Role of willow plants (Salix babylonica) in phytoremediation of perchlorate: a study of soil and hydroponic bioreactors at different perchlorate concentrations

Abstract

The role of willow plants (Salix babylonica) on phytoremediation of perchlorate was studied using hydroponic and soil bioreactors under greenhouse conditions. Hydroponic bioreactor experiments were conducted at three perchlorate concentration ranges namely; low (50-200 mg L^-1), high (300-700 mg L^-1) and very high (2,500-10,000 mg L^-1), with the former two concentration ranges run in parallel with non-planted bioreactors used as controls. Soil bioreactor experiment using sandy loam soil was conducted at perchlorate concentration range of 1,000-10,000 mg L^-1. Results from hydroponic experiment at low concentration treatments showed complete perchlorate removal within 5-10 days for both planted and non-planted bioreactors. Regression analysis showed that when Dissolved Organic Carbon (DOC) is not limiting in the solution, rate of degradation of perchlorate increased with increasing perchlorate concentration. At high perchlorate concentration treatments, planted hydroponics bioreactors removed perchlorate completely within 6-11 days regardless of initial perchlorate concentration while in the non-planted bioreactors, 9.68%, 17.89% and 61.07% of the initial concentrations for 300, 500 and 700 mg L^-1, respectively were still in solution when the experiment was terminated in day 11. Planted hydroponic bioreactors at very high perchlorate concentration were not able to completely remove perchlorate in 15 experimental days despite the presence of DOC. All plants suffered phytotoxicity at high perchlorate concentration ranges from day 2 and totally dried up in less than 15 days. Solution samples taken 7 days after leaf fall indicated that approximately 40.02%, 66.24%, 55.99%, 90.61%, 92.18% of the initial perchlorate doses were still in solution, for the 2,500; 4,000; 6,000; 8,000 and 10,000 mg L^-1 perchlorate treatments, respectively. Results from soil bioreactors dosed with 1,000; 5000, 10,000 and 12,000 mg L^-1 perchlorate also showed similar trend as hydroponics experiment at very high concentrations, showing a limited ability of plants to survive and remove perchlorate under high perchlorate concentrations. Analysis of leaf tissue samples from this experiment showed high concentration of perchlorate per dry weight, which imply that by the time plant leaves dropped off due to phytotoxicity, they had already phytoaccumulated high perchlorate loads in the tissues via the transpiration stream, due to the fact that there was abundant perchlorate available in the solution.

Keywords: Perchlorate, Dissolved Organic Carbon, Uptake, Hydroponics, Soil, Willows