Continuous Hydrogen Sulfide Monitor Based on Positive Fluorescence Green Chemistry. Application to Geothermal...

Renewable geothermal sources can be used for base-load power generation. Prior environmental impact assessment is needed, however, for H₂S emitted from wells and cooling towers. Detailed spatiotemporal H₂S concentration data need continuous monitoring by sensitive and fieldable H₂S analyzers. Fluorescein mercuric acetate (FMA), a toxic organomercuric reagent, has been a staple for H₂S measurement. We report a reagent containing copper 8-hydroxyquinoline-5-sulfonic acid (HQS) complex and Zn²⁺. As HS^- from sample air bearing H₂S reacts with such a reagent, thiophilic copper forms the hydrosulfide, and the released HQS forms a fluorescent zinc chelate. An automated fieldable H₂S analyzer based on this green chemistry, comprising a disposable gas scrubber with controlled liquid flow at 50 μL/min and a UV-laser based fluorescence detector, is reported. The instrument operates on a (4 min sample, 6 min zero) reporting the difference signal via a Raspberry Pi controller to the cloud every 10 min with a working range of 0.3 ppb (limit of detection) to >100 ppb. During each cycle a total of 0.8 L air and 1 mL aqueous solution containing 5 μmol NaOH, 10 μmol 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 40 nmol HQS, and 20 nmol each of Cu and Zn are consumed with a total chemical cost < USD $0.01. Performance characteristics and data from a geothermal field are reported. It provides far superior sensitivity and time resolution than conventional impinger absorption and colorimetry. It should find wide use in geothermal environmental assessment and for monitoring oil drilling and storage sites, paper-pulp and sewage treatment facilities, livestock management, etc.