GLY removal at pH 3
To optimize the concentrations/loadings of nZVI and H2O2, a number of experiments were performed at different pH values, to investigate GLY degradation efficiency (Fig. 1). Preliminary experiments at pH 3 were carried out, to optimize the loading of nZVI (1µg/L-0.3g/L) while minimizing GLY removal by nZVI alone (which could involve adsorption at least in part), thereby highlighting Fenton degradation as a consequence (Fig. 1b). The minimum tested amount of nZVI (1µg/L) was effective at minimizing adsorption, but it did not induce effective GLY degradation in the presence of H2O2. Moreover, such low loading could not be obtained by direct weighting, but rather by dilution of a stock nZVI suspension. Presumably due to difficulty in making the stock suspension fully homogeneous, the system lacked reproducibility in results. Therefore, a compromise loading (0.01 g/L, i.e., the lowest loading that could be achieved by direct weighting) was chosen for nZVI.
Note that adsorption is not the only phenomenon that could account for the removal of GLY shown in Fig. 1b. Indeed, the nZVI-Fenton process could be activated by O2 alone, without H2O2, as follows (Keenan and Sedlak 2008):
Fe0 + O2 + 2 H+ → Fe2+ + H2O2 (4)
Although complete GLY removal could be achieved by using 0.3 g/L nZVI (Fig. 1b), without H2O2, such a high loading would be impractical in water-treatment applications, due to elevated costs of chemical reagents (0.3 g/L nZVI would cost around 0.24 $/m3; Furia et al. 2021).
As shown in Fig. 1a, complete GLY removal could be achieved at pH 3 by using 0.01 g/L ZVI and 300 µM H2O2. Lower H2O2 concentrations were less effective, most likely because of lower hydroxyl radical (HO • ) generation when H2O2 is low. Higher H2O2 concentrations were not tested, because their use would increase treatment costs (300 µM H2O2 would cost around 0.009 $/m3; Furia et al. 2021), and because effectiveness would decrease, at a certain point, due to HO • scavenging by H2O2 itself (Buxton et al. 1988):
H2O2 + HO• → HO2• + H2O (5)
Gly Removal At Ph 4
Different loadings of nZVI (0.02g/L-0.4g/L) were tested to achieve the lowest removal of GLY on nZVI alone at pH 4. Minimum adsorption was observed with 0.02g/L nZVI, which was then used to perform further degradation experiments at different concentrations of H2O2 (200–400 µM). However, as shown in Fig. 2a, the maximum GLY removal by nZVI + H2O2 was at most only 72%, when using 400 µM H2O2 at pH 4.
An alternative strategy used, as proposed by Minella et al. (2019), consists in stepwise addition of H2O2. This procedure allows for the use of higher overall amount of H2O2 (thereby increasing the total amount of generated HO•), at the same time minimizing HO• scavenging by H2O2 itself. The rationale is that H2O2 gets progressively degraded as the reaction goes on and, if H2O2 is added stepwise, it never reaches excessive concentration values at any time point (Minella et al. 2019). By adding 150 µM of H2O2 at the beginning, plus an additional 50 µM at 30 minutes after the start of the reaction, we obtained 87% GLY removal with 0.02 g/L nZVI at pH 4 (Fig. 3).
Gly Removal At Ph 5 And 6
Several combinations of ZVI loading and H2O2 concentration, including multiple additions of different amounts of the latter, were tried to remove GLY at pH 5 and 6 by nZVI-Fenton. As shown in Table 1, no GLY degradation at all was observed, under any conditions. The most likely explanation is the decrease in efficiency of the Fenton reaction with increasing pH (and of the nZVI-Fenton one as well: Minella et al. 2019; Furia et al. 2021).
Table 1
Summary of the degradation experiments carried out at pH 5 and 6, in the failed attempt to achieve GLY removal.
ZVI (g/L) | H2O2 (µM) | pH | Remarks |
0.01 | 150 + 50 | 5 | No removal at all |
0.02 | 200 + 50 | 5 |
0.03 | 300 + 50 | 5 |
0.01 | 150 + 50 | 6 |
0.02 | 200 + 50 | 6 |
0.03 | 300 + 50 | 6 |
Gly Removal In Tap Water
GLY removal was then studied in GLY-spiked tap water at pH 3 and 4 (pH was adjusted by H2SO4; the original tap-water pH was 7.6–7.7), starting with the loadings/concentrations of nZVI and H2O2 already optimized in ultra-pure water. At pH 3, with 0.01 g/L nZVI and 300 µM H2O2, the GLY removal efficiency achieved 80%. Unfortunately, it was not possible to improve these removal percentages, even by employing the stepwise addition method (Fig. 4), or different initial concentrations/loadings of the Fenton reagents (data not shown). The lower GLY removal in tap water, compared to ultra-pure water, might be tentatively ascribed to the presence of common inorganic anions in tap water (Cl−, SO42−, NO3−, NO2−, HCO3−) (see Table 2). Some anions, and especially chloride, bicarbonate, and nitrite (although the latter did not occur in the tap water we used) are able to scavenge HO• to produce different reactive transient species, which are, however, significantly less reactive than HO• itself (Buxton et al. 1988). Therefore, the nZVI-Fenton efficiency would be lowered, as a consequence of these processes (Ahmed et al. 2021).
Very interestingly, it was possible to achieve 100% GLY removal in tap water at pH 4, by using 0.02 g/L nZVI, 150 µM of H2O2 at the beginning of the reaction, plus an additional 50 µM at 30 minutes reaction time. In these conditions, the GLY removal capacity of the ZVI-Fenton system can be calculated as at least 100 mgGLY/gZVI. This value exceeds the capacity of adsorption systems based on metals having high complexation capabilities towards P-containing ligands (Hu et al. 2011) and, to an even higher extent, the adsorption capacity of activated carbon (Dissanayake Herath et al. 2019), which is a widespread adsorption medium used in the tertiary treatment of any potabilization facility. On this premise, a preliminary, partial estimate of nZVI-Fenton process costs, in the framework of expenditure in chemical reagents per m3 of treated water, could be carried out when considering that 0.02 g/L nZVI costs 0.016 $ m−3, while 200 µM H2O2 (i.e., 150 µM + 50 µM) costs 0.006 $/m3 (Furia et al. 2021). The cost of H2SO4 can be estimated by carrying out tap-water titration, and measuring the amount of H2SO4 that is required to adjust water pH to any given value (pH 4 in the present case, see Fig. 5). On this basis, one can conclude that the adjustment of pH to 4 would require 1.8 mol H2SO4 m−3, which translates into a cost of 0.041 $ m−3 for H2SO4 (Furia et al. 2021). Moreover, again based on water titration data, one would also need 1 mol CaO m−3 to finally adjust pH to at least 6 for eventual discharge or reuse, which entails an added cost of 0.0067 $ m−3 (Furia et al. 2021). Overall, elimination of GLY from water with the ZVI-Fenton technique at pH 4 would cost 0.07 $ m−3 in chemical reagents. By comparison, treatment at pH 3 would cost 0.008 $ m−3 in ZVI, 0.009 $ m−3 in H2O2, 0.052 $ m−3 in H2SO4, and 0.010 $ m−3 in CaO, for a total of 0.079 $ m−3 (i.e., a slightly higher cost to achieve lower GLY removal).
The pH and conductivity values of GLY-spiked tap water were monitored during nZVI-Fenton treatment, as shown in Table 3, which summarizes the results of different experiments. Interestingly, initial tap-water conductivity was around 0.5–0.55 mS/cm, and it increased to around (or slightly over) 1 mS/cm upon acidification to pH 3. A further increase by ∼0.5 mS/cm units is expected to take place in the final neutralization step, intended to bring pH to at least 6. Despite the conductivity increase, it would still be possible to reuse treated water in several fields, including some of the most demanding in terms of water conductivity (e.g., agriculture; Salgot et al. 2006). In the case of the nZVI-Fenton treatment at pH 4, the framework was even more favorable, as conductivity increased to only around 0.6 mS/cm (Table 3).
Overall, ZVI-Fenton treatment of GLY at pH 4 seems to outperform treatment at pH 3 in most respects: lower cost, better performance, and lower water conductivity as a result of treatment.
Table 2
Concentration values of different anions in tap water.
Parameter | Measured Value (mg/L) |
Chlorides (Cl−) | 26 |
Sulfates (SO42−) | 44 |
Bicarbonates (HCO3−) | 221 |
Nitrates (NO3−) | 16 |
Nitrites (NO2−) | < 0.05 |
Table 3
Measured tap water parameters during GLY removal at pH 3 and 4
| Initial | Start of reaction | During reaction (30 minutes) | End of reaction (60 minutes) |
Tap water ZVI (0.01g/L)-H2O2(300 µM) | |
pH | 7.63 | 3.03 | 3.05 | 3.08 |
Conductivity (mS/cm) | 0.565 | 1.132 | 1.084 | 0.949 |
Temperature(°C) | 19.6 | 20.5 | 21.9 | 22.8 |
Tap water ZVI (0.01 g/L)- H2O2(400 + 50 µM) | |
pH | 7.77 | 3.02 | 3.13 | 3.17 |
Conductivity (mS/cm) | 0.575 | 0.909 | 0.714 | 0.617 |
Temperature(°C) | 21.6 | 22 | 22.9 | 23.6 |
Tap water ZVI (0.01 g/L)- H2O2(350 + 50 µM) | |
pH | 7.64 | 3.01 | 3..08 | 3.12 |
Conductivity (mS/cm) | 0.530 | 1.04 | 0.906 | 0.904 |
Temperature(°C) | 21.4 | 21.7 | 23.6 | 24.2 |
Tap water ZVI (0.01 g/L)- H2O2(150 + 50 µM) | |
pH | 7.72 | 3.03 | 3.07 | 3.09 |
Conductivity (mS/cm) | 0.565 | 1.037 | 0.942 | 0.898 |
Temperature(°C) | 19.9 | 21 | 22.7 | 24 |
Tap water ZVI (0.01 g/L)- H2O2(200 + 50 µM) | |
pH | 7.71 | 3.03 | 3.17 | 3.18 |
Conductivity (mS/cm)) | 0.564 | 1.039 | 0.899 | 0.883 |
Temperature(°C) | 19.8 | 21.1 | 24.5 | 25.5 |
Tap water ZVI (0.02 g/L)- H2O2(150 + 50 µM) | |
pH | 7.57 | 4.02 | 4.14 | 4.35 |
Conductivity (mS/cm) | 0.572 | 0.652 | 0.624 | 0.615 |
Temperature(°C) | 22.7 | 22.9 | 23.5 | 24.8 |
Gly Removal By Nzvi-metabisulfite
Metabisulfite (S2O52−), which is used for instance in wine preservation, has several potential advantages over H2O2, being both considerably less costly (which allows for the use of solutions that are 10x more concentrated than those of H2O2, thereby offsetting lower Fenton reactivity of S2O52−; Farinelli et al. 2019), and less dangerous in everyday use. For instance, compared to H2O2, concentrated metabilsulfite solutions are less irritant to either skin or eyes, less harmful when swallowed, and not explosive (in some circumstances, H2O2 could explosively decompose into H2O + O2).
Unfortunately, the nZVI-metabilsulfite system did not work satisfactorily towards GLY removal. We investigated addition of nZVI at several loadings, together with direct or stepwise addition of S2O52−. Maximum GLY removal (46%) was observed with 0.01 g/L nZVI and 150 µM metabisulfite at pH 3, while the attempts to add S2O52− stepwise did not improve process performance (see detailed results in the supplementary material section).