Aquatic plants are widely applied in regional bio-ecological restoration of water, however, there are some gaps in the understanding of phosphorus (P) removal mechanisms by aquatic plants at high temperatures in summer. Eight aquatic plants, including two floating species ( L. peploides and H. dubia ) and six emergent ones ( L. salicaria , S. sagittifolia , C. indica , S. stoloniferum , R. rotundifolia , and L. ovalis ), were treated for 5 weeks with five P solutions (3.0, 3.5, 4.0, 4.5, and 5.5 mg L −1 ) in a greenhouse in summer at air temperatures ranged of 25~35 ℃. Results showed that higher removal efficiencies (exceeded 95%) of water P were found by two floating plants of H. dubia and L. peploides and another two emergent ones of L. salicaria and S. sagittifolia , and the corresponding residual P concentrations of water were almost lower than the limit value 0.2 mg L −1 of Grade III in the environmental quality standards of surface water (GB3838-2002). There could be different removal paths for water P. H. dubia mostly enriched P by plant itself for P concentration of plant was increased significantly. As the culture time went by, water pH values had significant fluctuations in the fall and then the created H + was used to enrich P by H. dubia . L. peploides did not enrich P but it proliferated rapidly, further to removing P from water via fresh weight (FW) increment. In the growth process, the stem diameter and leaf length of L. salicaria were increased P concentrations in water or/and P contents but the height and root length L. peploides were restrained, respectively. Both L. salicaria and S. sagittifolia had the two paths of enrich-P and FW increment. Moreover, SOD and CAT activities were response to higher P concentrations of water or/and high temperatures, which against oxidative damage. These findings could offer both a theoretical foundation and pratical guiding for the choice of aquatic-plant species in water bio-ecological restoration projects in summer.
Swine wastewater (SW) treatment by Myriophyllum aquaticum is an important biotechnology for its resource utilization. However, some knowledge gaps remain in compound-pollutant removal in SW, especially in practical applications. To clarify the responses of M. aquaticum to the compound pollutants as well as the related operational parameters in SW treatment, three initial doses (0.5, 1.0, and 1.5 kg per pond in 150 L simulated SW) of M. aquaticum and a control (no plant; CK) were allocated to 12 ponds under a plastic roof in Nanjing city of Eastern China during 75 days in the summer of 2019. Results showed that M. aquaticum could be used as a pioneer plant to efficiently remove compounded pollutants of nitrogen (N), phosphorus (P), and especially for heavy metals in simulated SW. Compared with CK, M. aquaticum assisted in improving the total N, NH4+–N, NO3-–N, NO2-–N, and dissolved organic N by 30.1%, 100%, 100%, 97.6%, 20.2%, 39.8% whereas Cu, Zn, and Cd by 50.4%, 36.4% and 47.9% on average during the 75-day experiment in summer, respectively. Moreover, concentrations of Cu and Cd at day 75 were in the ranges of 1.92–2.82 and 0.64–1.47 g kg−1 DW, respectively, exceeding the corresponding limits of the heavy-metal hyperaccumulator. For the operational parameters, the optimized initial dose was 1.0 kg per pond with M. aquaticum harvested after 45 summer days, respectively. Given that M. aquaticum has been widely used as animal feed in recent years and limit values for Cu and Zn in animal feed are not set in China, the toxicities of Cu and Zn should be assessed and the guideline of their limit values needs to be established for safe feed production. Interestingly, NH4+–N could dominate the removal of heavy metals especially Cd in the simulated SW, however, related mechanisms are needed for further study.
The toxic accumulation of cadmium (Cd) in rice paddies is of concern. In this study, nine amendments were investigated for their effect on Cd-polluted paddy soil, including four organic (biochar (BI) and organic fertilizers with low (TL), medium (TM), and high (TH) C:N ratios), two inorganic (AXP and lime (LI)), and three organic–inorganic combined materials (BXP, AYF, and BYF). Soil underwent a 14-day unsubmerged period, followed by 14-day submerged incubation. The results showed that exchangeable Cd concentrations significantly decreased by 13.36% with AXP and 5.45% with BXP on day 7, by 6.67% with LI from day 14–28, and by 22.47% with BXP and 20.26% with BYF on day 28 (P < 0.05) Both TL and TM markedly increased the Fe-Mn oxide-bound and organic matter-bound Cd by 52.87% and 129%, respectively. Carbonate-bound Cd significantly decreased with all amendments on day 7 (except BXP, AYF, and LI) and with organic fertilizers after submersion (P < 0.05). Inorganic materials and BI significantly increased residual Cd on day 21. In general, amendments that worked better under submerged conditions may promote the translation of Cd. TM was more suitable under unsubmerged conditions for the safe Cd utilization, whereas LI could be used as a candidate amendment during the submerged period to decrease the ecological risk of soil Cd. The combination of organic and inorganic amendments, in a combination of powder and particle form, is beneficial for ensuring soil remediation, long-term cultivation of farmland fertility in paddy fields, and reducing ecological risks.
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