Contaminant-containing soil can be remediated by subjecting the soil to microwave irradiation to heat the soil to a combustion temperature of from 150° C. to 1200° C. to initiate combustion. A target amount of the contaminant in the soil is destroyed. An oxidizer gas can be flowed through the soil using a gas blower and at least one gas injection line and controlled at a rate such that a self-sustaining smoldering combustion front is formed that moves through the soil. A microwave absorber additive can be added to the soil to accelerate the heating of the soil. At least one waveguide connected to a source of microwave energy directs microwaves into the soil.