Comparison of Aerobic and Sulfate-Reducing Slurry Bioreactors for the Removal of Lindane from a Heavy Soil

摘要

Lindane or y-hexa-chloro cyclohexane (HCH) is an insecticide widely used in developing countries, particularly in Mexico, in spite of bans in first-world countries. Negative impacts of lindane on the environment and human health have been reported. Mexican agricultural soils characterized by high contents of clay and organic matter are often contaminated with lindane and other chlorinated pesticides and pose a challenge for their remediation. The aim of this work was to evaluate the effect of electron acceptor and supplementation of degradable organic co-substrate on lindane removal from a clayish agricultural soil with high levels of organic matter, using batch slurry bioreactors.A simple 2~2 factorial experiment was carried out with factor electron acceptor at two levels (aerobic and sulfate-reducing SB. i.e.. A-SB and SR-SB, respectively) and factor sucrose concentration at 0 to 1 g/L. An agricultural, mineral clayish soil with 8% organic matter spiked with 100 mg/kg lindane was treated in lab scale SBs. SBs were inoculated with biomass previously acclimated to lindane in suspended-growth, liquid phase, continuous bioreactors. Abiotic control SBs (sterilized soil and inocula) and biotic control SBs (noninoculated, nonsterilized soil) were also run. The A-SB with no sucrose removed nearly half of initial lindane in the first 10 day period whereas lindane removal was 86% at the end of 30 days. On the other hand and unexpectedly, the A-SB supplemented with 1 g/L sucrose showed a lower final 77% lindane removal. SR-SBs seemed to be the fastest (ca. 77% lindane removal in the first 10-day period) and most effective (82 and 88% lindane removals at 30 days, without and with added sucrose, respectively). On average, electron acceptor seemed to have a significant effect on lindane removal (sulfate-reducing higher than aerobic), whereas the addition of sucrose had a distinct effect that depended on the electron acceptor (statistical interaction). Indeed, in SR-SB, sucrose supplementation slightly improved lindane removal, while in A-SB sucrose addition significantly diminished lindane reduction. Final lindane-clastic bacterial counts were in the order of 8 (log CFU) for A-SB, independently of sucrose or no sucrose addition, and 7 (log CFU) in SR-SB. Biotic control A-SBs and SR-SBs exhibited reductions of lindane in the order of 65% suggesting that the soil native microflora itself had a significant capacity for degrading or transforming lindane. This is consistent with lindane-clastic bacterial counts around 6.7 and 5.5 (log CFU) in the aerobic and sulfate-reducing biotic controls, respectively. Inoculation with acclimated biomass seemed to increase lindane removals by an absolute 20% to 25% (relative 33%). On the other hand, removals of lindane in abiotic control SBs were very low, with an average of 11%. The excellent performance of SR-SBs for lindane removal could point out to an interesting alternative for the treatment in SB of heavy soils rich in sulfate salts.