Uptake, removal, accumulation, and phytotoxicity of phenol in willow trees (Salix viminalis)

Phenol is an intermediate in the metabolism of plants, but it also is a high-volume production compound frequently found in the environment. The relation between removal, uptake, accumulation, and toxicity of phenol in willow trees (Salix viminalis) was determined. Willows were exposed to phenol levels up to 1,000 mg/L in hydroponic solution and sand, respectively. The change on water transpiration of the trees was used to determine toxic effects. In both hydroponic solution and sand, only small inhibition of transpiration was detected up to phenol concentrations of 250 mg/L. For concentrations of 500 mg/L or greater, transpiration fell to 50% or less, and the trees wilted. The trees exposed to 1,000 mg/L wilted and eventually died. For initial nominal concentrations of 1,000 mg/L, a significantly higher amount of phenol remained at the end of the experiment compared to the amount remaining with all other concentrations. Phenol loss was approximately 90% for the trees that survived, except for trees exposed to the lowest concentration (10 mg/L). The loss of phenol was only 15 to 18% in experiments performed without trees. The results demonstrate a clear relation between removal, accumulation, and toxic effects of phenol in trees. Phenol levels of less than 250 mg/L could be degraded by "in planta" metabolism; therefore, no significant buildup of phenol nor any toxic effects in plants occurred. With higher levels, phenol was still degraded quickly, but an accumulation in trees accompanied by severe toxic effects occurred. Phytoremediation of phenol would be best with concentrations in water or soil solution of less than 250 mg/L, at which the degradation of phenol by willows or associated bacteria is rapid and efficient and the toxic effects on trees are negligible.