3.1 The leached Cd from the soils with Cd immobilized by activated carbon
As the volume of eluents increased from 120 to 370 mL (no leachate with 0-120 mL eluents), the leached Cd in 5 mL leachate from Cd immobilized by activated carbon increased and then decreased in each treatment (Fig. 1a-d). From activated carbon added hydromorphic paddy (HP/AC) soil, the Cd leached by Salt 0.1-2 increased from 0 to 3.76 ug and decreased to 0 ug (not detected) in 5 mL leachate with 120–150 mL and 150–370 mL eluents, respectively. While by eluents Salt 5 and 10, the leached Cd was greater than that of other treatments, increased from 0.54 to 4.95 ug, and gradually decreased to 0 or 1 ug with 120–150 mL and 150–370 mL eluent, respectively (Fig. 1a). On the other hand, the Cd leached by eluents at various pH (3.0–6.0) also increased from 1.58 to 6.82 ug and decreased to 0 ug (not detected) in 5 mL leachate with 120–210 mL and 210–370 mL eluents, respectively. The differences between treatments at various pH were not significant and not enough to demonstrate that lower pH leached out more Cd (Fig. 1b).
From activated carbon added gray fluvo-aqvic (GF/AC) soil, the Cd leached by Salt 0.1–10 increased from 2.61 to 28.10 ug and decreased to 0-5.16 ug in 5 mL leachate with 120–230 mL and 230–370 mL eluents, respectively. The difference in the results of leaching by various concentrations of salt showed the Cd leached by Salt 0.1-1 was not detected, while that by Salt 2–10 was 1.37–5.09 ug with the 230–370 mL leaching (Fig. 1c). On the other hand, the Cd leached by eluents at various pH (3.0–6.0) increased from 8.55 to 34.31 ug and decreased to 0 (not detected) or 9.15 ug in 5 mL leachate with 120–240 mL and 210–370 mL eluents, respectively. It was worth noting that the 160–370 mL eluent at pH 3.0 leached Cd at 8.84–15.61 ug in each 5 mL leachate, significantly higher than other treatments (Fig. 1d).
Figure 1 showed that there were more Cd leached from GF than HP under the same elution conditions. From HP soil, it seemed that leaching with salt at different concentrations was to produce differences in the leached Cd than acid leaching at different concentrations, while from GF soil, various acid concentrations of eluents led to different amounts of leached Cd.
3.2 The leached Cd from the soil with Cd immobilized by calcium superphosphate
As the volume of eluents increased from 120 to 370 mL (no leachate with 0-120 mL eluents), the leached Cd in 5 mL leachate Cd from calcium superphosphate (CS) added soil increased and then decreased in each treatment, and could be continuously detected (Fig. 2a-d). From calcium superphosphate added hydromorphic paddy (HP/CS) soil, the Cd leached by Salt 0.1-5 increased from 7.36 to 18.92 ug and decreased to 6.15 ug in 5 mL leachate with 120–130 mL and 130–370 mL eluents, respectively. Occasionally, the amount of Cd leached by Salt-5 and Salt-10 were significantly higher than that of other treatments and reached 25.16 and 24.53 ug with 140 and 160 mL Salt-10 eluent, respectively (Fig. 2a). On the other hand, the Cd leached by eluents at various pH (3.0–6.0) also increased from 8.14 to 25.37 ug and decreased to 5.44 ug in 5 mL leachate with 120–145 mL and 145–370 mL eluents, respectively. The Cd leached by eluent pH 3.0 was higher than or equal to that by other treatments with a various volume of eluents (Fig. 2b).
From calcium superphosphate added gray fluvo-aqvic (GF/CS) soil, the Cd leached by Salt 0.1-5 decreased from 21.97–41.94 ug to 9.14–14.46 ug in 5 mL leachate with 120–370 mL eluents, respectively. By the 120–145 mL eluent of Salt-10, there was higher Cd leached of 80.96-104.37 ug than other treatments in the 5 mL leachate (Fig. 2c). On the other hand, the Cd leached by eluents at various pH (3.0–6.0) increased from 10.13 to 25.64–58.19 ug and decreased to 11.02–28.27 ug in 5 mL leachate by 120–130 mL and 130–370 mL eluents, respectively. The Cd leached by eluent pH 3.0 and 3.5 were higher than or equal to that by other treatments with 270–370 mL of eluents (Fig. 2d).
Figure 2 showed that there were more Cd leached from GF than HP soil under the same elution conditions with the addition of calcium superphosphate. From HP and GF soils, leaching with eluents Salt-10 and pH 3.0 was more likely to produce differences in the leached Cd than other treatments. While Cd was always detected in the 370 mL leaching, which was different from that from the soil with activated carbon (Fig. 1), indicating that the immobilization by activated carbon was stronger than that by calcium superphosphate.
3.3 Comparison of total leached Cd
The total amount of leached Cd from the HP and GF soils without the addition of immobilizer was 4088 and 9179 ug, respectively (raw data not shown). By the leaching of eluent Salt-0.1 to Salt 10, the Cd immobilized ratio was 66%-96% in GF soil (161–388 and 1140–1399 ug Cd leached, compared to the 4088 ug), which lower than 93%-100% in the HP soil (7–93 and 537–625 ug Cd leached, compared to the 9179 ug). As shown in Table 1, the initial pH of HP and GF soils were 5.8 and 7.6, respectively (Fig. 3). The greater change of pH might lead to more activation of Cd, which may be the cause of a greater amount of leached Cd from GF soil.
From Fig. 3a, in HP and GF soils, AC (7–93 and 161–388 ug Cd leached) exhibited 3–77 times passivation ability than CS toward the salt leaching (537–625 and 1140–1399 ug Cd leached). This may be due to the large adsorption capacity of AC for the Cd cation. By Salt 0.1–10, leached Cd were 542, 560, 537, 580, 551, 619, 625 and 1179, 1140, 1149, 1188, 1198, 1212, 1399 ug from the CS added HP and GF soils, 7, 23, 28, 23, 29, 70, 92 and 161, 199, 198, 203, 322, 325, 388 ug from the AC added HP and GF soil, respectively. With the increase of concentration of salt in the eluents, the leached Cd from soils with AC added increased rapidly while increased slowly with the increase of concentration of salt from soils with CS added. That indicated the application of CS for Cd immobilization could adapt a wider range of salt concentration changes.
From Fig. 3b, as immobilizers in HP and GF soils, AC (17–39 and 214–676 ug Cd leached) exhibited better passivation properties than CS (499–719 and 1012–1244 ug Cd leached). By eluent pH 3.0–6.0, leached Cd were 719, 600, 499, 548, 521,535, 550 and 1244, 1224, 1071, 1069, 1012, 1037, 1021 ug, from the CS added HP and GF soils, respectively. By contrast, Cd leached were 33, 34, 27, 30, 38, 39, 17 and 676, 281, 259, 277, 275, 268, 214 ug, from the AC added HP and GF soils, respectively.
In all series of leaching experiments, the total amount of Cd in leachates followed the sequence: GF/CS(1012–1399 ug) > HP/CS(499–719 ug) > GF/AC(161–676 ug) > HP/AC(8–70 ug). This indicated that AC has a stronger ability to passivate Cd than CS, and HP soil could fix heavy metals greater than GF soil. The effect of pH of the eluents was not obvious in the leachate from the HP-AC, however, from the GF-AC, HP-CS and GF-CS, the eluents at pH 3.0 leached more Cd than other treatments. The effect of salt concentration of the eluents was obvious in the leachate from the HP-AC, GF-AC, HP-CH and GF-CH. The eluents Salt 10 leached more Cd than other treatments.
Overall, the differences in leaching caused by different acidities were greater than those caused by different salt concentrations. Under the conditions of continuous acid and salt leaching, the passivation effect of HP on Cd was better than that of GF, and the passivation ability of AC on Cd was stronger than that of CS.
3.4 The fraction of Cd in the soils after the leaching
As shown in the Fig. 4, for the HP soil leached by eluent pH 5.0 with standard salt concentration, compared with the control soil, AC addition resulted in a 60% and 19% reduction in the two states of acid-extracted and oxidizable, while the two forms of reduced and residue increased by 30% and 24%, respectively. CS addition resulted in a 47% and 27% decrease on both acid-extracted and reduced states and a 26% and 47% increase on oxidizable and residue states, respectively.
On the other hand, for the GF, the addition of AC resulted in a 27% decrease on the form of acid-extracted, while reduced, oxidizable and residue states increased by 62%, 12% and 39%, respectively. The addition of CS caused acid-extracted and reduced states decreased by 64% and 9%, while the states of oxidizable and residue increased by 100% and 121%, respectively.
AC addition tended to convert acid-extracted and oxidizable states to reduced and residue states (slightly increased oxidizable states in GF soil), while CS addition converted acid-extracted and reduced states to oxidizable and residue states. Whether the immobilizer was AC or CS, more residue states were formed in GF, indicated that the two passivators will have better application in GF soil rather than in HP for the long-term effect.