3.1 Migration of tetracycline and heavy metals
3.1.1 Migration of tetracycline on goethite-coated sand column
The penetration curve of TC on the GCS column is shown in Fig. 1a. Combined with Table S2, this shows that the penetration point PVb of TC was 15.3 at pH 3.0, and the adsorption equilibrium was reached rapidly after penetration, with a maximum adsorption capacity Q of 733.86 µg/g. At a pH of 5.5, the penetration point PVb of TC was 7.6, the effluent concentration after penetration first increased rapidly, and then slowed its increase until equilibrium was reached. Q was 452.48 µg/g at a pH of 8.0, PVb was 5.0, the trend of the penetration curve was similar to that at pH 5.5, and Q was 334.57 µg/g. The breakthrough curve trend of TC on GCS at pH 3.0 differed from that at pH 5.5 and pH 8.0, indicating that the adsorption mechanism of TC on GCS at pH 3.0 may be different from that at the other two pH levels. This is consistent with previous results of static adsorption experiments (Ma et al., 2019). The reason may be that complexation between TC and goethite was strong at pH 3.0. In addition, in the column experiment, the adsorption effect decreased with increasing pH, which differed from the static adsorption results. This difference was mainly due to the different experimental conditions between batch experiment and column experiment. Furthermore, the equilibrium state of TC in the GCS adsorption process under different pH levels was studied. The relative concentration of the effluent did not reach 100%, which may be caused by the uneven surface charge distribution of GCS and retention in the low flow velocity zone of the medium.
In addition, desorption of TC under various pH levels was also observed, with the relative concentration of the effluent first decreasing rapidly, which then slowed down. The ability to detect TC over a long period of time indicates that the desorption of TC on the GCS was affected by hysteresis. Moreover, the asymmetry of the penetration curve also indicates that the desorption of TC on the GCS was affected by hysteresis, which is consistent with the results of previous static adsorption desorption experiments (Jeong et al., 2012). Furthermore, the hysteresis of TC desorption was more apparent at pH 3.0. At a PV of 45, the relative concentration ratios of TC in the effluent of pH 5.5 and pH 8.0 were less than 10%, but the relative concentration ratio in the effluent of pH 3.0 remained at about 20%. This may either be because the adsorption force of GCS column to TC was stronger at pH 3.0, making it difficult to desorb, or it may be that the adsorption capacity of TC was stronger at pH 3.0, and the desorption rate was relatively low.
3.1.2 Migration of heavy metals on goethite-coated sand column
The penetration curve of Cu on the GCS column is shown in Fig. 1b. In combination with Table S3, it becomes clear that about 1.0 PV penetrated through the GCS column at pH 3.0. The effluent concentration quickly reached equilibrium after penetration, and the maximum adsorption capacity Q was 6.16 µg/g. At pH 5.5, penetration was about 4.1 PV, the breakthrough curve trend was the same as at pH 3.0, and the maximum adsorption capacity Q was 25.89 µg/g. The process of Cu penetration to 100% effluent concentration happened very fast, indicating that the adsorption of Cu on GCS is a very fast process. In addition, the penetration time and maximum adsorption capacity of Cu on GCS increased with increasing pH, indicating that the adsorption of Cu on GCS gradually increased with increasing pH. This is consistent with the results of static adsorption experiments (Ma et al., 2019).
In addition, the desorption of Cu from the GCS column can also be observed in Fig. 1b. At pH 3.0, the desorption of Cu was very rapid, and soon, the relative concentration of effluent decreased to zero. At pH 5.5, a certain concentration of Cu was still detected in the effluent after 18 PV, indicating a clear hysteresis effect of Cu desorption on the GCS column under this pH condition. The symmetry of the penetration curve at pH 3.0 and the asymmetry at pH 5.5 further indicate the existence of hysteresis in the desorption of Cu at a pH of 5.5. The possible reason is that at pH 5.5, the adsorption capacity of Cu on GCS column was larger than at pH 3, which made its desorption relatively slow.
The penetration curve of Cd on the GCS column is shown in Fig. 1c. In combination with Table S4, it becomes clear that about 1.0 PV penetrated through the GCS column at pH 3.0. The effluent concentration quickly reached equilibrium after penetration, and the maximum adsorption capacity Q was 11.47 µg/g. At pH 5.5, the penetration curve of 2.0 PV was about the same as that at pH 3.0, and the maximum adsorption capacity was 21.34 µg/g. At pH 8.0, 260 PV was required to achieve penetration. After penetration, the relative concentration of the effluent quickly reached 80%, and remained close to this value for a long time, before it reached equilibrium in a relatively short time, with a maximum adsorption capacity of 2798.82 µg/g. The penetration time and the maximum adsorption capacity at pH 3.0 and pH 5.5 were significantly lower than that at pH 8.0. This indicates that the adsorption of Cd on GCS is very weak under a low pH, while the adsorption effect is significantly enhanced under a high pH. This difference was mainly related to the hydrolysis of Cd under the condition of high pH. In addition, the breakthrough curve at pH 8.0 shows that there are two stages in the adsorption of Cd on GCS: in the first stage, GCS adsorbs the hydrolysates of positively charged Cd, while in the second stage, GCS adsorbs the hydrolysates of uncharged Cd. Adsorption at the first stage may provide an adsorption site for the second stage (Hanna et al., 2014).
In addition, the desorption behavior of Cd on GCS is also closely related to the pH value. At pH 3.0 and pH 5.5, the desorption rate of Cd on GCS was very fast, and the relative concentration of Cd in effluent rapidly decreased to zero. However, at pH 8.0, the desorption of Cd on GCS was very slow, and Cd could still be detected in the effluent of about 400 PV in the desorption experiment. This persistent Cd presence indicates that there was hysteresis in the desorption of Cd on GCS under this condition. The symmetry of the penetration curve at pH 3.0 and pH 5.5 and the asymmetry of the penetration curve at pH 8.0 also indicate that there was hysteresis in the desorption of Cd on GCS at pH 8.0. The above experimental results show that the adsorption of GCS to Cd was weak at pH 3.0 and pH 5.5 (which is why desorption occurred easily), while at pH 8.0, the adsorption of GCS to Cd was stronger (which is why desorption did not occur so easily). In addition, the desorption of Cd was carried out in stages at pH 8.0, which was consistent with the adsorption stage of Cd on GCS.
3.2 Effect of heavy metals on the migration of tetracycline on goethite-coated sand column
3.2.1 Effect of Cu on the adsorption of tetracycline on goethite-coated sand column
The penetration curves of TC on GCS in the presence of Cu are shown in Fig. 2a for each pH condition. When the GCS column adsorbs saturated Cu first, then TC is introduced into the system (Cu-TC), the penetration of TC is delayed, and the adsorption capacity is increased. This indicates that the existence of Cu can promote the adsorption of TC on the GCS column. The GCS column first adsorbs Cu, and when TC enters the GCS column, one part is adsorbed by GCS, while the other part is complexed with the Cu that is already adsorbed on GCS, and only then adsorbed by GCS. Finally, the ternary complex product of GCS-Cu-TC is formed, and in this process, Cu plays a bridging role. In addition, when TC is added to the GCS column, the Cu already adsorbed on GCS will be desorbed, and the desorbed Cu will complex with TC, forming a more adsorptive TC-Cu complex product and promoting TC adsorption. Therefore, there may be both competitive adsorption and complexation-promoting effects when TC and Cu coexist. When the complexation-promoting effect is weaker than the competitive effect, it appears as inhibited adsorption, but when the complexation-promoting effect is stronger than the competitive effect, it appears as promoted adsorption. In this experiment, the adsorption of Cu on the adsorbent is saturated first, which favors Cu to occupy the main adsorption site on GCS. Therefore, inhibition of TC adsorption may be stronger than under simultaneous Cu addition. In addition, when TC and Cu were added simultaneously (Cu + TC), TC adsorption was promoted to a different extent. At pH 3.0, the breakthrough point of the TC breakthrough curve was almost identical to that of TC alone, but the adsorption capacity increased and the Thomas rate constant decreased. At pH 5.5, simultaneous addition of TC and Cu increased the TC penetration from 7.6 PV to 63.3 PV, and the Thomas rate constant of the penetration curve of TC on GCS was two orders of magnitude smaller than that of TC only.
The above experimental results show that Cu and TC complexation is weak at pH 3.0 and therefore, the penetration of TC is not affected at first. With continuous accumulation of TC and Cu in the GCS column, the concentration of TC and Cu in the column increases gradually. This increase promotes the complexation of TC and Cu, and also promotes the adsorption of TC. At pH 5.5, the complexation of Cu and TC is enhanced, and the main positively charged complex product is CuHTC+. At this time, GCS is negatively charged, which greatly increases the adsorption of TC on the GCS column. At pH 5.5, compared with TC adsorption on the GCS column under the treatments of Cu-TC and Cu + TC, although TC penetration was delayed in Cu-TC, the delayed effect of Cu + TC on the penetration of TC was much stronger. The maximum adsorption capacity was also significantly increased. This indicates that when Cu and TC migrate on the GCS column, complexation of TC and Cu forms complex products. This is the main mechanism through which Cu promotes the adsorption of TC, rather than the bridging effect of Cu.
3.2.2 Effect of Cu on desorption of tetracycline on goethite-coated sand column
As shown in Fig. 2b, under certain conditions, the existence of the heavy metal Cu affects the desorption of TC on the GCS column. After adsorption of saturated TC by the GCS column, Cu was used for desorption. At pH 3.0, the desorption of TC by Cu showed little difference compared with the desorption with NaCl solution, and only showed a slight inhibitory effect. At pH 5.5, the introduction of Cu at the beginning of 6 PV did not show an apparent difference to the blank treatment. However, the relative concentration of TC in the effluent quickly dropped to zero, indicating that the existence of Cu can promote TC desorption. In addition, the desorption rate of TC was much slower under simultaneous adsorption of Cu and TC by the GCS column. Then, it desorbed with NaCl, indicating that the desorption of TC was significantly inhibited by the addition of Cu and TC. This inhibition increased with increasing pH.
These results show that the complexation between Cu and TC was weak at pH 3.0, indicating that Cu has little effect on the desorption of TC. At pH 5.5, with the desorption of TC, the concentration of TC decreased, and the concentration of Cu increased. This may be due to the competition for the desorption site of TC, thus promoting the desorption of TC. When Cu and TC are complexed first (before their addition), because the TC adsorbed on the GCS column mainly has the form of the complex of TC and Cu, it has a stronger affinity with GCS, and is therefore difficult to desorb. In addition, because complexation at pH 5.5 was stronger and more complexes are formed, desorption also takes longer. In summary, under certain experimental conditions, Cu can inhibit the migration of TC on GCS (including adsorption stage and desorption stage). With increasing pH, the complexation of Cu and TC as well as the inhibition are stronger.
3.2.3 Effect of Cd on the adsorption of tetracycline on goethite-coated sand column
The penetration curves of TC on GCS in the presence of Cd are shown in Fig. 3a. Cd saturates the GCS column before introducing TC into the system (Cd-TC). The penetration of TC on the GCS column is almost identical to that of the blank at pH 3.0 and pH 5.5, but at pH 8.0, the penetration of TC is significantly delayed. This indicates that at a high pH, adsorption of Cd on GCS column promotes adsorption of TC on GCS. This phenomenon may emerge because there was almost no complexation between TC and Cd at low pH levels, and the adsorption effect of Cd on GCS column was less effective. However, with increasing pH level, the adsorption of TC on the Cd-saturated GCS column increased to pH 8.0. The main reason is that GCS has very strong Cd adsorption at this pH, which results in a large Cd concentration increase in the GCS column and promotes the complexation of Cd and TC.
At all pH levels, when Cd and TC were added together, as TC penetrated the GCS column, no significant effect on the migration of TC was observed. This is due to the weak complexation of TC with Cd itself and the low concentrations of TC and Cd chosen for the experiment. At low pH, when the adsorption of Cd was weak, coexistence of Cd and TC had almost no effect on TC penetration. Although the adsorption of Cd by GCS was strong under a high pH, TC reached equilibrium within a relatively short time relative to Cd. Therefore, the concentration of Cd on GCS did not accumulate greatly, thus exerting little effect on the adsorption of TC. At pH 8.0, compared with the adsorption of TC on the GCS column, under the treatments of Cu-TC and Cu + TC, TC adsorption was greatly affected under Cu-TC, but not significantly affected under Cu + TC. The largest difference between both lies in the different concentrations of Cd in the GCS column. Therefore, when the pollution degree of Cd in the actual environment is very serious, the role of Cd should be considered when studying the migration of TC in the environment.
3.2.4 Effect of Cd on desorption of tetracycline on goethite-coated sand column
As shown in Fig. 3b, when desorbing TC in a GCS column with Cd, none of the pH conditions had a significant effect. When Cd and TC were added together and then desorbed with NaCl, there were no significant effects at pH 3.0 and pH 5.5. However, at pH 8.0, the desorption of TC in this case slowed and showed clear inhibition. This is because, at the concentration studied in this experiment, the weak complexation of TC with Cd did not contribute to the desorption of TC from the GCS column. At pH 8.0, Cd and TC were adsorbed and then desorbed on the GCS column. At the adsorption stage, the adsorption equilibrium of TC and Cd should be satisfied at the same time. The equilibrium of Cd on the GCS column was long. Therefore, under desorption, on the one hand, the concentration of Cd in GCS column was very high and NaCl desorbed TC, but also, when high Cd concentration was desorbed in the GCS column, the desorption of TC was delayed. On the other hand, in the process of co-adsorption of Cd and TC on the GCS column, complexation of Cd and TC was not strong at first. TC is adsorbed on the GCS column in the form of TC monomers. With continuous accumulation of Cd on the GCS column, adsorption of TC in the GCS column may change from the original monomer form to the complex form. The adsorption affinity of TC in the complex state was higher, which is why the desorption of TC was inhibited in the process of desorption. This was similar to the results of adsorption and desorption of TC on GCS column when Cu and TC coexist at pH 3.0. In summary, Cd can inhibit TC migration under high pH levels, which is mainly because GCS adsorbs Cd very strongly and promotes TC complexation with Cd.
A comparison of the effects of Cu and Cd on the migration of TC showed that under the same experimental conditions, the effect of Cu on the migration of TC on GCS was significantly stronger than that of Cd, which is similar to static desorption. This is because the complexation of TC with Cu was stronger than that between TC and Cd. In addition, the complexation product of TC and Cu was mainly the positively charged CuHTC+, while the complexation product of Cd and TC was CdTC0 without charge. At pH 5.5 and pH 8.0, the surface of GCS was negatively charged, which makes adsorption of the complex product of Cu and TC by GCS easier, and strengthens the effect of Cu on the migration of TC on the GCS column. Therefore, when evaluating the migration of antibiotics in the environment, the effects of various types of heavy metals on the migration of antibiotics should also be considered. In general, the stronger the complexation between antibiotics and heavy metals, the stronger the effect of heavy metals on the migration of antibiotics in the environment.
3.3 Effect of tetracycline on the migration of heavy metals on the goethite-coated sand column
3.3.1 Effect of tetracycline on the adsorption of Cu on goethite-coated sand column
As shown in Fig. 4a, when the GCS column adsorbs saturated TC first, penetration of Cu on the GCS column will be delayed, and the delay effect of the pH will be stronger. This shows that the existence of TC can promote Cu adsorption, and the promoting effect increases with increasing pH, because the complexation between TC and Cu was stronger at pH 5.5. When TC was added to the GCS column, it acted as a bridge, thus promoting Cu adsorption. However, when Cu was added to the system, part of TC was desorbed, the desorbed TC was complexed with Cu, and the products of the two complexes were adsorbed on GCS, thus facilitating the adsorption of Cu by GCS.
When TC and Cu were added simultaneously, the penetration time of Cu was the same as that in the blank treatment at pH 3.0. The concentration of effluent Cu after penetration of the water quickly increased to about 85%, but only after the next nearly 20 PV, the relative Cu concentration in the effluent increased to 100%. A pH of 5.5 delayed the penetration of Cu on the GCS and the Thomas rate constant decreased, suggesting that the adsorption on the GCS column was very slow. This is because the complexation between TC and Cu was weak at pH 3.0, but with continuous addition of TC and Cu, TC accumulated on the GCS column. Consequently, its concentration increased and the complexation between TC and Cu increased relatively. Therefore, more Cu was adsorbed on the GCS column through the complexation with TC until the equilibrium was reached. At pH 5.5, complexation of Cu and TC was very strong. When TC and Cu coexist, the main component in solution was CuHTC+. Because CuHTC+ has a stronger affinity for GCS than Cu2+, the adsorption rate was lower and the adsorption capacity was larger. Compared with the adsorption of Cu on GCS under the conditions of TC-Cu and TC + Cu at pH 5.5, the latter has a stronger influence on the migration of Cu on GCS. This indicates that the order of pollutants entering the environment greatly influences their migration in the environment.
3.3.2 Effect of tetracycline on desorption of Cu on goethite-coated sand column
The TC desorption of Cu on the GCS column is shown in Fig. 4b and has little effect on the desorption of Cu at pH 3.0. At pH 5.5, before 8 PV of TC, the relative concentration of Cu in the effluent was slightly higher than in the blank experiment. After 8 PV, the relative Cu concentration in the effluent was slightly lower than in the blank experiment. This shows that under the condition of pH, with the introduction of TC into the system, TC had a slightly inhibitory effect on the desorption of Cu at the beginning; with continuous desorption of Cu, TC accumulated, and the desorption of Cu by TC changed from inhibition to promotion. At pH 3.0, the adsorption of Cu on GCS was weak and the complexation of Cu with TC was also weak; therefore, TC had no effect on the desorption of Cu under this pH level. At pH 5.5, when TC was added to the system at the beginning, because of the low concentration of TC, there was no complexation with Cu. TC was adsorbed on GCS and occupied the re-adsorption site after Cu desorption, which made the relative concentration of the Cu effluent higher than in the blank experiment. With continuous accumulation of TC, TC complexed with Cu and promoted the desorption of Cu to be adsorbed on the GCS column again. Therefore, the relative concentration of Cu in the effluent decreased faster than in the blank.
In addition, migration of Cu on the GCS column was significantly inhibited when TC and Cu were added together. The desorption law of Cu was consistent with that of TC under this experimental condition. Under each pH level, Cu desorption was also inhibited compared with the blank. On the one hand, the GCS column adsorbed more Cu when TC and Cu coexisted; on the other hand, a stronger affinity complex product of TC-Cu was formed in the GCS column during the adsorption process, thus inhibiting the desorption of Cu. The inhibitory effect of Cu desorption at pH 5.5 was stronger than that at pH 3.0, because the adsorption of Cu at pH 5.5 was stronger than that at pH 3.0 and showed stronger complexation. In summary, the presence of TC inhibits Cu migration under certain experimental conditions. The stronger the complexation, the stronger the promoting effect on Cu migration.
3.3.3 Effect of tetracycline on the adsorption of Cd on goethite-coated sand column
As shown in Fig. 5a, at pH 3.0 and pH 5.5, TC was adsorbed and saturated first, which hardly affected the penetration of Cd, but the existence of TC advanced the penetration of Cd at pH 8.0. This indicates that TC has no effect on the adsorption of Cd on GCS under low pH conditions, but the presence of TC inhibits the adsorption of Cd on the GCS column at high pH levels. At low pH, GCS itself has a weaker adsorption capacity for Cd. This is why the GCS column first adsorbed saturated TC, which has less effect on Cd. However, with increasing pH, the adsorption of Cd by GCS gradually increased, and TC occupied the dominant site on GCS, which led to a decrease of the adsorption site of Cd and the inhibition of Cd adsorption.
In addition, when Cd and TC were added simultaneously, the penetration of Cd on the GCS column at pH 3.0 was not affected. At pH 5.5, there was no apparent change in the penetration point of Cd. After penetration, the relative concentration of the effluent quickly increased to 90%, and the first equilibrium emerged. After about 3 PV, the relative concentration of the effluent increased to 100%, indicating that TC promoted Cd adsorption on GCS at the near equilibrium stage. At pH 8.0, penetration of Cd on the GCS column happened significantly earlier than in the blank treatment, indicating that the adsorption of Cd was inhibited in this case. The results show that there was almost no complexation between TC and Cd at pH 3.0, which did not affect the penetration of Cd on GCS. At pH 5.5, the equilibrium time of Cd adsorption on GCS was shorter than that of TC. TC promoted Cd adsorption when it was close to equilibrium in the later stage because the concentration of TC in the GCS column was low at the beginning. With accumulating TC in the GCS column, complexation of TC and Cd was enhanced and the adsorption of Cd on GCS was promoted. At pH 8.0, the time required for TC to reach adsorption equilibrium on the GCS column was far shorter than the time required for Cd to reach adsorption equilibrium on GCS; therefore, TC first occupied the surface site of GCS, thus inhibiting Cd adsorption.
Compared with the adsorption of Cd in TC-Cd and TC + Cd treatments at pH 8.0, the penetration time of Cd on GCS under TC + Cd was much shorter, i.e., in the case of TC + Cd, the adsorption inhibition of Cd on GCS column was stronger. This is because the GCS column adsorbs saturated TC first, and then, following Cd input, part of the TC in the system will be desorbed and release adsorption sites to Cd. TC and Cd exist at the same time, because TC first balanced on the GCS column, and with continued input of the TC and Cd mixed solution, the concentration of TC did not decrease. Therefore, no sites were released, TC + Cd occupies more adsorption sites of Cd, and it has a stronger inhibitory effect on Cd.
3.3.4 Effect of tetracycline on desorption of Cd on goethite-coated sand column
The effect of TC on the desorption of Cd is shown in Fig. 5b. When desorbing Cd with TC at pH 3.0, TC has no effect on the desorption of Cd. However, with increasing pH, the desorption rate of TC to Cd was higher than that of the blank experiment, which shows that TC can promote the desorption of Cd under this experimental condition. The promoting effect of pH 8.0 was stronger than that of pH 5.5. Furthermore, at pH 8.0, compared with the blank experiment, the desorption of Cd decreased rapidly at first, then remained at a certain level, and finally began to decline relatively rapidly after a long plateau. In the desorption with TC, the decrease in the initial stage was more pronounced. There was almost no complexation between TC and Cd at pH 3.0, and the adsorption capacity of TC to Cd was also very poor; therefore, TC has almost no effect on the desorption of Cd under this pH condition. At pH 5.5, introduction of TC will desorb Cd, and occupy the re-adsorption sites after Cd desorption, thus resulting in accelerated Cd desorption. At pH 8.0, because the desorption of Cd takes longer, and the adsorption of TC on GCS has reached equilibrium, it occupies more re-adsorption sites after Cd desorption, and the promoting effect is more apparent compared to pH 5.5.
In addition, when Cd and TC were jointly added after the desorption of Cd, the migration of Cd had almost no effect at pH 3.0, but the migration of Cd was promoted at pH 5.5 and pH 8.0. The reason for this result is that the adsorption of Cd was weak at pH 3.0, and there was almost no complexation between TC and Cd. At pH 5.5 and pH 8.0, the desorption rate of TC was very low compared with that of Cd (Fig. 3b). The relative concentration of TC in 3 PV was almost at equilibrium, but at this time, the relative concentration of Cd was close to zero. Therefore, TC still firmly occupies the site on GCS, thus promoting Cd desorption.
Compared with the desorption of TC-Cd and TC + Cd at pH 5.5 and pH 8.0, although the desorption of Cd was promoted, TC + Cd promoted the desorption of Cd more. This is because TC already occupied the GCS site when TC + Cd began desorption, but TC-Cd began to occupy the site with the introduction of TC; therefore, TC + Cd occupied more sites than TC-Cd. TC + Cd promoted the desorption of Cd more. In summary, under certain experimental conditions, existence of TC will promote the migration of Cd. The main reason is that the complexation between TC and Cd is weak under the experimental concentration studied, while any TC in the solution will occupy the adsorption sites of Cd, resulting in promoted Cd migration. TC almost inhibited the migration of Cu on the GCS column, but almost promoted the migration of Cd on the GCS column. The main reason is that the complexation of Cu and TC was stronger than that of TC and Cd. In addition, it is also related to the migration characteristics of these pollutants themselves in GCS.