EFFECT OF SUPPLEMENTARY ORGANIC CARBON SOURCEAND ELECTRON ACCEPTOR ON THE REMOVAL OF2,4-DICHLOROPHENOXYACETIC ACID IN SLURRY BIOREACTORS

摘要

Ad situ remediation techniques appear as an interesting alternative for thetreatment of polluted solid matrices that are not amenable to in situ remediation techniques,such as sediments and soils characterized by high contents of clay and organicmatter coupled to low hydraulic conductivity and permeability. In this work, the removalof 2,4-D from a model, heavy solid matrix was evaluated in lab-scale slurry bioreactorsunder both aerobic and anaerobic (sulfate-reducing) conditions. Also, the effect of anadditional carbon source (sucrose) was assessed.A model solid matrix (48% clay, 4% organic matter) was sterilized and contaminatedwith a high dose of 300 mg 2,4-D/kg dry matrix. The polluted matrix was treated inboth aerobic and sulfate-reducing slurry bioreactors (SB-A and SB-SR, respectively),with and without 1 g/L sucrose. Each type of SB received a seed (20% v/v) acclimated to2,4-D from aerobic and sulfate-reducing continuous, complete mix reactors. In this study,aerobic conditions were more favourable for the degradation of 2,4-D in terms of theoverall removal efficiency (93%, 15 days treatment), compared to 25% under sulfatereducingconditions. However, the specific removal rate of SB-SR was significantlyhigher (ranging between 10-13 mg 2,4-D/(mg VSS seed.day)) than that in SB-A (2.7 mg2,4-D/(mg VSS seed.day)). This difference was ascribed to the fact that the VSS contentof the aerobic seed was much higher than the corresponding to the sulfate-reducinginoculum. The effect of the sucrose amendment on 2,4-D removal depended on theelectron acceptor used: aerobic removal was not affected by the sucrose supplementation,whereas the removal in SB-SR receiving sucrose was higher than in units withoutsucrose. These results could indicate different degradative pathways for aerobic andanaerobic conditions, as well as the existence of co-metabolism amongst a consortium ofmicroorganisms in the sulfate-reducing reactors. Yet, SB-SR without sucrose still showedan important 2,4-D removal (up to 18%) suggesting that 2,4-D could be used as substrate.The 2,4-D-clastic bacteria were present in all reactors dur ing the incubation: aerobic2,4-D-clastic bacteria ranged between 6 to 8 logs, whereas the sulfate-reducing 2,4-Dclasticpopulation ranged between 5 to 7 logs. Overall, the slurry bioreactor ex-situ bioremediationtechnique seems to be an effective alternative for the treatment of difficultsediments and soils (characterized by a fine texture and high contents of organic matter)polluted with agrochemicals.