3.2.1 Effect of initial pH of TC
Figure 3 exhibited that the sorption capacity of TC raised apparently as the initial solution pH increasing, and the maxim adsorption amount occurred at pH = 6. In aqueous solutions, TC will undergo protonation-deprotonation reactions and present different ionic species including cation (TC+), molecule (TC0) and anions (TC− and TC2−) as the dissociation constants (pKa) of TC are 3.3, 7.7, and 9.7 (shown in Fig. S1) . Apparently, when pH ≤ 3.0, the adsorption capacity of TC was very limited, due to both surface of prepared magnetic biochar and TC are positively charged. As pH increasing, the adsorption capacity sharply increased, and the adsorption performance of TC on magnetic biochar was maximized at pH = 6 (193.3 mg/g). Although the zeta potential of magnetic biochar was very small and TC mainly became neutral, the maximum adsorption capacity could be explained by the π-π electron donor-acceptor and hydrogen bonding interactions. whereas, when pH increased far from 6, both magnetic sludge biochar and TC (TC− and TC2−) were negatively charged, leading to a reduction of adsorption.
3.2.2 Effect of magnetic biochar dose
It was obvious that the adsorption capacity of TC increased with the rising of magnetic biochar dose (Fig. 4). The higher dose of magnetic biochar resulted in higher adsorption capacity of TC due to more available active sites. Specifically, the adsorption capacity of TC reached the maximum as the dosage of adsorbent was 0.6 g·L− 1, and decreased for further addition.
3.3 Effect of HA and FA on the adsorption behavior of TC
Figure 5 and Fig. 7 presented the effect of HA and FA in different concentrations on TC adsorption by magnetic sludge biochar. Overall, adding HA or FA to magnetic sludge biochar varied the adsorption behavior of TC significantly, but hardly affected its global regularity of pH dependency. HA and FA contain functional groups including carboxylic, phenolic and aromatic groups, which make them carry negative charge over the whole pH range . Therefore, HA and FA could be adsorbed on the surface of magnetic sludge biochar and change its surface properties or result in competition for surface sorption sites, so as to affect the adsorption behavior of TC. As shown in Fig. 6 and Fig. 8, dissolved HA and FA concentration at the end of experiment partly reduced, which indicated that dissolved HA and FA was partially adsorbed onto the magnetic sludge biochar over the pH range from 2.0 to 8.0. However, just simple electrostatic interaction mechanism could not completely account for the pH dependence of the sorption, because when pH > pHPZC (5.7), HA, FA and magnetic sludge biochar were all negatively charged, but as shown in Figs. 6 and 8 partial adsorption still occurred. Therefore, we suspected that the functional groups of HA and FA interacted with the components of magnetic sludge biochar which was responsible for the adsorption of HA and FA.
Low concentration of dissolved HA (8 ppm) promoted the sorption capacity of TC from 193.3 to 260.1 mg/g, then dramatically decreased to 95.4 mg/g with further increasing dissolved HA to 15 ppm, which could be caused by surface sorption sites competition of higher dissolved concentration HA. Compared with the HA samples, FA has less significantly influence in adsorption behavior of TC. Specifically, the presence of FA (5 ppm) enhanced the sorption capacity of TC from 193.3 to 220.6 mg/g, while higher concentration of dissolved FA inhibited. Moreover, HA and FA showed a major effect on the adsorption of TC at pH ≤ 6, and little effect at higher pHs. When pH ≤ 3, the adsorption capacity of TC increased dramatically in the presence of HA and FA, which could be attributed to adsorbed HA and FA on the surface of magnetic sludge biochar that could change its surface properties and make the repulsive force decreased. As the pH increased, the functional groups of absorbed HA and FA could enhance the π-π electron donor-acceptor and hydrogen bonding interactions. As a result, the sorption capacities of TC were enhanced. When pH above 6, the dominant form of tetracycline was anion, and HA, FA, and magnetic sludge biochar were more negatively charged, the amount of HA and FA adsorbed on magnetic sludge biochar decreased, which resulted in little effect of HA and FA on TC adsorption process.
To further investigate the competing role of HA and FA, the adsorption behaviors of HA and FA on magnetic sludge biochar were performed, and the results were shown in Fig.S2. Compared with the concentration of the residual HA and FA in the presence of TC (Fig. 6 and Fig. 8), the adsorption capacity of HA and FA on magnetic sludge biochar almost unchanged with relatively low concentrations (HA ≤ 8 ppm, FA ≤ 5 ppm) as well as pH < 3.0 or pH > 6.0. Nevertheless, the adsorption capacity of HA and FA decreased greatly at pH 3–6 with higher concentration. All these results (Fig. 6, 8, and S2) supported those higher concentrations of dissolved HA and FA at pH 3–6 mainly leading to the competition for surface sorption sites. The adsorbed HA and FA could on the one hand occupy active sites on the magnetic sludge biochar surface, on the other hand repel the approaching TC molecules sterically, thus exhibiting competing behavior against the sorption of TC.
HA and FA could efficaciously affect not only the adsorption capacity but also the adsorptive speed of TC. As shown in Fig. 9, when the initial HA concentration was below 8 ppm, the adsorption speed was improved and the adsorption equilibrium achieved at 12 h, while, with further increase of dissolved HA to 15 ppm, the adsorption speed was decreased and the adsorption equilibrium achieved at 18 h. Compared with the HA samples, FA affected adsorption speed of TC less significantly, but the influence trend was similar to HA.