Removal of paclobutrazol from irrigation water using granular-activated carbon

Abstract

A small-scale granular-activated carbon (GAC) system was evaluated for removal of the plant growth regulator paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)pentan-3-ol] from water. A solution with 50 µg L−1 of paclobutrazol was passed through canisters filled with 0.50–4.75 mm particle size (8 × 30 US mesh) granular-activated carbon at a flow rate of 6 L min−1. Paclobutrazol solution was exposed to varying amounts of contact time with GAC by increasing the number of filters in series. Analysis of samples using liquid chromatography–mass spectrometry (LC-MS/MS) found that paclobutrazol concentration decreased by 90 and 99% with a contact time of 12 and 59 s, respectively. In bioassay tests, broccoli hypocotyls at 14 days were 104% longer and begonia dry mass was 36% greater when treated with solutions that had a contact time of 59 s compared with the 0 s of GAC exposure. With the highest GAC contact time, begonia dry mass was the same as for plants treated with a zero paclobutrazol solution. Bituminous coal and coconut shell GAC sources were equally effective in reducing paclobutrazol concentration based on broccoli hypocotyl length, and paclobutrazol concentration measured using gas chromatography–mass spectrometry (GC-MS). Removal of paclobutrazol was not affected by solution pH from 4.0 to 10.0.