Pilot-Scale Evaluation of an Immobilised Rhodococcus–Dietzia Consortium on Agricultural Carriers for Petroleum-Contaminated Soil Remediation Under Arid Field Conditions in Kazakhstan

Oil contamination of soils in arid regions of Kazakhstan is a critical environmental problem, as extreme temperatures, low humidity and salinity limit traditional bioremediation. A pilot-scale 45-day field experiment at the Karazhanbas oil field (Mangistau Region, Kazakhstan) provided a first field validation of an adsorption-immobilisation bioremediation technology. A consortium of Rhodococcus erythropolis AT7 and Dietzia maris 22K was immobilised on buckwheat and rice husk carriers. Four treatments were tested on 1 × 1 m plots (initial petroleum products 3725 mg/kg): the consortium immobilised on buckwheat husks, on rice husks, free cells, and an untreated control. Petroleum products were measured by FTIR spectroscopy on days 0, 15, 30 and 45. The buckwheat husk variant showed the highest observed degradation efficiency among the tested treatments—94.0 ± 0.5% (3725 to 223 ± 18 mg/kg)—1.6 times higher than rice husk (57.9%) and 1.7 times higher than free cells at the standard dose (54.6%). Degradation followed first-order kinetics (k = 0.0355 day−1; t1/2 = 19.5 days), and hydrocarbon-oxidising microorganisms increased to 3.4 × 108 CFU/g in the immobilised treatments. The higher performance of buckwheat husks may be associated with lower lignin content and improved carrier properties, with phenolic compounds such as rutin as a plausible additional factor. These pilot results provide a basis for larger-scale validation in Western Kazakhstan.