Phosphate, an essential
molecule in biochemistry, is not abundant
in modern oceans and would have been even less abundant in early Earth’s
oceans. One possible mechanism for concentrating phosphate for prebiotic
reactions is adsorption onto iron (oxy)hydroxide minerals, which would
have precipitated from interactions between iron-rich oceans of the
early Earth and near neutral-alkaline hydrothermal fluids. In this
work, we synthesized ferrous and ferric (oxy)hydroxides to test their
adsorptivity toward phosphate under early Earth oceanic conditions
(anoxic, dissolved Fe2+, pH 6–9, and low phosphate
levels). Prebiotically relevant amino acids (cysteine, histidine,
and arginine) were added to test their effect on phosphate adsorption.
Colorimetry techniques coupled with nuclear magnetic resonance and
statistical analysis were utilized to determine how experimental conditions
influenced the adsorption reaction. We observed an 80–90% reduction
of ferric to ferrous iron minerals in the presence of cysteine; we
hypothesize that iron and cysteine underwent a redox reaction to produce
cystine. In addition, phosphate was readily adsorbed onto iron (oxy)hydroxide
minerals, but their efficacy depended on the iron redox state and
pH at which the minerals were precipitated. Phosphate adsorption was
the greatest with ferrous (oxy)hydroxide minerals precipitated at
pH 9, reaching a maximum average adsorption of 45%. Under these conditions,
the addition of organics significantly enhanced phosphate adsorption
by an additional ∼30%; differences due to the amino acid side
chain were not statistically significant. This work shows how environmental
conditions (redox state, pH, and presence of organics) influenced
adsorption in a simulated mineral system; such systems merit further
study under increasingly complex conditions in order to better understand
phosphate dynamics on wet-rocky worlds such as early Earth or Mars.