Materials

Two types of powdered activated carbon (PAC) were investigated; the first (analytical grade, article 102186) was purchased from Merck Eurolab (Darmstadt, Germany), and the other (Hydraffin WG, for water and wastewater treatment) from Lurgi (Frankfurt am Main, Germany). Properties of the carbons are summarized in Table 1. Titanium dioxide (Aeroxide P25) was provided by Evonik Industries AG (Hanau–Wolfgang, Germany), TetraEGDME (synthesis grade, purity >98%) and phenol (analytical grade) by Merck Eurolab. A grab sample of biologically pretreated greywater was taken from the effluent of a subsurface vertical-flow constructed wetland, with intermittent feeding of greywater separately collected in the eco-settlement Lübeck–Flintenbreite. Operation of the constructed wetland is described elsewhere in greater detail Citation3. The non-purgeable TOC (np-TOC) concentration of the clear constructed wetland effluent was 7.0±0.2 mg L⁻¹, and total inorganic carbon (TIC) concentration was 78.5±2 mg L⁻¹. A second grab sample used for recording an adsorption isotherm exhibited an np-TOC of 6.1±0.2 mg L⁻¹ and a TIC of 73.1±0.4 mg L⁻¹. The np-TOC concentration of a third sample taken from the same source in order to quantify different classes of organic compounds by liquid chromatography coupled with organic carbon detection was 6 mg L⁻¹.

Concentration of biologically treated greywater by vacuum evaporation

The effluent of the constructed wetland was subjected to vacuum evaporation in a rotary evaporator at 80°C. After evaporating 20 L to a final volume of 2 L, the concentrate was re-diluted with 2 L of original biologically pretreated greywater in order to obtain sufficient fivefold concentrate for PCO experiments in duplicate.

UV irradiation experiments

The following samples were subjected to UV irradiation at ambient temperature in 1 L batches:

•	1 mM phenol (94 mg L−1) solution in deionized water or tap water,
•	0.225 mM TetraEGDME (50 mg L−1) solution in deionized water or tap water, and
•	a fivefold concentrate of biologically pretreated greywater.

The pH of the model wastewaters and the greywater concentrate was not further adjusted but was recorded during UV irradiation. Solutions were placed in slim 2 L beakers (inner diameter 10.8 cm) which were stirred by means of 7 cm magnetic stirring bars at a speed of 300 min⁻¹ (Figure 1). Prior to UV irradiation the suspensions were stirred for 1 h in the dark, according to Citation7. UV irradiation was performed with face tanners (HD 172, Philips, Hamburg, Germany) placed 20 cm above the liquid surface. UV intensity was measured using a pyranometer (CMP 3, Kipp & Zonen, Delft, Netherlands) and was 15 W m⁻² at the liquid surface. The face tanners exhibited an emission maximum at 352 nm. Although not thermostated, the temperature of the reactor contents was sufficiently constant at C, slightly above room temperature due to heat emission from the UV lamps. Different kinds of irradiation experiments were performed, including photolysis (without addition of TiO₂ and PAC), PCO (with addition of 2.5 g L⁻¹ TiO₂ P25), and PCO in the presence of PAC (with addition of 2.5 g L⁻¹ TiO₂ P25 and 0.5 g L⁻¹ of one of the two investigated activated carbon types).

Samples (50 mL) were taken from the stirred reactors at different times. Water evaporating from the reactors was replenished with deionized water prior to sampling. All samples were membrane-filtered (Magna Nylon filters, pore width 0.45 μm, Carl Roth GmbH, Karlsruhe, Germany) prior to DOC, DIC and phenol analysis. Some experiments were performed in multiplicate as indicated in the results and discussion section. Apparent first-order rate constants, k, were derived from the slopes of the regression lines recorded for the different experiments: where c is concentration of phenol, DOC or np-DOC, c ₀ the respective concentration at the beginning of UV irradiation and t is the irradiation time. As the reaction rate depends on temporary organic concentrations, and concentration of the organics was increasing due to water loss by evaporation during UV irradiation, Equation (2) is a simplification. However, as enrichment factors due to water evaporation between two sampling events were less than 1.1, this simplification was regarded as tolerable.

Adsorption kinetics in the dark

1 L suspensions of 0.5 g L⁻¹ PAC in solutions of phenol (94 mg L⁻¹) or TetraEGDME (50 mg L⁻¹) were magnetically stirred in closed bottles in a thermostated water bath to maitain the temperature at 27°C for up to 48 h. Light was excluded from the bottles by wrapping them in aluminium foil. At different time intervals, 30 mL samples were pipetted from the stirred solutions and filtered over a folded paper filter. Filtrates of phenol solutions were subjected to photometric phenol analysis, while the TetraEGDME solution filtrates were analysed for TOC.

Greywater adsorption isotherm

Different masses of the Merck PAC were suspended in 50 mL portions of non-concentrated biologically pretreated greywater, giving PAC concentrations between 0.1 and 20 g L⁻¹. These suspensions were agitated for 5 days at 21°C on a mechanical shaker at 150 min⁻¹. Subsequently the PAC was allowed to settle and the supernatant liquor was filtered over 0.45 μ m porosity membrane filters. Filtrates were analyzed for np-DOC.

Analyses

Phenol concentrations were analyzed photometrically by recording the absorbance of membrane-filtered samples in quartz glass cuvettes between 200 and 400 nm using a double-beam photometer (V–550, Jasco, Gross-Umstadt, Germany). The height of the absorption maximum at 270 nm was corrected by subtracting the height of the baseline drawn between the two valleys neighbouring the maximum. The baseline-corrected absorbances were calibrated against phenol solutions of known concentration.

DOC concentrations of phenol and TetraEGDME solutions were measured using the difference method. Due to the high TIC concentrations, greywater samples were analyzed for np-DOC. Both analytical procedures were completed in accordance with German standard methods Citation26 using a TOC analyser multi-N/C 3000 (analytikJena AG, Jena, Germany) under the following conditions: furnace temperature, 850°C; catalyst, CeO₂; incineration gas, CO₂-free air. For np-DOC analysis, samples were acidified to pH 1–2 with concentrated HCl and purged 20 min with CO₂-free air prior to DOC analysis. TIC and DIC were analyzed in the same analyzer following the difference method.

Different classes of organic greywater constituents (polysaccharides, humic substances, building blocks, low molecular weight organic acids) in constructed wetland effluent samples were analyzed subsequent to membrane filtration (0.45 μm pore width). Liquid chromatography was employed, with organic carbon detection (LC–OCD; DOC–Labor Dr. Huber, Karlsruhe, Germany) as described in Citation27 using a size exclusion column HW–55S and FIFFIKUS software for quantification. The LC–OCD analyzer was equipped with a 190 nm irradiation thin-film reactor organic carbon detector and a UV detector.

Dissolved oxygen concentration and pH in the reactors were determined using the respective probes (WTW, Weilheim, Germany).

Matrix properties

Table 2 shows the range of pH and dissolved oxygen concentration established in the different reaction mixtures during UV irradiation. Dissolved oxygen concentration was only recorded for phenol degradation in a deionized water matrix (six experiments on photolysis, two experiments on PCO and four experiments on the combination of PCO with Merck PAC addition). It was clearly demonstrated that stirring of the reaction mixtures supplied sufficient dissolved oxygen (6 to 7 mg L⁻¹).

Comparison of the pH of phenol solutions in deionized water during photolysis to pH of phenol/TiO₂ suspensions in deionized water during PCO shows that TiO₂ addition decreased the pH due to its acidic surface functional groups. Addition of Merck PAC slightly raised the pH, while addition of Hydraffin WG PAC decreased the pH by about 0.5 units. This is in accordance with the more acidic character of Hydraffin WG (Table 1). The matrices of the solutions investigated largely influenced pH; while TiO₂ suspensions in phenol solutions with deionized water caused pH between 3.9 and 4.8, the same suspensions in a tap water matrix exhibited a pH range of 7.6 to 8.4 due to the dissolved hydrogen carbonate in tap water (TIC concentration 21.6 mg L⁻¹). A similar pH increase was observed when deionized water was replaced by normal tap water with TetraEGDME as the solute. The highest pH was found in TiO₂ suspensions in concentrated biologically pretreated greywater, which showed a TIC concentration of 432 mg L⁻¹. While the pH at the start of the experiment was 8.2 to 8.4, it increased to above 9 during PCO, irrespective of the presence of PAC.

Reference experiments with phenol solutions and the impact of activated carbon type

The results of photometric phenol analysis might be slightly affected by aromatic phenol PCO intermediates (1,2-dihydroxybenzene and 1,4-dihydroxybenzene as predominant intermediates, and 1,2,3-trihydoxybenzene, 1,2,4-trihydroxybenzene Citation28, and benzoquinone Citation29 as minor products). Most of these substances exhibit absorption maxima sufficiently remote from the 270 nm absorption maximum of phenol, and the valley-to-valley maximum baseline subtraction prevents overestimation of phenol concentration. Only 1,2-dihydroxybenzene (maximum at 277 nm) might interfere, since it shows a shoulder on the phenol maximum. However, valley-to-valley correction minimized this interference.

It was intended to determine whether the PAC-derived synergy for phenol PCO observed in other studies Citation7 Citation8 could be reproduced in a reactor system with lower UV intensity, different UV emission spectrum and different reactor geometry at higher temperatures. The experiments of Matos et al. Citation7 Citation8 were therefore repeated with a modified experimental setup, as described in the methodology section, but with the same type of photocatalyst and PAC and similar concentrations of phenol, photocatalyst and PAC. All the results given in Figure 2 are displayed at a starting time of −1 h. The period between −1 h and 0 represents the stirring phase in the dark prior to each UV irradiation experiment. UV irradiation was started at time 0. Analogously, the first hour of the adsorption experiment in the dark is displayed in the time interval between −1 h and 0. The graphs shown in the upper part of Figure 2 represent (a) the kinetics for phenol removal, and (b) for DOC removal, during photolysis (UV irradiation without photocatalyst and PAC), PCO without PAC, and phenol adsorption by Merck PAC and PCO in the presence of the Merck PAC. Figure 2(c) shows phenol concentrations during phenol adsorption by Hydraffin WG and during phenol PCO in the presence of this PAC. Figure 2(d) presents the DOC concentrations obtained in the same experiments.

In contrast to the results of Matos et al. Citation7 Citation8, photolysis of phenol was more pronounced in the present study, probably explained by the different types of UV lamp employed. Matos et al. Citation7 Citation8 used a high-pressure mercury lamp giving an emission spectrum with predominant intensities at 365 and 366 nm, whereas the face tanner used in the present study was characterized by a broad emission range between 320 and 420 nm with a maximum at 352 nm, which might cover a part of the absorption maxima of phenol and its photolysis intermediates. The broad UV emission maximum of the face tanner reproduces more realistically the sky and solar radiation in the UV range than the UV line spectra of a high-pressure mercury lamp.

The equilibrium of phenol adsorption to Merck PAC in the absence of TiO₂ and UV irradiation was not completely reached within the first hour of stirring at 27°C (Figure 2(a) and (b)). This is in contrast to phenol adsorption on the same activated carbon, but at 20°C Citation7, when at 20°C equilibrium was reached after 15 min. The anomaly is explained by slower mass transfer at a higher temperature due to a smaller driving force, which was evidently not compensated by an increase in the mass transfer coefficient with increasing temperature. However, the major part of the phenol was adsorbed within the first hour of stirring, as the phenol concentration decreased from 94 to 42.3 mg L⁻¹, equivalent to a reduction in DOC concentration from 72 to 34.2 mg L⁻¹ (Figure 2(b)). During the following 6 h the phenol concentration was reduced by 4.6 mg L⁻¹ and in the final 41 h by a further 2.5 mg L⁻¹. Although some phenol removal was detectable in the phase after the first hour of stirring, this does not require correction of the data collected for phenol PCO in the presence of Merck PAC, since the decrease of phenol concentration in the second phase was small compared to the phenol removal by combining PCO with Merck PAC (open and closed diamonds in Figure 2(a)). While after 47 h stirring in the dark 17% of the phenol concentration present at t=0 (at the end of the first hour equilibration period) was removed from the liquid phase by further adsorption by Merck PAC, almost 70% of the phenol was removed by the PCO/Merck PAC hybrid process over the same time. In contrast to Merck PAC, the Hydraffin WG PAC reached the adsorption equilibrium at 27°C after only 1 min (Figure 2(c) and (d)). However, the equilibrium phenol concentration in the aqueous phase was much higher (74 mg L⁻¹) than with Merck PAC. The adsorbability of phenol on both PAC types showed the opposite trend to the BET surface and the iodine number (Table 1). On the other hand, this result is in accordance with recent findings indicating that phenol is adsorbed to a greater extent by activated carbons with a lower ash content Citation30; the Merck PAC contained less than 1% ash, while the ash content of the Hydraffin WG PAC was about 5% (Table 1).

Figure 3 illustrates the rate constants (based either on phenol concentrations or on DOC) for all the UV irradiation experiments and the adsorption in the dark for phenol solutions in deionized water. By analogy with the PCO experiments, in which the first hour without UV irradiation was used to establish the adsorption equilibrium as completely as possible, the liquid phase phenol concentrations which were recorded in the first hour during adsorption by the two activated carbons in the dark were not considered for calculating rate constants. Rate constants for phenol adsorption in the dark, ignoring the first hour of stirring (0.0035 h⁻¹ for Merck PAC and 0.0003 h⁻¹ for Hydraffin WG PAC), were small compared to the rate constants recorded in the corresponding PCO/PAC experiments. The addition of Merck PAC increased the PCO rate constant based on phenol degradation from 0.0125 to 0.0253 h⁻¹, greater than by merely adding the adsorption rate constant to the PCO rate constant, which would result in 0.0160 h⁻¹. Even when the standard deviations for PCO and PCO/Merck PAC rate constants were added to this sum, the result (0.0225 h⁻¹) was still 11% below the rate constant of the PCO/Merck PAC hybrid process. At least the synergetic trend observed in other investigations Citation7 Citation8 was reproduced in these experiments.

Matos et al. Citation7 found rate constants for phenol PCO (based on phenol concentrations) of 0.336 h⁻¹ (without PAC) and 0.834 h⁻¹ (with Merck PAC), while the rate constants achieved in the present study were only  h⁻¹ without PAC and  h⁻¹ in the presence of Merck PAC (Figure 3). The larger rate constants obtained in Citation7 are clearly attributable to a stronger UV lamp (125 W power uptake) located very close to a much smaller illuminated volume (20 mL) of the phenol/TiO₂ (PAC) suspensions in comparison to the present study (75 W power uptake, 20 cm distance between lamp and suspension surface, 1000 mL suspension volume). The SF derived from phenol-based apparent first-order rate constants in the present study was 2.02±0.75, while Matos et al. Citation7 found a slightly larger SF of 2.5 with the same concentrations of phenol, TiO₂ and PAC, but applying different irradiation conditions. However, the SFs obtained in both studies were not very different after allowing for the uncertainties inherent in the results presented here.

DOC removal rate constants in the PCO experiments presented were lower than the rate constants for phenol removal (Figure 3). However, the rate constant for DOC removal was also increased by adding Merck PAC to the PCO process. The sum of the DOC-based rate constants for PCO (0.0093 h⁻¹) and for adsorption on to Merck PAC (0.0037 h⁻¹), ignoring the first hour adsorption equilibration period, was definitely smaller than the rate constant of the PCO/Merck PAC hybrid process (0.0168 h⁻¹). On the other hand, when the standard deviations of PCO and PCO/Merck PAC rate constants are added, the result (0.0169 h⁻¹) is larger than the DOC-based rate constant for the PCO/Merck PAC hybrid process. This means that the synergy of Merck PAC addition to phenol photocatalysis is not completely clear once all the organics in the reaction mixture, including phenol and temporary oxidation intermediates, are considered.

The SF calculated from DOC-based rate constants was slightly lower than the SF based on phenol analysis , indicating that the synergy achieved by Merck PAC addition is more marked for PCO of the mother molecule than for PCO of oxidation intermediates. According to the discussion above, it cannot be ruled out that the enhancement of DOC-based PCO rate constant by the addition of Merck PAC depends only on additional (residual) adsorption during the 50 h irradiation interval, because phenol adsorption equilibrium was not completely established within this period.

As explained in the Introduction, synergy between activated carbon adsorption and photocatalysis depends on the activated carbon being in full contact with the photocatalyst. This includes the surface functional groups responsible for interaction with the photocatalyst as well as the extent of order of graphene layers enhancing activated carbon conductivity. Accordingly, the second PAC tested (Hydraffin WG) did not cause a pronounced synergy in phenol PCO (Figure 3). The SF for Hydraffin WG PAC addition to phenol PCO was 1.13 (calculated from rate constants based on phenol concentrations). The rate constant in the presence of Hydraffin WG PAC was smaller than the sum of PCO rate constant and its standard deviation. Poor phenol adsorption to Hydraffin WG PAC is supposed to contribute to lack of synergy for this PAC type, as the synergy implies adsorption of the organics on the activated carbon. On the other hand, addition of Hydraffin WG did not impair the PCO rate constants. Therefore, even with a SF of around 1, dosing of Hydraffin WG PAC offers a benefit, inasmuch as it contributes to phenol removal from the liquid phase by adsorption additional to PCO; after 50 h, PCO alone resulted in a residual phenol concentration in the liquid phase of 38 mg L⁻¹ (Figure 2(a)), while PCO in the presence of Hydraffin WG PAC led to a residual phenol concentration of only 31 mg L⁻¹ (Figure 2(c)).

Impact of the type of organic compound and inorganic matrix on synergy

Results of experiments performed with 50 mg L⁻¹ TetraEGDME (instead of 94 mg L⁻¹ phenol) solution in deionized water are shown in Figure 4. As in Figure 2, the period from −1 h to time 0 is the 1 h stirring phase without UV irradiation. TetraEGDME photolysis was negligible in comparison to photolysis of phenol. Figure 4 shows that 68% of TetraEGDME was adsorbed at 27°C, not very different from the 62% phenol adsorption to Merck PAC at this temperature (Figure 2(a) and (b)). However, the initial TetraEGDME concentration on a mass base was only about half as large as phenol concentration before PAC addition. On a molar base, TetraEGDME concentration was even 75% lower than the phenol concentration. In contrast to phenol, TetraEGDME adsorption reached equilibrium already 30 min after adding the Merck PAC (at −0.5 h in Figure 4). Dosing of 0.5 g L⁻¹ Merck PAC to PCO of the TetraEGDME solution had the advantage that DOC was rapidly diminished by more than 60% (as also shown for adsorption alone), but the PCO rate in the presence of PAC was much smaller than without PAC (Figure 4). Following a 50 h UV irradiation period in the presence of Merck PAC, the DOC was not markedly lower than in the absence of PAC. This contrasts with the results obtained with phenol solutions in deionized water (compare Figure 2(b)).

In Figure 5 the DOC-based apparent first-order rate constants for photolysis, adsorption on Merck PAC, PCO with and without Merck PAC of phenol and TetraEGDME solutions in deionized water are compared. Rate constants from PCO and PCO/Merck PAC experiments but with tap water as the matrix are also shown. TetraEGDME was removed by PCO in the absence of PAC with a more than threefold greater rate constant compared to phenol, irrespective of the matrix. In tap water, the DOC-based PCO rate constant was 29% lower for phenol and 26% lower for TetraEGDME than in deionized water. This can be explained by the presence of the radical scavenger in tap water (tap water TIC concentration: 21.6 mg L⁻¹, which is equivalent to 1.8 mmol L⁻¹ hydrogen carbonate ion).

PAC addition obviously prevented phenol PCO from inhibition by the radical scavenger hydrogen carbonate, as indicated by similar rate constants for the PCO/PAC hybrid process for phenol in deionized and tap water. A possible explanation may be that hydroxyl radicals do not play a predominant role in the hybrid process for organics when the SF is above 1. Hypothetically, aromatic molecules (such as phenol) adsorbed on the inner activated carbon surface by charge-transfer interactions, e.g. with quinone-like PAC surface functional groups or with graphene layers, might be oxidized directly by electron holes injected from an illuminated attached TiO₂ particle into the PAC grain and transferred via well-ordered graphene layers to the adsorption site. Thus, hydroxyl radicals may not be necessary for phenol oxidation in the PCO/PAC hybrid process when the PAC has been properly selected.

Baransi et al. Citation12 have discussed π–π interactions of phenolic compounds with graphene layers of activated carbon and that these might lead to an increased photocatalytic degradation in the photocatalyst/activated carbon/UV system, even when these organic compounds are not in direct contact with the photocatalyst but are adsorbed within the inner pores of the activated carbon particle to which a hole has been injected by an adjacent illuminated photocatalyst particle. This assumption is based on the observation that nearly 90% of polyphenols were removed from an anaerobically pretreated olive mill wastewater by the PCO/PAC hybrid process, while COD was only eliminated by less than 60%, indicating that the adsorbable polyphenols were removed by the hybrid process to a greater extent than the other organic constituents of this wastewater. With TetraEGDME, PAC addition did not protect from hydroxyl scavenging by hydrogen carbonate. This can be concluded from the much lower PCO/Merck PAC hybrid process rate constant in the tap water matrix (0.0091 h⁻¹) than in deionized water (0.0158 h⁻¹), as shown in Figure 5.

Of course, PCO rate constants in the tap water matrix might be affected not only by the radical scavenger hydrogen carbonate but also by pH. The pH of the TiO₂ suspensions in the tap water matrix was around 8, while pH of TiO₂ suspensions in deionized water was between 3.4 and 5.5 (Table 2). It is assumed that phenol as well as carboxylic acid oxidation intermediates of both model compounds (phenol and TetraEGDME) are adsorbed more efficiently to the photocatalyst at lower pH and are thus more efficiently oxidized; at pH > point of zero charge (PZC), the TiO₂ photocatalyst exhibits negative surface charges, repelling phenolate and carboxylic acid anions which are predominantly negatively charged when the pH exceeds the pK_a of the respective compounds. Obviously, this inhibition by pH was not particularly pronounced for phenol, because the pKa of phenol is 9.98 Citation31.

The most striking finding shown in Figure 5 is the antagonistic action of PAC addition on PCO of TetraEGDME. While DOC-based SFs achieved by Merck PAC addition to phenol PCO were 1.81±0.64 in deionized water, and even 2.95 in tap water, the respective factors for DOC removal from TetraEGDME solutions were 0.5 in deionized water and as low as 0.39 in tap water. This clearly shows an inhibition of photocatalytic TetraEGDME removal when the Merck PAC was added. Inhibition was obviously a consequence of additional photocatalyst shading by PAC particles in the suspension. The inhibitory effect of photocatalyst shading by PAC particles was probably more than compensated by synergy in case of phenol and Merck PAC. It can be assumed that there was even a small degree of compensation of the shading effect by synergy in case of phenol and the Hydraffin WG PAC, because according to Figure 3 the SF here was around 1 (no inhibition).

A general conclusion to be drawn from the experiments is that aliphatic organic compounds such as TetraEGDME are not susceptible to the synergetic effect delivered by Merck PAC addition to PCO. Accordingly, synergy was so far observed exclusively for aromatic compounds Citation7–15. Since charge carriers injected into activated carbon particles by illuminated TiO₂ particles can migrate through the activated carbon, oxidative reactions initialized by holes are assumed to take place also on the activated carbon surface in the PCO/activated carbon process via activated carbon functional groups/aromatic adsorbate π–π interactions, as explained above. This assumption has been corroborated by the recent finding that special types of activated carbon themselves, i.e. in the absence of known photocatalysts such as TiO₂, are able to catalyze phenol photodegradation Citation32 Citation33.

Experiments with greywater concentrate

Figure 6(a) shows np-DOC removal kinetics for the fivefold concentrate of biologically pretreated greywater. Although the np-DOC removal rate in the presence of Merck PAC was lower than without PAC, due to a lower dissolved organic concentration, the PCO rate constant was not markedly affected by PAC addition for greywater organics (Figure 6(b)); the SF was 1.07±0.26. This is different from the synergy obtained with phenol. On the other hand, PAC addition did not lead to impairment of greywater concentrate PCO efficiency, in contrast to findings with TetraEGDME. Also in the case of greywater, a SF of around 1 is beneficial for the hybrid process as activated carbon removes additional DOC by adsorption. Accordingly, a recent study Citation34 has indicated that addition of 1 g L⁻¹ Hydraffin WG PAC to PCO of biologically pretreated greywater leads to more efficient np-DOC elimination than PCO alone, even when the PAC/TiO₂ mixture has been reused 10 times. From these data it was estimated that the insolation area for solar PCO of biologically treated greywater can be reduced by a factor of 7 to achieve the same purification result when 1 g L⁻¹ Hydraffin WG PAC has been added. Efficient regeneration by UV irradiation has also been demonstrated for a TiO₂/activated carbon composite used for removal of coloured substances from secondary municipal effluent Citation35.

Moreover, it is noteworthy that the apparent rate constant for photocatalytic removal of np-DOC from greywater concentrate (0.0083 h⁻¹) was nearly as high as that for DOC removal from phenol solutions in deionized water in the absence of PAC (0.0093 h⁻¹; Figure 3) and markedly higher than for DOC removal from the phenol solution in tap water (0.0062 h⁻¹; Figure 5). This was not expected, since there was a high concentration of the radical scavenger hydrogen carbonate in the greywater concentrate; TIC was 432 mg L⁻¹ which is equivalent to 36 mmol L⁻¹ hydrogen carbonate. On the other hand, the TiO₂/DOC mass ratio in the PCO experiments with greywater concentrate was more than twice as high as during photocatalytic phenol degradation. It cannot be excluded that the catalyst/organics mass ratio also influences the SF.

A great many of the organic constituents of biologically treated greywater are represented by humic substances Citation36. In another effluent sample of the same constructed wetland for greywater treatment, the following groups of organics were quantified by LC–OCD analysis: polysaccharides (100 μ g DOC L⁻¹), humic substances (3530 μ g DOC L⁻¹), building blocks (880 μ g DOC L⁻¹), and low molecular weight organic acids (790 μ g DOC L⁻¹). Humic substances contain aromatic structures. Therefore, the absence of synergy for biologically treated greywater was not expected. On the other hand, no synergy in humic acid PCO (using TiO₂ prepared by the sol–gel method) was detected when adding a nut kernel-derived activated carbon (70–100 mesh) Citation37. It cannot be excluded that the reason for the absence of synergy in that study was an activated carbon type not exhibiting the properties required for synergy, as discussed above (particular oxygen-containing functional surface groups, undisturbed graphene layers, sufficiently large contact interface between activated carbon and photocatalyst). However, the rate constant for photocatalytic humic acid mineralization was increased when the same activated carbon was coated with TiO₂ by a sol–dipping–gel method Citation37. Therefore, TiO₂-coated activated carbon seems to have advantages over simple mixtures of activated carbon and photocatalyst. A disadvantage of composite catalysts is that they are not common industrial products and have to be specially synthesized, involving additional know-how and expense.

Another reason for the absence of synergy in the PCO/Merck PAC hybrid process treating the greywater concentrate might be the size of a particular fraction of the organic molecules contained in greywater. Figure 7 shows an adsorption isotherm for the Merck PAC recorded with another sample of biologically pretreated greywater, which was not however concentrated by evaporation. In order to ensure complete establishment of adsorption equilibrium, a very long agitation time of five days was selected. Once the PAC concentration exceeded 2 g L⁻¹, no further np-DOC reduction was observed with increasing PAC concentration.

This means that there is a residual organic concentration in the biologically treated greywater which is not adsorbable by the Merck PAC. The non-adsorbable concentration is about 3.3 mg np-DOC L⁻¹ or 54% of the original np-DOC of the non-concentrated biologically pretreated greywater. It can be seen that this percentage was lower for the greywater concentrate (41%) when comparing np-DOC at the start of the PCO experiment (i.e. subsequent to the 1 h stirring period in the dark) without PAC addition and at t=0 of the hybrid process PCO/Merck PAC (Figure 6(a)). It has to be considered that these np-DOC concentrations at irradiation time zero were also affected by adsorption on the photocatalyst.

The organic molecules representing the non-adsorbable residue (among them humic substances formed during biological greywater treatment) are simply too large to enter the pores inside the activated carbon particles. Of course, these non-adsorbed substances would be excluded from being oxidized by holes injected from illuminated TiO₂ particles into the activated carbon as discussed above. Therefore, very large humic matter molecules are not susceptible to the synergetic effect provoked by addition of the Merck PAC. The consequence of the lack of adsorbability of a considerable percentage of organics contained in biologically treated greywater is that separation of adsorption and subsequent off-line regeneration of the loaded adsorbent by (solar) photocatalytic oxidation in two stages as suggested in Citation38 is not feasible. Therefore, the advantage of separated adsorption and regeneration stages, which would allow for decoupling the mass flow of organic constituents from the volume flow of the water, cannot be utilized, at least in the case of biologically pretreated greywater. As Figure 6(a) indicates, simultaneous PCO and PAC adsorption (with at least partial in situ regeneration, (as demonstrated in Citation34 for a different PAC type) is advantageous because the hybrid process achieves lower np-DOC concentrations than each of the individual processes alone.