Cr(VI) removal from aqueous systems using pyrite as the reducing agent: batch, spectroscopic and column experiments

Area of Science:

• Environmental Chemistry
• Geochemistry
• Water Remediation

Background:

• Chromium(VI) is a toxic contaminant in aqueous systems.
• Pyrite is a potential reactive material for in situ permeable reactive barriers (PRBs).
• Understanding Cr(VI) removal mechanisms by pyrite is crucial for effective remediation.

Purpose of the Study:

• To elucidate the reaction mechanisms and kinetics of Cr(VI) removal by pyrite.
• To evaluate pyrite's performance under static and dynamic flow conditions relevant to PRBs.
• To investigate the influence of pH and concentration on Cr(VI) removal efficiency.

Main Methods:

• Laboratory batch and column experiments.
• Geochemical calculations.
• Spectroscopic analysis (X-ray photoelectron spectroscopy - XPS).
• Zeta potential measurements.

Main Results:

• Cr(VI) removal occurs via reduction to Cr(III), coupled with pyrite oxidation (Fe(II) to Fe(III), S2(2-) to SO4(2-)).
• Pyrite surface charge is pH-dependent and influenced by Cr(VI) presence.
• Cr(VI) removal efficiency decreases significantly with increasing pH due to precipitation of metal hydroxides and surface passivation.
• Reaction kinetics follow a first-order model, with rate constants decreasing as pH increases.
• Column experiments confirmed Fe(II) release and sulfate production, with evidence of product accumulation and pressure buildup.

Conclusions:

• Pyrite is an effective material for Cr(VI) reduction in aqueous systems.
• Surface passivation by precipitation products limits long-term efficiency, particularly at higher pH.
• The study provides insights into the geochemical processes governing Cr(VI) remediation using pyrite in PRBs.