posted on 2023-11-20, 05:48authored byMaggie
A. Cooper, Bruno C. Batista, Oliver Steinbock
Multi-compound precipitates are important in various
processes,
from recovering resource-limited metals to forming chimney structures
at deep-sea hydrothermal vents. Their growth often occurs rapidly,
involving steep concentration gradients, fluid flow in the surrounding
solution, and transient reaction conditions. Our study systematically
controls these complex factors using a microfluidic device that produces
co-flowing laminar streams of NaOH and (Ni,Mg)Cl2 solutions.
The resulting precipitate membranes thicken exclusively in the direction
of the mixed metal solution and consist of Ni(OH)2 and
Mg(OH)2. Energy-dispersive spectroscopy (EDS) and mass
spectrometry show that, for the investigated metal solution ratios
and a constant growth time, the average [Mg2+]/[Ni2+] product ratio is proportional to the [Mg2+]/[Ni2+] reactant ratio. The Mg levels in the membrane are high
overall. Over the initial 1.5 h, the membrane width is proportional
to the square root of time, yielding effective diffusion coefficients
that increase with increasing [Mg2+]/[Ni2+]
reactant ratios. EDS maps also show a strong compositional gradient
across the membrane, where early formed layers contain the highest
levels of Mg. These results demonstrate strong deviations from equilibrium
behavior and are discussed in the context of kinetic factors favoring
Mg(OH)2 precipitation. A simple model is proposed to explain
the observed compositional gradient, attributing it to a cation-exchange
process that becomes more effective during the slow late growth of
the membrane.