Continuous biodegradation of phenoxyalkanoate herbicides bySphingomonas herbicidovorans MH in a PU-supplied bubble reactor

Abstract

The continuous aerobic degradation of phenoxyalkanoate herbicides by Sphingomonas herbicidovorans MH was investigated in a bubble reactor filled with modified polyurethane-foam (PU 90/51) as a carrier for the adsorptive immobilization of the bacterial cells. The PU-foam was applied in the form of plates (5 × 10 × 10 mm) and the amount added was equivalent to a PU-load of 1.25% [w/v]. Strain MH is capable of detoxifying the dichloro-substituted phenoxyalkanoates 2,4-DP, 2,4-D and 2,4-DB and the methylchloro-substituted phenoxyalkanoates MCPA, MCPP and MCPB. Degradation of the respective substrate was followed by HPLC analyses and by determination of the chloride release. No intermediates of the degradation pathways or dead end products were detected by HPLC analyses. The PU-bubble reactor with immobilized 2,4-DP-pre-grown cells was run continuously at 30 °C at the high dilution rate of D = 0.5 h -1 with 2,4-DP (0.2 g/l), and with subsequent changes to each of the other phenoxyalkanoates as a single substrate in the feed and with an intermittent return to 2,4-DP. Finally, after an intermediate substrate accumulation, 2,4-D, 2,4-DP, MCPA and MCPP could be degraded under the aforementioned conditions corresponding to a maximum degradation rate of Q phen = 100 mg/l × h. In the case of 2,4-DB, a slightly reduced conversion rate of about 94% could be calculated. In contrast to these results, 0.2 g/l of the more recalcitrant MCPB could not be metabolized at this high dilution rate of D = 0.5 h - 1 by the biofilm of Sphingomonas herbicidovorans MH, but it was degradable at a reduced dilution rate of D = 0.25 h - 1 . Complete detoxification of a stoichiometric mixture of the dichloro- and the methylchloro-substituted phenoxyalkanoates including MCPB, respectively, at a total concentration of 0.2 g/l was achieved at D = 0.25 h -1 , corresponding to a degradation rate of Q tot = 50 mg/l × h. Finally, the efficiency of the PU-immobilized cells of Sphingomonas herbicidovorans MH in detoxifying mixtures of all six herbicides could be increased to Q tot = 75 mg/l × h by the further addition of PU-foam particles corresponding to a final PU-load of 2.5% [w/v]. This PU-bubble reactor was successfully operated for more than 12 months to clean up synthetically concocted waste waters with fluctuations in phenoxyalkanoate concentration and composition.