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Enhanced cometabolic transformation of 4-chlorophenol in the presence of phenol by granular activated carbon adsorption.

Substrate inhibitions that manifest within the cometabolism system of 4-chlorophenol (4-cp) and phenol were alleviated through the application of granular activated carbon (GAC) in batch biodegradation. It was found that 4-cp was preferentially adsorbed over phenol by the GAC and that 50% to 70% of the adsorption was achieved within the first two hours of contact. The kinetics of 4-cp adsorption was also much faster than that of phenol, even when the co-existing phenol was of a significantly higher initial concentration. As a result, competitive inhibition between the two compounds was minimized. Adsorption also caused a lowering of the phenol concentration in solution with a concomitant reduction in the substrate inhibition effect on cell growth. The addition of GAC benefited the biotransformation process through shortening the total degradation time for 600 mg [L.sup.-1] phenol and 100 mg [L.sup.-1] 4-cp from 42 h to 12 h; and it also made it possible for cells to survive and transform 600 mg [L.sup.-1] phenol and as high as 400 mg [L.sup.-1] 4-cp in free suspension cultures. Repeated operations in which GAC was reused showed that GAC could be regenerated by the cells, thus rendering the GAC incorporated process amenable to long term operations.

Les inhibitions de substrat qui se manifestent dans le systeme de cometabolisme du chlorophenol-4 (cp-4) et du phenol ont ete reduites par l'application de charbon actif granulaire (GAC) lors d'une biodegradation discontinue. On a trouve que le cp-4 etait le premier a etre adsorbe sur le phenol par le GAC et que de 50 a 70 % de l'adsorption avait lieu dans les deux premieres heures du contact. La cinetique d'adsorption du cp-4 est egalement beaucoup plus rapide que celle du phenol, meme lorsque le phenol coexistant est d'une concentration initiale substantiellement plus elevee. En consequence, l'inhibition competitive entre les deux composes est minimisee. L'adsorption cause egalement une baisse de la concentration de phenol dans la solution avec une reduction concomitante de l'effet d'inhibition du substrat sur la croissance des cellules. L'ajout de GAC a un effet benefi que sur le processus de biotransformation en raccourcissant le temps de degradation total pour 600 mg [L.sup.-1] de phenol et 100 mg [L.sup.-1] de cp-4 de 42 h a 12 h; et cela permet egalement aux cellules de survivre et de transformer 600 mg [L.sup.-1] de phenol et jusqu'a 400 mg [L.sup.-1] de cp-4 en cultures de suspensions libres. Des operations repetees dans lesquelles le GAC est reutilise montrent que le GAC pourrait etre regenerees par les cellules, rendant ainsi le procede d'incorporation du GAS propice pour des operations a long terme.

Keywords: cometabolism, adsorption, phenol, 4-chlorophenol, Pseudomonas putida, granular activated carbon

Phenolic compounds, many of which are known to be toxic to many living organisms, are commonly found in the environment as a result of the rapid pace of industrialization. The oil refining, textile, pesticide, as well as the pulp and paper industries have always produced high concentrations of phenol and chlorinated phenolic compounds in either their primary products, or their waste effluents. These compounds do not only taste and smell unpleasant, but they are also known to be toxic and carcinogenic. Biological treatment of such waste water and industrial effl uent usually involves utilization and transformation of mixed substrates. In some cases, the phenolic compounds cannot support cell growth (non-growth substrates), and cometabolism is encountered.

In cometabolism, biotransformation of the non-growth substrate is achieved either by growing cells in the presence of a growth substrate, by resting cells in the absence of a growth substrate, or by resting cells in the presence of an energy source (Criddle, 1993). In order for a biological treatment process involving cometabolism to be sustainable, growth substrates, which provide energy for cell growth and maintenance, must inevitably be present. One well-known cometabolism system that has been extensively studied is the biodegradation of phenol (growth substrate) and 4-chlorophenol (4-cp) (non-growth substrate) by Pseudomonas putida (Saez and Rittmann, 1991; Loh and Wang, 1998). Phenol is converted by monooxygenase to catechol and then by catechol 2, 3-dioxygenase to 2-hydroxy muconic semialdehyde (HMSA), after which HMSA is further oxidized serially to ultimately yield C[O.sub.2], energy for biosynthesis and maintenance and biomass (Yang and Humphrey, 1975). For the biotransformation of 4-cp, Saez and Rittmann (1991) postulated that the fi rst two enzymes for the degradation of phenol can also catalyze 4-cp to 4-chlorocatechol and then to 2-hydroxy-5-chloro-muconic semialdehyde (HCMSA), which remains in solution as a dead-end metabolite. In this case where both phenol and 4-cp share the same key enzymes, competitive inhibition exists and cometabolic transformation of 4-cp is strongly affected by the presence of phenol. On the other hand, 4-cp also inhibits metabolism of phenol because of its toxicity and recalcitrance, thereby decreasing cell growth and retarding biodegradation (Criddle, 1993; Saez and Rittmann, 1993). Furthermore, cell growth on phenol has been observed to display substrate inhibition effect at high phenol concentrations (Yang and Humphrey, 1975).

To avoid competitive inhibition two-stage or sequencing reactor systems have been proposed to decouple cell growth and transformation of the non-growth substrate. For example, Alvarez-Cohen and McCarty (1991) have proposed the use of a two-stage bioreactor that utilizes cometabolic biotransformation of trichloroethylene (TCE) by methanotropic cells in the presence of methane (a growth substrate) while Hecht et al. (1995) have investigated the feasibility of a bioscrubber to cometabolically degrade TCE. However, from a practical viewpoint, such multistage processes are not applicable when treating co-existing pollutants, like phenol and 4-cp.

Activated carbon adsorption has been extensively studied and applied as a physical technique for the removal of aromatic hydrocarbons from waste water (Flora et al., 1994; Moreno-Castilla et al., 1995; Chatzopoulos and Varma, 1995; Furuya et al., 1997; Haghseresht et al., 2002; to name a few). In the realm of biological treatment, activated carbon has also been used as a cell immobilization matrix (Ehrhardt and Rehm, 1985; Kindzierski et al., 1992; to name a couple). Ehrhardt and Rehm (1985) noted the survival of the immobilized bacteria in spite of the addition of normally toxic phenol concentrations and the subsequent utilization of most of the adsorbed phenol. They concluded that quick adsorption of phenol on the activated carbon enabled the bacteria to be exposed for only a short time to the toxic phenol concentrations, and this seemed to be applicable for the treatment of waste water containing temporarily high phenol concentrations. According to Kindzierski et al. (1992), the activated carbon served as an immobilization buffer and protected the immobilized micro-organisms by adsorbing toxic phenol concentrations and gradually set free low quantities of the adsorbed phenol for biodegradation.

In a recent study, Furuya et al. (1997) reported that the adsorption isotherms of phenolic compounds on activated carbon could be signifi cantly different depending on the electron cloud density of the compounds, and their consequential interactions with the surfaces of the carbons. Based on this, we anticipated that there might be a possibility of separating phenol and 4-cp utilizing activated carbon adsorption so as to reduce the extent of competitive inhibition between the two compounds, and at the same time, lower the phenol liquid phase concentration, thereby decreasing the substrate inhibition effect.

The objective of this study was to exploit the feasibility of using granular activated carbon (GAC) in a batch reactor to simultaneously biodegrade a mixture of phenol and 4-cp. To this end, equilibrium adsorption isotherms of phenol and 4-cp on a coconut-shell-based GAC, both singly and in combination, were obtained. In addition, the kinetics of adsorption was also investigated. Subsequently, the bio-availability of adsorbed phenol and 4-cp was investigated to evaluate the use of GAC for phenol biodegradation and consequently cometabolic transformation of 4-cp in the presence of phenol in a sustained treatment operation.

EXPERIMENTAL

Micro-Organisms

Throughout this study, Pseudomonas putida ATCC 49451 was used, and stock cultures were maintained on nutrient agar slants (Oxoid, Hampshire, U.K.) and stored at 4[degrees]C. For preparation of inocula, cells from the nutrient agar slant were induced with the basal medium (described below) supplemented with 200 mg [L.sup.-1] phenol as the sole carbon source. All batch cultures were grown with agitation at 200 rpm and at 30[degrees]C. The working volume in each run was 250 mL, in cotton plug fitted 500 mL Erlenmeyer flask. All cell culture experiments were conducted in triplicates.

Culture Medium

The chemically defined culture medium used in this study consisted of a mineral salt medium and a trace mineral solution, the compositions of which have been reported by Loh and Wang (1998). 10 mL of the trace mineral solution and 30 mL of the mineral salt medium were added to each litre of the medium. The sole carbon source, phenol, was added accordingly, to obtain the desired concentrations for each experiment.

Chemicals

All chemicals used in this study were of analytical grade. Phenol and 4-cp, obtained from Merck (Darmstadt, Germany), were each dissolved in 0.02M NaOH solution to make 10 000 mg [L.sup.-1] stock solution. The granular activated carbon (GAC) used throughout this study was a coconut-shell-based type (1~1.5 mm in diameter) and obtained from Casitan (Petaling Jaya, Malaysia). The GAC was washed with Milli Q water to remove all the fine particles, and then heated to 70[degrees]C for 24 h, and finally stored in a desiccator for subsequent use.

Adsorption Kinetics and Isotherms

Kinetics of the adsorption was obtained by soaking 0.5 g GAC in 250 mL of phenol or 4-cp at two initial concentrations, 300 mg [L.sup.-1] and 600 mg [L.sup.-1]. At predetermined time intervals, the concentrations of phenol or 4-cp in solution were measured from 1 mL sample taken from the fl ask. All experiments were performed in triplicates.

To obtain the equilibrium adsorption isotherms of phenol and 4-cp on the GAC, 0.5 g of GAC were immersed in 250 mL of varying initial concentrations of phenol (100-1600 mg [L.sup.-1]) and 4-cp (100-1000 mg [L.sup.-1]), either alone or in combinations. The phenol and 4-cp solutions were prepared in culture mineral solution. The suspension was left for 72 h for equilibrium to be established. The amount of phenol or 4-cp adsorbed was determined from material balances after measuring the equilibrium concentration of the components in solution.

Cell Density Measurement

Cell concentration was determined by measuring the optical density (OD) at 600 nm using a Shimadzu UV-Visible Spectrophotometer UV-1601 and 1-cm-path length quartz cuvettes with deionized water as reference. Optical Density (OD) obtained was converted to cell mass based on the correlation established earlier: X(mg [L.sup.-1])=314.5*O[D.sub.600] (Wang and Loh, 1999).

Phenol and 4-cp Measurement

Phenol and 4-cp concentrations were determined in accordance with the protocol reported by Loh and Wang (1998). The detection limit of the gas chromatography method was within 1 mg [L.sup.-1].

RESULTS AND DISCUSSION

Single Component Adsorption The kinetic profi les of phenol and 4-cp adsorption respectively, on GAC during the fi rst two hours of the adsorption were fi rst investigated. Previous studies on the rate of adsorption of phenols on GAC have indicated that the adsorption rate was rather fast at the beginning. In their study, Zogorski et al. (1975) reported that the amount adsorbed in the initial stage of adsorption was linearly related to the square root of time ([t.sup.1/2]), and that more than 60% to 80% of the adsorption occurred within the first hour. Nevskaia et al. (1999) reduced kinetic expressions derived from models based on interaction between the surface and the adsorbate, to the same parabolic relationship as that shown in Equation (1):

q (t) = [alpha] x [t.sup.1/2] (1)

where [alpha] is an initial concentration dependent coefficient. Equation (1) was used to fit the experimental kinetic data obtained (data not shown) with excellent agreement ([r.sup.2] > 0.98).

The top half of Table 1 summarizes the values of [alpha]for different initial concentrations of phenol and 4-cp. We found that [alpha] for 4-cp was much larger than that for phenol at both initial concentrations studied, suggesting that the uptake rate of 4-cp was faster than that of phenol. During the 2 h period in which the kinetics experiments was conducted, the adsorbed phenol levels were 51% and 53% of the equilibrium capacity for initial concentrations of 300 mg [L.sup.-1] and 600 mg [L.sup.-1], respectively, while the corresponding percentages for 4-cp, were 71% and 57%. In general, equilibrium adsorption was reached after 24 h.

The experimental adsorption data for phenol and 4-cp on GAC were fitted to the Langmuir-Freundlich isotherm (Equation (2)), also with excellent agreement ([r.sup.2] > 0.97; data not shown).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

In Equation (2), [q.sub.e], [q.sub.s], and b are defined as the equilibrium solid phase concentration of the adsorbate, the saturated solid phase concentration representing the adsorption capacity and the Langmuir isotherm constant, respectively; v is a parameter related to the heterogeneity of the active sites on the adsorbent, and when v is unity, the Langmuir-Frendlich model is reduced to the Langmuir isotherm for an energetically homogeneous adsorbent (Jaroniec and Madey, 1988). The Langmuir-Freundlich isotherm was chosen because liquid-solid adsorption data could be better modelled especially when interactive effects of multiple substrates were involved.

The second half of Table 1 tabulates the model parameters for phenol and 4-cp. The obvious difference between the value of [q.sub.s] for phenol and 4-cp indicates the different adsorption capacities: that GAC has higher adsorption capacity for 4-cp over phenol. Moreover, the affinity constant, b, for 4-cp was also larger than that for phenol, indicating that the GAC used has a greater adsorption affinity for 4-cp over phenol. The adsorption phenomena of phenolic compounds on GAC has yet to be thoroughly understood due to the great variety of activated carbons available and the complexity of the adsorption process itself (Arafat et al., 1999). It has long been assumed that the adsorption of phenolic compounds on activated carbon is due to the interaction of the [pi] electrons of the aromatic ring and the surface of the activated carbon. Radovic et al. (1997) has reported that the adsorption of phenolic compounds on activated carbon also depends on the number and types of substituents on the aromatic ring although the carbon atoms attached to the chlorine atoms in chlorophenols did not seem to interact with the active site on the adsorbent.

The fitting results also gave a lower v for 4-cp compared with phenol, which indicates that the active sites were more heterogeneous for phenol adsorption than for 4-cp adsorption.

Binary Component Adsorption

The kinetics of adsorption in the binary component system was first examined. Results from these experiments are shown in Figure 1. These indicate that the kinetics of adsorption for both phenol and 4-cp were little affected by their mutual presence in the mixture. Furthermore, it can be seen again that a significant portion of the adsorption was accomplished within the first two hours of contact with the GAC.

[FIGURE 1 OMITTED]

In considering the use of the adsorption process to facilitate cometabolism of phenol and 4-cp, the adsorption rates and the differences in the rates are very important characteristics. This is because the overall biodegradation rate could very likely be dominated by the lag phase, which could vary greatly with different concentrations of both the non-growth substrate and the growth substrate, due to their toxicity effect on the degrading cells. Therefore, a faster adsorption rate of the organic compounds is required to help the cells in overcoming the toxicity of the substrate through shortening the contact time for direct contact with the highly concentrated and toxic solution.

The competitive effects that exist between phenol and 4-cp adsorption on the GAC were investigated by performing adsorption experiments on mixtures of phenol and 4-cp. Figure 2 features the resulting isotherms.

[FIGURE 2 OMITTED]

It can be seen that the inhibition of 4-cp on the equilibrium adsorption of phenol was greater than the converse; the equilibrium adsorption of 4-cp was not affected to as great an extent by the presence of the equal initial concentration of phenol.

The adsorption data were fitted to modified interactive isotherms (Ruthven, 1984) given by Equation 3:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where i, j = 1, 2 for phenol and 4-cp as the case may be, and all other notations have their usual meanings. Specifically, k provides an indication of the relative inhibition effect between the two substrates.

Figure 2 also shows the corroboration of the model and the experimental data. The only adjustable parameters were obtained as [k.sub.2] (inhibition due to 4-cp) = 6.3 (dimensionless) and [k.sub.1] (inhibition due to phenol) = 1.8 (dimensionless). The fits were very good with [r.sub.2] in excess of 0.97. For comparison, the single component adsorption isotherm was also plotted. It can be seen that phenol adsorption was relatively more significantly decreased in the presence of 4-cp than the converse. The stronger relative inhibition exerted by 4-cp on phenol adsorption can be quantitatively assessed by the ratio:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where subscript 1 is for phenol, and 2 is for 4-cp. For all the concentrations studied, this ratio ranged from 1.2 to 1.5 (data not shown), confirming the stronger inhibition of 4-cp to phenol adsorption.

What was more significant in these results was that for the same starting concentrations of each of phenol and 4-cp, due to the higher adsorption capacity of GAC for 4-cp, the equilibrium concentration of 4-cp in the solution mixture was significantly lower than that of phenol. For example, for starting concentrations of 1000 mg [L.sup.-1] each of phenol and 4-cp, the equilibrium concentration of phenol was 600 mg [L.sup.-1] while that of 4-cp was only 300 mg [L.sup.-1]. If the starting concentrations of phenol were much higher than that of 4-cp which is typically the case in industrial waste water (e.g. 1000 mg [L.sup.-1] phenol and 100 mg [L.sup.-1] 4-cp; Loh and Wang, 1998), this difference in equilibrium concentrations would be more dramatic. When phenol and 4-cp co-exist in a mixture and the 4-cp concentration is lower than the phenol concentration, GAC can adsorb most of the 4-cp, leaving a significant amount of phenol in the solution. This result implies an effective separation of phenol and 4-cp in the waste water, and the release of the competitive inhibitory effect of 4-cp on phenol degradation in the phenol-4-cp cometabolism system.

Bioavailablity of Adsorbed Phenol

Before investigating the feasibility of using GAC in the cometabolism system, studies were conducted to analyze the bioavailability of the adsorbed phenol for cell growth. Typically, suspension cells could not grow in phenol concentrations exceeding 800 mg [L.sup.-1] due to substrate inhibition effects (Wang and Loh, 1999). Figure 3 shows the results of a representative experiment for 600 mg [L.sup.-1] initial phenol concentration. At this phenol concentration, in the absence of GAC, the suspension cells experienced a short lag phase of about 5 h before phenol was gradually degraded, concomitant with cell growth. The specific growth rate in this case was 0.29 [h.sup.-1], and the maximum cell density attained was 314 mg [L.sup.-1]. Total degradation of phenol was achieved in 21 h.

[FIGURE 3 OMITTED]

With GAC, the phenol concentration profile was very different. There was a sharp decrease in the first hour due to rapid adsorption of the phenol on the GAC. This was followed by a gradual removal of phenol to an undetectable limit. Cell growth did not show significant lag, and exponential growth immediately ensued. The specific growth rate, in this case, was 0.4 [h.sup.-1]. Cells grew faster because of the lower (less toxic) phenol concentration in the solution. Disappearance of phenol from the solution was achieved in a much shorter time, only 15 h. It is important to note that in contradistinction to the control experiment, in which the cell density immediately declined upon depletion of phenol, cells continued to grow, albeit slowly even though phenol was undetected in the solution in the presence of GAC. During this period, there was still a small amount of adsorbed phenol, which slowly desorbed from the GAC to support cell growth.

All the experiments conducted in this section gave similar results, and it can therefore be concluded that the adsorbed phenol indeed did desorb from the GAC, and was therefore available for cell growth. However, there remained a doubt on whether all of the adsorbed phenol could reversibly desorb from the GAC. To investigate this, the cell yield on phenol, based on the suspension cell density obtained and assuming complete consumption of phenol, was calculated for all the experiments.

Figure 4 compares the cell yield on phenol in the presence and absence of GAC. In the absence of GAC, cell yield noted a decrease with increase in phenol concentrations, while in the presence of GAC, we observed an increase in cell yield with increase in phenol concentrations. Notwithstanding, the cell yield obtained with GAC present was in all cases lower than that without GAC. Superficially, it seemed that some of the adsorbed phenol had been irreversibly bound on GAC, and hence not available for cell growth, hence resulting in lower final cell density.

[FIGURE 4 OMITTED]

However, it has been reported that GAC is a good immobilization matrix for cells (Kindzierski et al., 1992), and therefore the cells could have grown on the adsorbed phenol on the GAC, but not registered in the cell density measurements of solution turbidity. Moreover, it has also been observed that cells continued to grow for a further duration beyond phenol depletion in solution, implying that there was continued desorption of phenol. In the absence of reliable analytical technique to determine the total cell density present in each of the experiment, it was difficult to ascertain if all of the adsorbed phenol was presented to the cells during the biodegradation process. In subsequent sustainability studies, however, it was confirmed that a small but constant amount of phenol (and 4-cp) remained irreversibly bound to the GAC after the first exposure. This will be discussed in a later section.

Regardless, the presence of GAC has allowed for cell immobilization, hence increasing phenol tolerance (Keweloh et al., 1989), a lowering of the phenol concentration in solution (at high initial phenol concentrations) with decreased substrate inhibition on cell growth, and consequently a much shorter time taken for phenol removal.

Impact of 4-cp Toxicity on Degradation Time

The inhibitory effect of 4-cp on phenol degradation has been studied extensively (Saez and Rittmann, 1993; Wang and Loh, 1999). It has been proven that 4-cp could severely retard phenol degradation and cell growth. The presence of 4-cp caused a long lag phase for phenol degradation resulting in a longer overall degradation time. In our study, we found that the lag phase for phenol degradation of 300 mg [L.sup.-1] phenol was extended from 6 h to 35 h when 4-cp concentration increased from 100 mg [L.sup.-1] to 200 mg [L.sup.-1]. Of course, the level of the phenol concentrations in the mixture also influenced the lag phase significantly since phenol exerts substrate inhibition effects. Due to competitive inhibition between phenol and 4-cp, typically 4-cp would only start to transform upon complete removal of phenol (Wang and Loh, 1999). Therefore, the total degradation time would inadvertently be longer than the time taken for complete phenol depletion. The concentration of 4-cp, nevertheless, has a more dominating effect on the total degradation time due to its toxicity on the cells. For example, we have found that complete removal of 600 mg [L.sup.-1] phenol with 100 mg [L.sup.-1] 4-cp took 41 h, which was still 10 h shorter than the time taken to remove 300 mg [L.sup.-1] phenol and 200 mg [L.sup.-1] 4-cp. Although pure cultures of P. putida could provide a rather high specific growth rate, the lag phase--as long as 35 h (in presence of 200 mg [L.sup.-1] 4-cp)--makes the total degradation period longer than 50 h, thus causing the potential application to deteriorate.

To alleviate the competitive inhibition of the growth substrate (phenol in our study) and the non-growth substrate (4-cp in our study) as well as the extension of the lag phase for degradation due to non-growth substrate toxicity in simultaneous degradation of the two compounds, many researchers have suggested the sequential degradation of the growth substrate, followed by the non-growth substrate by resting cells (Alvarez-Cohen and McCarty, 1991; Hecht et al., 1995).

Considering the special adsorption features of GAC that have been demonstrated earlier, it is possible that simultaneous degradation could be improved by simply adding GAC, which could result in the considerable lowering of the 4-cp concentrations in solution. Before studying the impact of GAC on phenol and 4-cp, a study on sequential degradation of phenol and 4-cp was conducted, to assess the effectiveness of using GAC later. P. putida was grown in different concentrations of phenol, to cell confluence through phenol depletion and 100 mg [L.sup.-1] 4-cp was added to the solution. At that stage, no growth substrate was present in the medium and 4-cp was transformed at the expense of the resting cells. Table 2 tabulates the experiments conducted and the results of the degradation times for complete removal/ transformation of the two compounds.

The complete removal of 100 mg [L.sup.-1] 4-cp (in reasonable time durations) required cells to be grown in initial phenol concentrations of as high as 300 mg [L.sup.-1] in order that sufficient resting cells were available for transforming 4-cp. The times taken for the complete removal of phenol and 4-cp in these experiments, however, were much lower than that for simultaneous degradation. For example, the time taken for removing 500 mg [L.sup.-1] phenol and 100 mg [L.sup.-1] 4-cp in sequential degradation was 17 h and this was much shorter than that for simultaneous degradation, which took 27 h (data not shown). However, when the 4-cp concentration was as high as 300 mg [L.sup.-1], even cells raised in 500 mg [L.sup.-1] could not transform it. It can be inferred that the activity of resting cells could be inhibited when they were suddenly exposed to such a high 4-cp concentration.

Application of GAC in the Phenol-4-cp Cometabolism System

The feasibility of using GAC for the simultaneous degradation of phenol and 4-cp was examined by conducting the biodegradation experiments in the presence of 2 g [L.sup.-1] of GAC. Figure 5 shows the phenol and 4-cp concentration profiles for all the experiments. For easy reference, indexes for these experiments are P for phenol only, PC for phenol + 4-cp mixtures, G for experiments with GAC and the number at the end represents the concentration of 4-cp in hundreds mg [L.sup.-1]. In all of these, there was a sharp drop in concentration during the first couple of hours of fast adsorption. Following that, depending on the initial concentration of 4-cp, there was either no lag in degradation (at low 4-cp concentrations) or a short lag in degradation (at high 4-cp concentration). GAC was effective in selectively removing 4-cp from the solutions rendering improved phenol degradation rates.

[FIGURE 5 OMITTED]

From Figure 5, comparing PC1 and GPC1, in the presence of GAC, the lag phase for degradation was significantly lowered from 33 h to 6 h and consequently total degradation time decreased from 42 h to 12 h, or almost 4 times reduction in time taken. With increasing 4-cp concentration, the lag phase also increased despite the presence of GAC, but the overall effectiveness was definitely enhanced. In particular, when cells were provided with 4-cp concentrations above 300 mg [L.sup.-1] in the absence of GAC, no biotransformation occurred regardless of phenol concentration; in the presence of GAC, even 400 mg [L.sup.-1] 4-cp could be removed within a reasonable duration.

It can be concluded from the above results that the separation of 4-cp and phenol by GAC adsorption could effectively relieve the inhibitory effect of 4-cp on phenol degradation and maintained phenol at a sufficient level to ensure a reasonable cell growth rate. Consequently, the total time needed for removal of phenol decreased from 42 h (PC1) to 12 h (GPC1) for the same phenol and 4-cp concentrations. The question, however, remained whether 4-cp was merely adsorbed by the GAC or that it was bioavailable to the cells for transformation. In the final section of this research, the GAC used in the experiments was reused to seek an answer to this question.

Reusability of GAC in the Cometabolism System

Multiple degradation runs were conducted to evaluate the reusability of the GAC. In this reusability test, the same GAC was repeatedly used in multiple operations of the adsorption-degradation-desorption cycle. During the test, 800 mg [L.sup.-1] of phenol mixed with 200 mg [L.sup.-1] of 4-cp, was used together with 2 g [L.sup.-1] GAC. The degradation and cell growth profiles of cycles 1, 3, 4, 5 and 6 were recorded. It is important to mention that each of the experimental runs was maintained for 10-15 h beyond the complete exhaustion of both phenol and 4-cp to ensure that all reversibly bound substrates had been released before starting the next cycle.

According to Figure 6, it can be seen that part of the GAC adsorption capacity for both phenol and 4-cp was irreversibly lost. The lost capacity was around 100 mg [g.sup.-1] GAC for phenol and 40 mg [g.sup.-1] GAC for 4-cp, accounting for 57% and 58% of the GAC adsorption capacities of phenol and 4-cp, respectively, in the first 4 to 6 h. However, it was found that once GAC had lost this part of its adsorption capacity, there was no more irreversible adsorption. It was found that during the third to the sixth run, the same phenol and 4-cp removal profiles were recorded, despite an extension of the lag phase by about 10 h after the first run.

[FIGURE 6 OMITTED]

The lost capacity of phenol and 4-cp could be due to several reasons, including adsorbate oxidation, and hysteresis. Our results, nevertheless, show that the reversible capacity of GAC could be maintained at a certain level in a sustained application.

This implies a potential for application in industrial waste water treatment involving cometabolic transformations. In this study, cell growth also occurred in quite a similar manner in the third to sixth runs. After approximately a 35 h lag phase, exponential growth occurred and ended with consistently about 283 mg [L.sup.-1] cell mass. The similarity of the results in the last four runs suggests that the process could reach a steady state operation when the GAC adsorption capacity has been stabilized.

CONCLUSIONS

In order to sustain cometabolic biodegradations in waste water treatment, a number of substrate inhibitions have to be taken care of: (a) substrate inhibition of the growth substrate phenol; (b) toxicity of the non-growth substrate 4-cp; and (c) competitive inhibition between the two substrates. Granular activated carbon (GAC) has been exploited in batch biodegradation for minimizing these inhibitions so that removal efficiency could be enhanced. GAC has been found to be effective for selectively removing 4-cp over phenol in synthetic waste water containing the two substrates. Furthermore, adsorption of the substrates reduced the solution concentration of both substrates. The feasibility of using GAC has been demonstrated to not only reduce the substrate inhibitions involved in the cometabolism system, but the overall degradation effi ciency has also been improved, with a consequential reduction in the overall degradation time. Long term operation using this technique has also been ascertained through repeated operations in which the GAC was reused.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the graduate scholarship to Ye Wang provided by The National University of Singapore.

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Manuscript received October 10, 2005; revised manuscript received December 5, 2005; accepted for publication December 6, 2006.

Kai-Chee Loh * and Ye Wang

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Dr. 4, Singapore 117576, Singapore

* Author to whom correspondence may be addressed.

E-mail address: chelohkc@nus.edu.sg
Table 1. Kinetics and Langmuir-Freundlich parameters for phenol
and 4-cp adsorption on GAC in single component system

Component Kinetics ([alpha])
 (mg[L.sup.-1][h.sup.-1])

 Initial concentration

 300 mg[L.sup.-1] 600 mg[L.sup.-1]

Phenol 69 108

4-cp 104 167

 Langmuir-Freundlich parameters

 [q.sub.s]([mgg.sup.-1]) b v

Phenol 1430 0.04 0.30

4-cp 2150 0.10 0.15

Table 2. Effect of 4-cp on degradation time in sequential degradation
of phenol and 4-cp

Phenol 4-cp Time for phenol
(mg[L.sup.-1]) (mg[L.sup.-1]) degradation (h)

50 100 7

75 100 7

100 100 8

200 100 11

300 100 9

500 100 12

500 300 20

600 200 24

Phenol Time for 4-cp Total time of
(mg[L.sup.-1]) removal (h) processes (h)

50 N/A -

75 N/A -

100 78 86

200 66 77

300 9 18

500 5 17

500 N/A -

600 5 29

N/A: 4-cp was not appreciably transformed.
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Author:Loh, Kai-Chee; Wang, Ye
Publication:Canadian Journal of Chemical Engineering
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Date:Apr 1, 2006
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