Phytoremediation of arsenic-contaminated water by Lemna Valdiviana: An optimization study.

Phytoremediation is a technique in which plants are used to treat contaminated media. The objective of this study was to monitor the influence of the parameters pH, phosphate concentration, and nitrate concentration in the process of arsenic absorption by Lemna valdiviana Phil. The response surface methodology was used to analyze the data to subsidize actions that maximize the phytoremediation process. A central composite rotational design (CCRD) was used with 3 variables including 6 axial points and 6 repetitions at the central point, totaling 20 trials. The plants were exposed to a constant concentration of arsenic in the optimization test of 0.5 mg L-1 (NaAsO2) and varied levels of pH, P-PO4, and N-NO3 in a period of 7 d. At the end of the experiment, the mass of arsenic removed from water and arsenic accumulated in the plants, the arsenic species present, the relative growth rate of plants (RGR), the tolerance index (TI), and the bioaccumulation factor (BAF) were calculated. Lemna valdiviana absorbed a greater amount of As when cultivated under pH conditions between 6.3 and 7.0, readily available phosphorus (P-PO4) concentration of 0.0488 mmol L-1, and nitrogen in the form of 7.9 mmol L-1 nitrate. Under these conditions, the plants were able to accumulate 1190 mg kg-1 As (in dry weight) from the aqueous media and reduce 82% of its initial concentration. Therefore, Lemna valdiviana has been shown to be an arsenic bioaccumulating macrophyte with high phytoremediation potential for media contaminated with the metalloid.

Optimization of arsenic phytoremediation using Eichhornia crassipes.

This study aimed to evaluate the pH, phosphate, and nitrate in the process of arsenic absorption by Eichhornia crassipes (water hyacinth), using the surface response methodology, in order to optimize the process. The plants were exposed to a concentration of arsenic of 0.5 mg L-1 (NaAsO2) over a period of 10 days. The results indicated optimal levels for the absorption of arsenic by E. crassipes at pH equal to 7.5, absence of phosphate, and minimum nitrate level of 0.0887 mmol L-1. For the tested concentration, E. crassipes was able to accumulate 498.4 mg kg-1 of As (dry base) in its plant tissue and to reduce 83% of the initial concentration present in the aqueous medium where it was cultivated. The concentration of phosphorus in solution linearly increased the phosphorus content in the plants and negatively influenced the absorption of arsenic. The concentration of 0.5 mg L-1 of As did not significantly affect the relative growth rate (RGR) and the tolerance index (TI). 94% of As (III) initially solubilized in water was converted by the end of the experiment period into As (V). The water hyacinth was important in the phytoremediation of arsenic when cultivated under optimal conditions for its removal.

Kinetics of arsenic absorption by the species Eichhornia crassipes and Lemna valdiviana under optimized conditions.

This work aimed to study the kinetics of arsenic absorption by Eichhornia crassipes and Lemna valdiviana under pre-established conditions of pH phosphate and nitrate in the nutrient solution. Additional aims were to evaluate the conversion kinetics between As(III) and As(V), and the effect of arsenic concentrations on development of the species. The plants were cultivated in nutrient solutions containing different arsenic concentrations: 0, 0.56, 0.89 and 1.38 mg L-1 for the water-hyacinth, and 0, 0.13, 0.48, 0.99 and 1.4 mg L-1 for Lemna. Monitoring of arsenic removal by the plants was performed by sampling at intervals of 0, 4, 8, 16, 24, 48, 96, 144, 192 and 240 h for the water hyacinth, and 0, 4, 8, 16, 24, 48, 96, 144 and 168 h for Lemna. The samples were submitted to analysis of total arsenic, As(III), As(V) and phosphorus. The first-order kinetics was fit to the arsenic removal kinetics by the plants, and it was observed that the decay coefficient (k) decreased with the increase of its initial concentration in the nutrient solution. For the, absorption was observed after 96 h of culture, the time coinciding with the greatest As(V) concentrations. For Lemna, the metal was only absorbed by the plant after decay of the phosphate levels of the medium, which occurred at 48 h. Concentrations above 1 mg L-1 implied deleterious effects in both plant species and in the phytoremediation process, and the bioaccumulation factor decreased for concentration above this for both E. crassipes and L. valdiviana.

Immobilization of Eu and Ho from synthetic acid mine drainage by precipitation with Fe and Al (hydr)oxides.

Use of lime to mitigate acid mine drainage is, in general, accompanied by precipitation of iron (Fe) and aluminium (Al) (hydr)oxides which may increase the removal of trace elements from water. This work aimed to evaluate the precipitation of Fe/Al (hydr)oxides to remove rare earth elements (REE) from contaminated water and the stability of precipitates. Two sets of 60-day syntheses were carried out using different Fe/Al/REE molar ratios, for europium (Eu) and holmium (Ho). The pH was periodically adjusted to 9.0, and the stability of the resulting precipitates was evaluated by water-soluble and BCR extractable phases, namely (1) acid soluble, extracted by 0.11 mol L-1 acetic acid; (2) reducible, extracted with 0.5 mol L-1 hydroxylamine hydrochloride; and (3) oxidisable, extracted with 8.8 mol L-1 hydrogen peroxide efficiencies of the water treatments for both Eu and Ho that were higher than 99.9% irrespective to the Fe/Al/REE molar ratios. Water-soluble phases of Eu and Ho were lower than 0.01% of the total contents in the precipitates. Recoveries from precipitates by Bureau Communautaire de Référence (BCR) sequential extractions increased with increasing concentrations of Eu and Ho. Acetic acid extracted higher amounts of REE, but Eu recovery was superior to Ho. Lepidocrocite was formed as Eu concentration increased which decreased its stability in the precipitates.