion model (SDM) [3]. The estimated values of kf, Dp, and Ds for P-CPA  are listed in Table 2. Table 2 shows that the magnitude of the three estimated values, Ds being much smaller than other two. This implies that the diffusion inside a particle is a rate-controlling step.

Since the flow rate is an important factor in a fixed bed design, its effects were studied by testing different fluid velocities. Fig. 3 shows that the break time was shorter for higher flow rate. Usually, the intraparticle diffusivity is believed to be independent of flow rate. However, the breakthrough curves were found to be steeper at higher flow rate as shown in Fig. 3. This phenomenon is attributed to external film mass transfer resistance. This resistance is smaller when flow rate is higher, so that the length of mass transfer zone is reduced and a shaper breakthrough curve is generated.

The choice of a certain regeneration method should depend upon the physical and chemical characteristics of both the adsorbate and the adsorbent. In this study, distilled water was used as desorbate for GAC. As shown in Fig. 4, desorption of P-CPA was about 90% only using distilled water of usually pH 7. The effluent pH decreased in the earlier adsorption stage, and then increased to influent pH as adsorption proceeded, as represented in Fig. 4. At pH 3.5 and P-CPA concentration of 0.45mol/m3, the effluent pH sharply decreased up to about nearly 3.5 and then slowly increased.

Acknowledgment

This research was financially supported by the Korea Research Foundation (KRF-Y00-316).

Reference

1. Cabral, M. G., Viegas M.. C., Teixeira and Sa-Correia, Chemosphere, 51, 1 2003

2. Kim, S. J., Kim, T. Y., Kim, S. J. and Cho, S. Y., Korean J. Chem. Eng., 19, 6 2002

3. Kim, T. Y., Kim, S. J. and Cho, S. Y., Korean J. Chem. Eng., 18, 5 2001

Table 1. Adsorption equilibrium constants of P-CPA on to activated carbon at 25oC

Isotherm type	Parameters	pH 3.5	pH 7.0
Langmuir	qm b Error(%)	2.25 6.79 2.29	0.38 9.90 1.98
Freundlich	k n Error(%)	3.22 1.72 7.99	0.91 2.44 5.26
Sips	qm b n Error(%)	2.25 6.81 0.99 3.33	0.80 10.31 0.78 1.71

Table 2. Kinetic parameters of P-CPA in a batch adsorber at 25oC

pH	[eqn: { k}_{f } TIMES { 10}^{5 }] m/s	Dp × 1010 m2/s	Ds × 1013 m2/s
3.5	7.67	2.90	6.80
7.0	2.94	0.278	0.729
10.0	1.91	0.244	0.177