| The combined pollution of heavy metal arsenic and cadmium is a typical category of water body pollution in China.Phytoremediation technology is a fast-developing method for the remediation of polluted water bodies in recent years.It purifies polluted water bodies through the heavy metal enrichment capacity of plants.Meanwhile,it has come to the attention of people that the heavy metal enrichment process of plants is commonly characterized by antagonistic or additive heavy metal interactions.The impact of heavy metal interactions on the heavy metal enrichment capacity of plants and wastewater remediation techniques is of increasing concern.Pontederia crassipes is a widely applicable plant for the remediation of polluted water bodies,the thesis uses Pontederia crassipes as the research object.The study determines whether there is an interaction between arsenic and cadmium enrichment through several groups of comparative experiments,analyzes the effect of the interaction on the growth,dry weight,and mineral element content of Pontederia crassipes,and researches the guiding significance of the interaction between arsenic and cadmium in Pontederia crassipes for water remediation technologies.The hydroponic experiments on Pontederia crassipes were divided into three groups of culture solution: arsenic(arsenic concentration: 0.5 mg/L,1 mg/L,2 mg/L,4mg/L and 8 mg/L),cadmium(cadmium concentration: 1 mg/L,2 mg/L,4 mg/L,8 mg/L and 16 mg/L),and combined arsenic and cadmium(combined arsenic and cadmium concentration ratios: As 0.5 mg/L + Cd 1 mg/L,As 1 mg/L + Cd 2 mg/L,As 2 mg/L +Cd 4 mg/L,As 4 mg/L + Cd 8 mg/L,and As 8 mg/L + Cd 16 mg/L)as well as 6 plant blank controls and 15 plant-free controls.The hydroponic experiment lasted for 5 weeks,and a total of 81 plants of Pontederia crassipes were removed on 3 d,6 d,12 d,21 d,and 31 d and altogether 465 culture samples were collected on 1 d,2 d,3 d,4 d,5 d,6d,8 d,10 d,12 d,15 d,18 d,21 d,24 d,27 d,and 31 d.The arsenic and cadmium contents,dry weight,and mineral element contents of roots,stems,and leaves of the three culture solutions were measured.Then,the enrichment factors and transfer factors of arsenic and cadmium in Pontederia crassipes were calculated.Based on these,the effects,and mechanisms of the interaction between arsenic and cadmium on the enrichment characteristics and growth and development of Pontederia crassipes were studied.Major research findings are as follows:Arsenic and cadmium enrichment characteristics and growth and development of Pontederia crassipes are listed below:(1)As the concentration of arsenic and cadmium in the culture solution increased,the arsenic and cadmium contents of roots,stems,and leaves also gradually increased.Pontederia crassipes displayed stronger enrichment capacity for cadmium than for arsenic.When the concentration of both arsenic and cadmium in the culture solution was 8 mg/L,the maximum arsenic and cadmium contents of the plants were 492 mg/kg(dw)and 6,166 mg/kg(dw),respectively.With the extension of the experimental time,both arsenic and cadmium in the plants were gradually transferred from the roots to the stems and leaves.The maximum value of the arsenic transfer factor was 3.01 and the maximum value of cadmium transfer factor was only 1.16 when the two cultures of arsenic and cadmium were hydroponically cultivated separately,so Pontederia crassipes had greater transfer capacity for arsenic than cadmium.(2)Both arsenic and cadmium had a low concentration-promoting and high concentration-inhibiting effect on the dry weight growth of Pontederia crassipes.When the arsenic concentration was 0.5 mg/L,at the end of the experiment,the dry weight of the plant reached the maximum value,up by 52% compared with the plant blank control.At this moment,the plants could still absorb mineral elements from the culture solution,although the uptake of mineral elements was inhibited.As the concentration of arsenic and cadmium increased,the accumulation of the dry weight of plants was significantly inhibited and the content of mineral elements in plants decreased significantly.The total concentration of mineral elements in the culture solution increased,and the maximum value could be increased from the initial value of 45 mg/L to 80 mg/L.Effects of arsenic and cadmium interaction on arsenic and cadmium enrichment and growth and development of Pontederia crassipes.(1)The cadmium had a significant inhibitory effect on arsenic enrichment in Pontederia crassipes.Compared with the arsenic culture solution,the arsenic content of the combined arsenic and cadmium culture decreased by 13% on average in the hydroponic solution of Pontederia crassipes.(2)The arsenic had little effect on the enrichment of cadmium in Pontederia crassipes.The average cadmium content in the combined arsenic and cadmium culture hydroponic solution of Pontederia crassipes dropped by 1% compared with that in cadmium culture hydroponic solution.(3)The antagonistic effect between arsenic and cadmium reduced the toxicity of arsenic and cadmium to Pontederia crassipes to some extent.Also,the interaction between arsenic and cadmium showed an antagonistic effect on the mineral element content of the plants.Comprehensive research shows that Pontederia crassipes can enrich both arsenic and cadmium.The enrichment capacity of the plant for cadmium is much greater than that of arsenic,and the transfer ability for arsenic is greater than that of cadmium.The interaction of arsenic demonstrates antagonistic effects on arsenic and cadmium enrichment and growth and development of Pontederia crassipes.Such antagonistic effects have mitigated the toxic effects of high concentration arsenic on Pontederia crassipes.The experiments are of great significance for the practical application of Pontederia crassipes in the remediation of heavy metal compound pollution in water bodies. |
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