Optimization of Nd(III) removal from water by Ulva sp. and Gracilaria sp. through Response Surface Methodology

Abstract

This study focused on the impact of 3 key variables (macroalgae mass, water salinity, and Nd(III) initial concentration) on the performance of two living macroalgae (Ulva sp. and Gracilaria sp.) in the removal and bioaccumulation of Nd(III), finding the optimal operating conditions through the Response Surface Methodology (RSM). Experiments were carried out in batch conditions stablished using a Box-Behnken design of 3 factors-3 levels (macroalgae mass of 0.5, 3.0 and 5.5 g dm-3; water salinity of 10, 20 and 30; and Nd(III) initial concentration of 10, 255 and 500 µg dm-3). The linear, quadratic and combined effects of the variables were described by RSM through a second order polynomial function. Optimized conditions were calculated by nonlinear regression using the Nelder-Mead simplex algorithm. Results evidenced the great capacity of both species to remove Nd(III), with removals around 80 % for the conditions: initial concentration of Nd(III) of 255 µg dm-3, water salinity 10, and macroalgae mass of 5.5 g dm-3. 3-D Response Surfaces showed that after 72 h of experiment, the most impactful variable on the removal process was the macroalgae mass. Regarding bioaccumulation, for the conditions studied, the higher the Nd(III) initial concentration the greater was its accumulation. The RSM tool made it possible to obtain the optimal conditions for both species, for contact times of 72 and 120 h. The proposed methodology can be applied in the recovery of Nd (which is a critical element) from contaminated waters, with the aim of its reintroduction in industrial production processes (recycling).