Is it possible to valorize bicarbonates from reclaimed wastewater by CO2 electroreduction into formic acid? Investigation under low bicarbonate concentration and low-conductivity solutions

Abstract

The electroreduction of CO₂ into value-added compounds from low bicarbonate concentration solutions typical of treated wastewater has been explored for the first time, unlike existing studies that focus on high bicarbonates concentrations and pressurized gas CO₂ as sources. This study investigates the feasibility of producing formic acid (FA) from low initial total inorganic carbon (TIC₀) concentration referring to total inorganic carbon (TIC) after acidification to pH 4 (15-50 mg-C L⁻¹) under microfluidic conditions in a filter press cell, from simulated to real reclaimed wastewaters. The effect of TIC₀ demonstrated that an optimal TIC₀ of 20 mg-C L⁻¹ at j_app = 5 mA cm⁻² balanced the conversion yield (56 %), faradaic efficiency (FE) (9 %), and specific energy consumption (E_sp) (1.96 kWh mol CO₂⁻¹). Reducing the j_app from 5 to 2 mA cm⁻² resulted in a 14 % increase in FE, although accompanied by a decrease in an overall conversion efficiency, highlighting the compromise that exists between selectivity and productivity. A kinetic model based on TIC degradation and FA formation accurately captured experimental trends. Results from real effluents showed successful FA production.

These findings highlighted the interdependence between mass transport (k_m) limitations, TIC availability, and competing reactions such as hydrogen evolution reaction (HER). The observed variations in k_m (1.4–1.6 × 10⁻⁵ m s⁻¹) indicated that mass transport constraints played a critical role in CO₂ reduction efficiency, where an increased diffusion layer ($\delta$) at higher TIC₀ limited CO₂ availability to the cathode surface shifting the balance between CO₂ reduction and HER. FTIR analyses further revealed an intensification of FA bands with increasing current.