Strength Enhancement and Lead Immobilization of Sand Using Consortia of Bacteria and Blue-Green Algae

Abstract

Metabolites generated from microorganisms have a high potential for the strength improvement and contaminant remediation of soils. The consortium of naturally available bacteria and microalgae in soils provides a sustainable, carbon-neutral, economical, and environmentally friendly approach for both biocementation and contaminant remediation. This combined approach has not been explored earlier. The study aims to improve the strength and reduce leachable lead in cohesionless soil by providing optimal nutrients and chemicals to expedite the metabolic activity for the formation of calcite crystals in between the sand grains. Laboratory experiments were conducted using a poorly graded sand that was artificially contaminated with lead (toxic heavy metal). Algae Nostoc commune (blue-green algae) was used individually and in consortium with Bacillus (B.) sphaericus and Sporosarcina (S.) pasteurii in sand. The uncontaminated and artificially contaminated soil specimens were inoculated with different consortia combinations and treated with two different cementation media concentrations up to 16 days. After biocementation, unconfined compressive strength, direct shear, permeability, and leachability tests were conducted to analyze the engineering properties of the sand and the contaminant (lead) immobilization. The amount of calcite content was analyzed based on a calcimeter test. The leachability of lead from biocemented sand was analyzed using toxicity characteristics leaching procedure (TCLP), which showed 94%–99.2% immobilization of lead. The formation of CaCO3 and PbCO3 was identified by microcharacterization, which showed immobilization of leachable lead. The results revealed that biotreatment with the consortium of B. sphaericus and Nostoc commune increase strength, decrease permeability, and increase contaminant immobilization of sands.