Removal of 4-chlorobenzoic acid from spiked hydroponic solution by willow trees (Salix viminalis)

Chlorobenzoic acids (CBA) are intermediate products of the aerobic microbial degradation of PCB and several pesticides. This study explores the feasibility of using basket willows, Salix viminalis, to remove 4-CBA from polluted sites, which also might stimulate PCB degradation. The removal of 4-CBA by willow trees was investigated with intact, septic willow trees growing in hydroponic solution and with sterile cell suspensions at concentrations of 5 mg/L and 50 mg/L 4-CBA.

Nutrient solutions with different levels of ammonium and nitrate were prepared to achieve different pH levels. The concentration of 4-CBA was tracked over time and quantified by HPLC. At the low level of 4-CBA (5 mg/L), willows removed 70% (pH 4.2) to 90% (pH 6.8), while 48% (pH 4.2) to 52% (pH 6.8) of the water was transpired.

At the high 4-CBA level (50 mg/L), the pH varied between 4.4 and 4.6, and 10% to 30% of 4-CBA was removed, but only 5% to 9% of the water. In sterile cell suspensions, removal of 4-CBA by fresh biomass was much higher than removal by dead biomass. The results indicate that 4-CBA is toxic to willow trees at 50 mg/L.

The removal of 4-CBA from solution is by both passive processes (uptake with water, sorption to plant tissue) and metabolic processes of the plants. Plants, such as willow trees, might assist in the degradation of PCB and their degradation products CBA.