Optimizing irrigation water use for cost-effective rhizoremediation of oil-contaminated soils with special reference to arid regions.

Supply of irrigation water represents a major challenge for rhizoremediation of oil-contaminated soils in arid areas. A microcosm experiment was conducted to evaluate the cost-effectiveness of rhizoremediation for oil-contaminated soils with regard to the different irrigation water supply rates. The results show that the growth performance of the test plant (Italian ryegrass) and the microbial activities in the soils during the 6-month experiment improved with the increasing rate of irrigation water supply from 30 to 70 mL per kg of soil per 2 days. The enhanced plant root development and microbial inoculation boosted the growth of PHC-degrading bacteria, mainly Macondimonas, Ramlibacter and Quasibacillus, which accelerated the degradation of PHCs in the soils. Different soil-borne PHC fractions responded differently to the change in irrigation water supply rate with increased water supply and microbial inoculation being more important for the decomposition of longer-chain PHCs (C34-C40) compared to shorter-chain PHCs (C16-C34). Over 70 % of C16-C34 fractions were removed even at low irrigation rates, while C34-C40 degradation rates ranged from 5.86 % to 61.73 % depending on treatment. However, the cost-effectiveness of rhizoremediation in arid areas decreased with increasing rate of irrigation water supply; the irrigation water cost for removing 1 kg of soil-borne PHCs increased by 46 % or 77 % when the irrigation water supply increased from the low rate to medium rate or high rate, respectively. To minimize the remediation costs, a 2-phase treatment approach with the focus in the first phase being placed on the removal of short-chain PHCs under non-inoculated and low water supply conditions, followed by intensive rhizodegradation or chemical treatment of long-chain PHCs is proposed.