| It is a great challenge to remediate Arsenic-contaminated cropland in some parts of China. Chemical soil amendments in situ have been considered as a low cost and efficient method for remediating As-contaminated soils. In this study, seven materials were amended into a red soil at different rates. The soils were then incubated in both pot and bag together with Brassia campestris and water spinach. The biomass and As content were determined in the plants in order to understand the impacts of these chemicals plant growth and As accumulation in plant tissue. The ultra-structural analysis was used to reveal the inhibition of As-uptake by plant. The methods of XRD, FT-IR, SEM-EDS were used to explore the binding mechanism of As with the added chemicals. The obtained main results are as follows:1. Adding either sepiolite, red mud, iron grit, phosphogypsum, ferrihydrite or iron phosphate induced in an increase of B. campestris biomass by more than14.2%, with an exception of LDO amendment in which no significant effect was found. In contrast with the increase of plant biomass, the As concentration in both plant tissues were reduced by8.9%-52.7%. The reduction order was ferrihydrite> phosphogypsum> LDO> iron phosphate> sepiolite> red mud> and iron grit. It was evident that ferrihydrite had highest inhibition of As absorption by the plant.2. The biomass of water spinach increased with increasing rates of either ferrihydrite or phosphogypsum, but LDO had not significant impact on water spinach biomass. At1%addition rate of the chemicals, phosphogypsum had highest biomass of water spinach,125.3%higher than that of control. The As concentration of water spinach was reduced with increasing the chemical addition rates. The addition of LDO, ferrihydrite and phosphogypsum at1%rate resulted in reduction of As concentrations in the edible parts of water spinach by77.4%,78.1%and23.4%respectively, compared with the control. Furthermore, all the three amendments had aftereffect in different level. The after effect was enhanced with increasing addition rates. The suitable addition proportion for LDO was0.25%to0.5%, but1%for ferrihydrite and phosphogypsum.3. The root-bag experiment data showed that LDO had a negative effect on plant growth of both B. campestris and water spinach. In contrary, both ferrihydrite and phosphogypsum promoted the plant growth, with better effect of phosphogypsum. All of the three amendments reduced As content, depending on crop. LDO had the highest inhibition As absorption by water spinach, but phosphogypsum acted on B. campestris. The changes of As chemical forms in the soil showed that LDO, ferrihydrite and phosphogypsum reduced the As bioavailability and thus reduced plant uptake.4. The root ultrastructure results indicated the meristematic tissue cells of B, campestris root tips in unamended soil were severely damaged, such as severe deformation of the vacuoles, plasmolysis, deep color of plasm and no other organelles besides nucleus and vacuole. Similar damage was observed in the root tip of water spinach. Arsenic caused less cell disorder, small vacuole, dim mitochondria structure, starch and oil particles in plastid. However, adding the chemicals, significantly alleviated the poisoning symptoms of the plants, much less plasmolysis, normal size of vacuole, full cell wall, clear organelles, and more mitochondria in B. campestris root, and thick cell wall, large vacuole, more mitochondria with clear structure in water spinach root tips. It was evident that As harmed cell wall structure and then organelles. Adding these chemical alleviated As toxicity through reducing As bioavailability and uptake by plant.5. The characterization data implied that LDO adsorbed and locked As to the surface and inside inter-layers of the double oxides through both anion and ligand exchanges. Whileas, ferrihydrite formed bidentate corner-sharing complexes by binding As with strong covalent bond. Phosphgypsum formd surface precipitation with As and thus reduced As bioavailability. |
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