Clean Biocide Project: Halophyte Extracts as Natural Corrosion Inhibitors in Water Systems

Farmland is becoming more saline worldwide. By 2050 it is estimated that 50% of all farmable land will be too saline for most plants to grow (200 mM NaCl). Halophytes (Fig. 1a) are one type of plant which are already being cultivated as food crops in coastal areas. Halophytes have been proposed as a salt remediation strategy in salt-affected farmland. To survive in saline conditions, halophytes produce valuable phytochemicals. Some are antimicrobials and have potential use as biocides/corrosion inhibitors in microbiologically influenced corrosion (MIC) mitigation.
Water-based extracts (Fig. 1b) were produced from four different halophyte species in the laboratory. A series of anaerobic flasks with Postgate B Medium were inoculated with a mixed anaerobic microbial culture. Each flask was treated with one of the extracts at four concentrations (5, 10, 15, and 20% v/v). At 20%, ATP and H2S measurements showed that adding the extract resulted in decreased ATP concentrations and no H2S production. This indicated a selective inhibition of sulfate-reducing bacteria (SRB). Subsequent 16S rRNA amplicon sequencing, measurements of coupon weight loss, and 3D-topography of coupons have confirmed this hypothesis. The sequencing data showed that the relative abundance of SRB decreased below the detectable limits. Only a few species, which have not been linked to MIC, were able to survive the treatment. Subsequent tests of the extract in an anaerobic, closed-loop CDC Biofilm Reactor showed a capability to break apart an established anaerobic biofilm.
This study used the multiple lines of evidence (MLOE) approach to demonstrate that halophyte-based extracts have potential as a mitigation strategy for MIC. However, these crude plant extracts are chemically complex. This currently poses problems with legislation as the entire chemical composition is not strictly defined and may vary between batches.