We studied the use of sequential batch reactors under oxygen limitation to improve and maintain consortium ability to biodegrade hydrocarbons. Air-agitated tubular reactors (2.5 L) were operated for 20 sequential 21-day cycles. Maya crude oil – paraffin mixture (13 000 mg/L) was used as the sole carbon source. The reactors were inoculated with a consortium from the rhizosphere of Cyperus laxus, a native plant that grows naturally in weathered, contaminated soil. Oxygen limitation was induced in the tubular reactor by maintaining low oxygen transfer coefficients (k_La < 20.6 h^–1). The extent and biodegradation rates increased significantly up to the fourth cycle, maintaining values of about 66.33% and 460 mg·L^–1·d^–1, respectively. Thereafter, sequential batch reactor operation exhibited a pattern with a constant general trend of biodegradation. The effect of oxygen limitation on consortium activity led to a low biomass yield and non-soluble metabolite (0.45 g SS/g hydrocarbons consumed). The average number of hydrocarbon-degrading microorganisms increased from 6.5 × 10⁷ (cycles 1–3) to 2.2 × 10⁸ (cycles 4–20). Five bacterial strains were identified: Achromobacter (Alcaligenes) xylosoxidans, Bacillus cereus, Bacillus subtilis, Brevibacterium luteum, and Pseudomonas pseudoalcaligenes. Asphaltene-free total petroleum hydrocarbons, extracted from a weathered, contaminated soil, were also biodegraded (97.1 mg·L^–1·d^–1) and mineralized (210.48 mg CO₂·L^–1·d^–1) by the enriched consortium without inhibition. Our results indicate that sequential batch reactors under oxygen limitation can be used to produce consortia with high and constant biodegradation ability for industrial applications of bioremediation.Key words: sequential batch reactors, oxygen limitation, consortium, hydrocarbon biodegradation.

This work focused on the determination of boron diffusion coefficient through the Fe ₂ B layers on AISI 1026 steel using a mathematical model. The suggested model solves the mass balance equation at the (Fe ₂ B/substrate) interface. This thermochemical treatment was carried out in the temperature range of 1123–1273 K for a treatment time ranging from 2 to 8 h. The generated boride layers were characterized by different experimental techniques such as light optical microscopy, scanning electron microscopy, XRD analysis and the Daimler-Benz Rockwell-C indentation technique. As a result, the boron activation energy for AISI 1026 steel was estimated as 178.4 kJ/mol. Furthermore, this kinetic model was validated by comparing the experimental Fe ₂ B layer thickness with the predicted one at a temperature of 1253 K for 5 h of treatment. A contour diagram relating the layer thickness to the boriding parameters was proposed to be used in practical applications.