3. 1 N and P of pore water in sediment
It can be seen from Fig. 3 that before 130 d, the contents of TP and SRP of pore water in the sediment in CS and TC show an increasing trend, and maintain a high level of P content. It indicates that with the increase of temperature, and the P in the sediment releases to the surrounding water. The amount of SRP released reaches the maximum at 40 d. At this time, the content of SRP in the CS and TC is 122.97% and 74.32% bigger than that in TC + AMZON, respectively. The amount of TP released reaches the maximum at 70 d, and the content of TP in the CS and TC is 260.14% and 218.23% bigger than that of TC + AMZON, respectively. The contents of TP and SRP in TC + AMZON increased slightly before 70 days, but both are less than those in the CS and TC. The main reason is that during the anaerobic period in summer, Fe-P in the sediment is reduced to form a large amount of SRP, resulting in the strong release of P in the sediment21. However, the addition of the Al-modified zeolite with oxygen nanobubbles makes the sediment locally aerobic, and oxidizes Fe (Fe2+) in the sediment to iron oxide (Fe3+). Then it forms an iron oxide passivation layer, and Fe2+ is converted into Fe3+. SRP/Fe-P is strictly fixed in the sediment, and effectively reduces the release of P. However, CS and TC is under the anaerobic conditions without the addition of Al-modified zeolite with oxygen nanobubbles, and a very high concentration gradient of Fe-P is on the sediment surface, so it leads to a strong release of P in the sediment. When the Al-modified zeolite with oxygen nanobubbles in TC + AMZON is added to the surface of the sediment, the nanobubbles continuously releases oxygen and forms an oxide layer on the surface of the sediment, which not only reduces the concentration of P and Fe but also greatly reduces the concentration gradient of P on the sediment surface. Thereby the release of P in the sediment is inhibited. Fortunately, the oxygen nanobubble on Al-modified zeolite plays an important role. In addition, zeolite has a large specific surface area and strong adsorption22,23. The natural zeolite is rich in Al3+ after modification with aluminum salt, and the Al3+ is hydrolyzed to form Al(OH)3 colloid, which plays an important role for the adsorption of PO43+ in the water24,25. It can reduce the release of P in sediment and improve the water quality of the upper water. Therefore, it achieves the dual purpose of controlling P pollution and preventing eutrophication of upper water. At the same time, the available P in the pore water is absorbed by the aquatic plants, so that the P in the sediment is effectively removed. The river sediment in-situ passivation with phytoremediation is a P pollutant treatment technology with high efficiency, low cost, environmental protection and aesthetics26.
The TN and NH4+-N contents of the pore water in CS and TC sediments also show a trend of first increasing and then decreasing. The influence of TC + AMZON on TN and NH4+-N contents is great, and they are significantly lower than those in CS and TC. It shows that the N in the sediment is also released to the surrounding water, and Al-modified zeolite with oxygen nanobubbles inhibits the release of N in the sediment. Studies have shown that nanobubbles have negative charges at the gas-liquid interface, which can interact with positively charged pollutants in water, and the free radicals and vibration waves generated when they burst can promote the removal of pollutants27. Therefore, the pollutants discharged from urban non-point source pollution to rivers and lakes can be repaired by in-situ Al-modified zeolite + phytoremediation technology in the sediment, which can achieve the effect of pollutant control and beautify the city. It is the most effective technical means at present.
3.2 DO and ORP of pore water in sediment
It can be seen from Fig. 4 that the addition of Al-modified zeolite with oxygen nanobubbles can significantly increase the dissolved oxygen(DO) content of the pore water in the sediment. During the experimental period, the DO in the CS is kept between 2.9 and 4.3 mg/L, and it is varied from 5.3 to 7.5 mg/L in TC. However, the DO in the TC + AMZON is kept at 6.2–10.6 mg/L. Before 60 days, the DO content in TC + AMZON is significantly bigger than that in the CS and TC. On the 0th day, the DO content in TC + AMZON reaches 10.6 mg/L, which is 178.95% and 112.0% bigger than that in the CS and TC. With the experimental continual, the DO content of the pore water in the TC + AMZON is gradually decreased, which is mainly caused by the continuous oxygen consumption of reducing substances and microbial activities in the sediment. 60 days later, the DO concentration in TC + AMZON decreases to below 5.5 mg/L, and it is consistent with that of the TC, and gradually becomes stable. The results show that the Al-modified zeolite with oxygen nanobubble has a good ability to increase the oxygen at the sediment-water interface. Therefor the addition of Al-modified zeolite with oxygen nanobubbles is beneficial to increase the DO content in the pore water of the sediment.
The oxidation-reduction potential (ORP) on the sediment surface of the CS is maintained at -35-20 mV, and it is maintained at 5–35 mV in the TC, however the ORP in TC + AMZON is maintained at 12–95 mV. Therefore, the ORP of pore water in the TC + AMZON is significantly increased. Moreover, the change process of ORP in the sediment is consistent with that of DO, and it indicates that the distribution of DO content in the sediment affects the level of ORP. Which is consistent with the research results of Shi M. et al19. The transformation and diffusion of most dissolved substances in sediments are affected by their ORP.
3. 3 N, P and COD in upper water
It can be seen from Fig. 5 that the TP content in the upper water decreases with the time. The TP content in the three tanks is in the order of CS > TC > TC + AMZON, and TP content in the TC and TC + AMZON is all smaller than that in the CS. It shows that the release of P in the sediment will not increase the P content in the upper water. The main reason is that the Al-zeolite particles rapidly adsorb the dissolved active P in the sediment. Relevant studies have shown that aluminum ions in modified zeolites are hydrolyzed to form positively charged Al(OH)3 colloids, which can strongly adsorb negatively charged bacteria and ions in water28,29, such as PO34−. At the same time, Al(OH)3 colloids is the suspended matter with light weight and is not easy to precipitate30,31. But the Al(OH)3 can increase its mass after adsorbing ions in water, and gradually settle to the bottom of the water, reducing the possibility of resuspension. In addition, the hydrolyzed product Al(OH)3 provides bridging adsorption to adsorb suspended solids in water32,33. Al3+ is hydrolyzed to form a high molecular polymer with a linear structure, one side of the high molecular polymer can adsorb a particle far away, and the other side extends into the water to absorb another particle. The particle is bridged by the polymer adsorption that make the particles gradually become bigger and bigger34. With the increasing of the particle adsorbed by Al3+, it gradually moves to the bottom of the water, and it can absorb the suspended particles in the water during the sinking process35. After the colloid settles to the surface of the sediment, a covering layer is formed on the surface of the sediment to prevent the pollutants in the sediment from being released to the upper water.
The TN content in the upper water shows a decreasing trend with the test time, and the NH4+-N shows a trend of increasing first and then decreasing gradually. The TN and NH4+-N contents in the TC and TC + AMZON is all smaller than that in the CS. Before 60 days, the contents of TN and NH4+-N in the TC + AMZON are slightly lower than those in TC, and the differences gradually decreased 60 days later. The main reason is that zeolite has an adsorption effect on NH4+-N, which reduces the N content in the upper water. In addition, the absorption of available N by aquatic plants can also reduce the N content in water. Therefore, when using Al-modified zeolite to remediate pollutants in sediment, it is necessary to plant aquatic plants to absorb pollutants in sediment and water36.
Al-modified zeolite has little effect on COD in the upper water. In the early stage of the experiment, there is minor differences of COD in the upper water among the three tanks. In the later stage, the COD content of the upper water in the TC + AMZON and TC is slightly lower than those in CS. It is mainly because the adsorption of soluble organic pollutants by plants that reduce the COD content. Therefore, the application of Al-modified zeolite has no obvious improvement effect on COD in black and odorous water.
3. 4 Transparency and chlorophylla in upper water
It can be seen from Fig. 6 that the transparency of the upper water in the TC + AMZON and TC gradually increases with the experimental time, while it shows a trend of decreasing first, then remains stable in the later period in the CS. And the difference become bigger and bigger 60 days later. According to statistics, the transparency in the TC + AMZON remains at 17.6–30 cm 60 days later, and it varies from 15.5 to 22.4 cm in the TC. However, the transparency in the CS remains at 11.6–15.7 cm. Thus compared with the CS and TC, the transparency of the upper water in the TC + AMZON is increased by 130.76% and 58.73%, respectively at the sampling same. Therefore, the application of Al-modified zeolite with oxygen nanobubbles has a significant effect on water purification.
The content of chlorophylla in the upper water generally increases at first and then decreased, and keeps stable finally. Tt begins to increase 30 days later, and reach the maximum value 50 days later. The main reason is that the experiment starts at the end of May. 30 days later, the temperature is high. There are more prokaryotic blue-green algae (cyanobacteria) and eukaryotic algae in the test tanks except for the green plants. The algae can synthesize some organic substances through photosynthesis, which converts light energy into chemical energy. Thereby increasing the content of chlorophylla in water37,38. 80 days later, the content of chlorophylla of the upper water in the TC + AMZON is significantly less than those in TC and CS. And the content of chlorophylla in the TC + AMZON remaines between 15 and 21 µg/L, but it varies from 22 to 32 µg/L and and from 29 and 36 µg/L in the TC and CS, respectively. The content of chlorophylla in the TC + AMZON is decreased by 50.0% and 55.56% compared with TC and CS at the same sampling time. It shows that the content of chlorophylla in water can be reduced by the addition of Al-modified zeolite with oxygen nanobubbles. 105 days later, the chlorophylla content in the three tanks gradually becomes stable.