The Effect of Both Different Light Conditions and the pH Value on the Capacity of Salvinia minima Baker for Removing Cadmium, Lead and Chromium

Abstract

Salvinia minima is a tropical aquatic floating plant, which has become a weed in the tropical and subtropical regions due to its high productivity and tolerance to a wide range of temperatures. The objective of this work was to evaluate the effect of two different light conditions (extreme light limiting conditions and non-limiting light conditions) and the pH (at different values, depending on the metal) on the capacity of Salvinia minima BAKER for accumulating and removing Cd (II), Pb (II) and Cr (VI) from aqueous solutions. The experimental units were exposed either to light and temperature controlled conditions (60 μmol/m 2 s and 25 °C) or to the outdoor conditions of the City of Xalapa, Veracruz (subtropical region), during the period of April through June. It was found that under controlled conditions, the concentration of cadmium in the whole plant tissue was 10,930 mg/kg and the bioconcentration factor (BCF) was 2,718. In the case of outdoor conditions (1,044 μmol/m 2 s), the Cd concentration in the tissue reached 11,262 mg/kg and the BCF 2,694, for the same metal initial concentration (4 ± 0.18 mg/l). Lead bioaccumulation was also high for both light conditions. Under controlled conditions, the metal concentration in the plants supplied with 3.0 ± 0.27 mg/l was 9,780.0 mg/kg and the BCF was 3,304. With an average light intensity of 336 μmol/m 2 s, lead concentration reached 7,705 and the BCF was 2,822. Under controlled conditions, a bioaccumulation of Cr (VI) of 3,111 mg/kg at an initial concentration of 4.0 ± 0.03 mg/l and a corresponding BCF of 784 were attained. The pH had a significant influence on the capacity of Salvinia to remove Cd (II). At an initial concentration of 4 mg/l, this effect was highest at pH 7.0 (84.58 ± 1.43%) under controlled conditions. However, under outdoor conditions, the pH did not exert a significant influence on the removal capacity of Salvinia for Cd (II), being in the range of 81.00 to 82.59% for the range of pH from 5.0 to 8.0. Cadmium removal by Salvinia minima occurred very quickly during the first 5 hours of incubation, regardless of the initial Cd (II) concentration or the light conditions, indicating the possible occurrence of adsorption to the root's surface. Lead removal by S. minima was independent of the pH in the range of pH 3.0 to 5.0, but it was affected by the light intensity. A maximum removal of 97.44% was achieved at pH 4.0; under outdoor conditions at an initial concentration of 3 mg Pb (II)/1. The removal percentage of Cr (VI) at pH 3.0 to 5.0 was also very limited.