The central control of mineral weathering rates on biogeochemical systems has motivated studies of dissolution for more than 50 years. A complete physical picture that explains widely observed variations in dissolution behavior is lacking, and some data show apparent serious inconsistencies that cannot be explained by the largely empirical kinetic ''laws.'' Here, we show that mineral dissolution can, in fact, be understood through the same mechanistic theory of nucleation developed for mineral growth. In principle, this theory should describe dissolution but has never been tested. By generalizing nucleation rate equations to include dissolution, we arrive at a model that predicts how quartz dissolution processes change with undersaturation from step retreat, to defect-driven and homogeneous etch pit formation. This finding reveals that the ''salt effect,'' recognized almost 100 years ago, arises from a crossover in dominant nucleation mechanism to greatly increase step density. The theory also explains the dissolution kinetics of major weathering aluminosilicates, kaolinite and K-feldspar. In doing so, it provides a sensible origin of discrepancies reported for the dependence of kaolinite dissolution and growth rates on saturation state by invoking a temperature-activated transition in the nucleation process. Although dissolution by nucleation processes was previously unknown for oxides or silicates, our mechanism-based findings are consistent with recent observations of dissolution (i.e., demineralization) in biological minerals. Nucleation theory may be the missing link to unifying mineral growth and dissolution into a mechanistic and quantitative framework across the continuum of driving force.silica ͉ kinetics ͉ mineralization O ver long time scales, the geochemistry of earth systems is, in large part, controlled by the kinetics of silicate mineral dissolution. Because waters contain a wide variety of solute types and concentrations, including significant levels of aqueous silica, there is considerable need to understand the dependence of silicate mineral dissolution rates on chemical driving force, as measured by the extent of undersaturation. This need has motivated intense investigations of both mineral weathering and the corrosion behavior of silica-based glasses.Basic thermodynamic principles predict that mineral dissolution rates should increase with increasing driving force or chemical potential; however, experimental studies of major silicate minerals show that this dependence is complex. A further complication is the so-called ''salt effect'' reported for quartz, SiO 2 , whereby the dissolution rate of this oxide end-member to all silicates is increased up to 100 times in the presence of the major cationic solutes found in natural waters (Na ϩ , K ϩ , Ca 2ϩ , Mg 2ϩ ) (1-3). In contrast, dissolution rates of silicate minerals have only a weak sensitivity to the introduction of electrolytes (4). To explain these behaviors, many of the widely used rate models are based on variants of transition state t...

Polyaspartate domains are a prominent feature of proteins associated with biogenic carbonates and have been implicated in modifying crystal morphology through specific interactions with step edges. Here, we show that the morphology and growth kinetics of calcite are modified in a systematic way when a series of poly-L-aspartates, Asp 1-6 , are introduced into solution. In-situ measurements of step propagation rates by atomic force microscopy reveal these effects are strongly chain-length dependent and specific to the crystallographically distinct, obtuse and acute step types. Direct observations of differential roughening and rounding of the step edges demonstrate that, while Asp 1 and Asp 2 have stronger effects on acute step edges, a crossover occurs for the longer Asp 4,5,6 peptides that preferentially affect obtuse steps. Independent analysis of Asp n -step edge interactions by semiempirical quantum mechanical modeling gives estimates of aspartate-step edge binding energies and predicts that the crossover should occur at n ) 2. The switch occurs because, upon Asp n binding, the energy required to dehydrate acute steps is greater than that at the obtuse steps when n ) 3-6.Step velocity measurements show that the concentration of Asp n needed to stop growth scales exponentially and inversely with the calculated binding energies. A simple model of Asp n adsorption to the steps is derived from these results. These findings suggest a process by which small fluctuations in primary structure in proteins can control calcite shape.

Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Here we quantify the kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates under controlled conditions. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. Chitosan presents a low-energy barrier to nucleation because its near-neutral charge favors formation of a substrate-crystal interface, thus reducing substrate interactions with water. Progressively higher barriers are measured for negatively charged alginates and heparin that favor contact with the solution over the formation of new substrate-crystal interfaces. The findings support a directing role for PS in biomineral formation and demonstrate that substrate-crystal interactions are one end-member in a larger continuum of competing forces that regulate heterogeneous crystal nucleation.biomineralization | calcium carbonate | free energy | algae | crustacean E fforts to decipher patterns of biomineralization have identified proteins and polysaccharides (PSs) as major components of the organic matrices associated with sites of calcification. In mollusks and other organisms including the red algae, coccolithophores, and foraminifera (1, 2) (Table 1), calcifying macromolecules are dominated by functional groups with an acidic character-proteins that are rich in carboxylated amino acids (14) and PSs that are highly sulfated and carboxylated (15,16). Although a physical picture of these interactions is not well developed, this recurring affiliation of carboxylate and sulfate groups with zones of mineralization in organisms suggests specific roles for macromolecules in nucleation and growth. Early studies have led the biomineralization community to generally assume that charged proteins actively regulate mineralization. In contrast, PSs, such as the chitin found in the insoluble fraction of the mollusk shell, are thought to provide an inert scaffolding to support these proteins (17,18).Recent studies challenge this assumption with qualitative evidence that PSs can also promote calcium carbonate (CaCO 3 ) mineralization (19,20) (Table 1). For example, specific orientations of chitin fibers with neutral functional groups promote the templating of CaCO 3 in the lobster carapace (13, 21), crab cuticle (21), and nautilus shell (22). More generally, the monosaccharide sequences along PS chains can influence biological function and their interactions with proteins (23). This suggests PS chemistry and interactions with proteins could regulate patterns of mineralization.Anecdotal observations from in vitro studies also support the thinking that PSs influence mineral formation, but their specific effects are unclear. For example, PSs with higher carboxyl and sulfate content promote either faster (24, 25) or slower (2...

The mechanisms by which amorphous silica dissolves have proven elusive because noncrystalline materials lack the structural order that allows them to be studied by the classical terrace, ledge, kink-based models applied to crystals. This would seem to imply amorphous phases have surfaces that are disordered at an atomic scale so that the transfer of SiO(4) tetrahedra to solution always leaves the surface free energy of the solid unchanged. As a consequence, dissolution rates of amorphous phases should simply scale linearly with increasing driving force (undersaturation) through the higher probability of detaching silica tetrahedra. By examining rate measurements for two amorphous SiO(2) glasses we find, instead, a paradox. In electrolyte solutions, these silicas show the same exponential dependence on driving force as their crystalline counterpart, quartz. We analyze this enigma by considering that amorphous silicas present two predominant types of surface-coordinated silica tetrahedra to solution. Electrolytes overcome the energy barrier to nucleated detachment of higher coordinated species to create a periphery of reactive, lesser coordinated groups that increase surface energy. The result is a plausible mechanism-based model that is formally identical with the classical polynuclear theory developed for crystal growth. The model also accounts for reported demineralization rates of natural biogenic and synthetic colloidal silicas. In principle, these insights should be applicable to materials with a wide variety of compositions and structural order when the reacting units are defined by the energies of their constituent species.

Calcite and other crystalline polymorphs of CaCO 3 can form by pathways involving amorphous calcium carbonate (ACC). Previous studies of ACC provide important insights, but apparent inconsistencies in the literature indicate the relationships between ACC composition, local conditions, and the subsequent crystalline polymorphs are not yet established. This experimental study quantifies the control of solution composition on the transformation of ACC into crystalline polymorphs in the presence of magnesium. Using a mixed flow reactor to control solution chemistry, ACC was synthesized with variable Mg contents by tuning input pH, Mg/Ca, and total carbonate concentration. ACC products were allowed to transform within the output suspension under stirred or quiescent conditions while characterizing the evolving solutions and solids. As the ACC transforms into a crystalline phase, the solutions record a polymorph-specific evolution of pH and Mg/Ca. The data provide a quantitative framework for predicting the initial polymorph that forms from ACC based upon the solution aMg 2+ /aCa 2+ and aCO 3 2-/aCa 2+ and stirring versus quiescent conditions. This model reconciles apparent discrepancies among previous studies that report on the nature of the polymorphs produced from ACC and supports the previous claim that monohydrocalcite may be an important, but overlooked, transient phase on the way to forming some aragonite and calcite deposits. By this construct, organic additives and extreme pH are not required to tune the composition and nature of the polymorph that forms. Our measurements show that the Mg content of ACC is recorded in the resulting calcite with a ≈1:1 dependence. By correlating the composition of these calcite products with the Mg tot /Ca tot of the initial solutions, we find a ≈3:1 dependence that is approximately linear and general to whether the calcite is formed via an ACC pathway or by the classical step-propagation process. Comparisons to calcite grown in synthetic seawater show a ≈1:1 dependence. The relationships suggest that the local Mg 2+ /Ca 2+ at the time of precipitation determines the calcite composition, independent of whether growth occurs via an amorphous intermediate or classical pathway for a range of supersaturations and pH conditions. The findings reiterate the need to revisit the traditional picture of chemical and physical controls on CaCO 3 polymorph selection. Mineralization by pathways involving ACC can lead to the formation of crystalline phases whose polymorphs and compositions are out of equilibrium with the local growth media. As such, classical thermodynamic equilibria may not provide a reliable predictor of observed compositions.

The physical basis for how macromolecules regulate the onset of mineral formation in calcifying tissues is not well established. A popular conceptual model assumes the organic matrix provides a stereochemical match during cooperative organization of solute ions. In contrast, another uses simple binding assays to identify good promoters of nucleation. Here, we reconcile these two views and provide a mechanistic explanation for template-directed nucleation by correlating heterogeneous nucleation barriers with crystal-substrate-binding free energies. We first measure the kinetics of calcite nucleation onto model substrates that present different functional group chemistries (carboxyl, thiol, phosphate, and hydroxyl) and conformations (C11 and C16 chain lengths). We find rates are substrate-specific and obey predictions of classical nucleation theory at supersaturations that extend above the solubility of amorphous calcium carbonate. Analysis of the kinetic data shows the thermodynamic barrier to nucleation is reduced by minimizing the interfacial free energy of the system, γ. We then use dynamic force spectroscopy to independently measure calcitesubstrate-binding free energies, ΔG b . Moreover, we show that within the classical theory of nucleation, γ and ΔG b should be linearly related. The results bear out this prediction and demonstrate that low-energy barriers to nucleation correlate with strong crystal-substrate binding. This relationship is general to all functional group chemistries and conformations. These findings provide a physical model that reconciles the long-standing concept of templated nucleation through stereochemical matching with the conventional wisdom that good binders are good nucleators. The alternative perspectives become internally consistent when viewed through the lens of crystal-substrate binding.B iological systems are unique in their ability to organize minerals into functional materials with complex patterns and architectures. A substantial body of evidence suggests specialized macromolecules, particularly proteins (1, 2) and carbohydrates (3, 4), provide preferential sites for nucleation to direct the placement, timing, and orientation of crystals (5), both intra-and extracellular. Within the biomineralization community, the conventional view of biologically directed nucleation is that macromolecular matrices present an interfacial match to the crystal lattice that assists in forming the crystal nucleus. This cooperative view of directed nucleation is rooted in the collective action of multiple residues that guide the organization of ions into a configuration defining the energetic minimum for the system. A series of in vitro observations have reinforced this picture by showing that highly ordered organic monolayers can control the location and orientation of calcite crystals precipitated from solution (6). In this approach, good templates are revealed through a direct functional assay, i.e., nucleation. Over the years, this view of mineralization, both in the context of natural stru...

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The fate of metals in soils where soluble organic compounds are present may be strongly influenced by the degree to which they are complexed by organic ligands. We undertook this study to determine the combined effect of molecular weight (MW) and hydrophobicity on the Cu‐binding ability of dissolved organic compounds in biosolids (i.e., sewage sludge). Dissolved organic matter (DOM) from anaerobically digested sewage biosolids was fractionated by using a combination of MW fractionation and XAD‐8 resin chromatography (to separate the dissolved organic compounds according to hydrophilic and hydrophobic characteristics). The Cu‐binding abilities of the DOM fractions were obtained by using a Cu2+‐ion‐selective electrode (Cu‐ISE) technique. The Cu‐binding ability of fractionated DOM decreased significantly with increasing molecular weight, indicating that low‐MW DOM had more metal‐binding sites than high‐MW DOM. Within each MW fraction, the hydrophilic and the hydrophobic components also exhibited differences in Cu‐binding ability. For the DOM with MW 500–3500 Da, the hydrophilic fraction showed a greater Cu‐binding capacity than did the hydrophobic fraction, whereas the hydrophobic acid components were most important in binding Cu for DOM with MW > 3500 Da. The maximum Cu‐binding capacities of different biosolids‐derived DOM fractions, estimated by employing a Langmuir model, ranged from 1.85 to 14.3 mmol Cu mol−1 dissolved organic C (DOC), which is the same order of magnitude as similar measurements of DOM from other sources.