(280e) Selective and Continuous Lead Removal By Shock Electrodialysis Device

Excessive exposure to heavy metals like lead affects the normal function of the human body and leads to heavy metal poisoning. Exposure to lead may occur in a variety of ways, one of which is by consumption of lead ions present in drinking water. Lead usually contaminates drinking water upon the corrosion of plumbing materials that comprise this element, such as pipes, fixtures, faucets, and parts joined by lead solder. For example, households in Flint, Michigan and public schools across the US---many of which still rely on legacy lead pipes for plumbing and transporting water---were recently exposed to dangerously high levels of lead in their drinking water. This public health crisis has been combated predominantly by distributing bottled water and expensive filters to residents and schools, though more sustainable and cost-effective solutions are needed to end this crisis. Conventional methods for the removal of lead---typically present in trace quantities---from tap water include filtration though these systems are costly and require either regular chemical regeneration or frequent replacement.

In this article, we adapt an emerging method for electrokinetic deionization known as shock electrodialysis (or shock ED) to continuously and selectively remove lead from water in the presence of excess sodium. The main part of the shock ED system is a negatively charged macroporous material sandwiched by two cation exchange membranes (CEMs). As electrical current is applied to the system (through water splitting at the two Pt electrodes), concentration polarization occurs in the porous material. When the current exceeds diffusion-limiting current (enabled by surface conduction and electroosmosis), a deionization shock can propagate from the cathode-side membrane to the anode-side membrane. Then a cross feed stream into such a system can be splitted into a brine and a fresh stream at the outlet. Recent experimental works showed that shock ED can selectively and effectively remove multivalent cations, which makes shock ED promising for heavy metal ion removal.

In this work, we used the shock ED prototype to remove lead from a model tap water consisting of 28 ppb Pb and 4.2 ppm Na. We prepared two devices, one made of borosilicate frit and one of SiC. We added 50 mM HCl in the cathode stream to prevent lead precipitation (in theory anolyte and catholyte can be recycled), so both ion exchange and electric current can work to remove lead. At zero current, we showed that the lead removal and water recovery were respectively about 80% and 55% for the frit device (five-day data) and about 95% and 77% for the SiC device (30-day data). When further increasing the current for the frit device (5 days a current), we found more than 95% lead removal with about 60%–70% water recovery at energy consumption of 0.02 kWh/m³. However, at very high current lead ions seemed to be stuck in the device. Then we applied the depth-averaged model that we have developed before to explain the experimental data. The simulation results are well consistent with experimental results in most cases. Two effects should explain the selective lead removal: the affinity of lead to charged walls where velocity is small due to non-slip boundary, and the decrease of lead concentration in frit at increased current due to combined transport mechanisms.

To conclude, this work showed that shock ED device can selectively, effectively, and continuously remove lead by ion exchange (zero current needed), or a very low electrical energy cost. However, the process is still far from well-optimized. In the future, we plan to replace the acid by reusable buffer solution, build up stacks to further increase the current efficiency, and seek better materials to prevent lead precipitation.