Magnetic separations at very low magnetic field gradients (<100 tesla per meter) can now be applied to diverse problems, such as point-of-use water purification and the simultaneous separation of complex mixtures. High-surface area and monodisperse magnetite (Fe3O4) nanocrystals (NCs) were shown to respond to low fields in a size-dependent fashion. The particles apparently do not act independently in the separation but rather reversibly aggregate through the resulting high-field gradients present at their surfaces. Using the high specific surface area of Fe3O4 NCs that were 12 nanometers in diameter, we reduced the mass of waste associated with arsenic removal from water by orders of magnitude. Additionally, the size dependence of magnetic separation permitted mixtures of 4- and 12-nanometer-sized Fe3O4 NCs to be separated by the application of different magnetic fields.

Contaminants of environmental concern commonly reside in the sediment or solid phase. The extent and rate of desorption has heretofore been particularly unpredictable. In the present research, the adsorption and desorption of seven organic compounds with water solubilities ranging from 0.005 to 517 mg/L have been studied in natural sediments. In every case, a fraction of the adsorbate was adsorbed irreversibly (i.e., desorption was not the opposite of adsorption, yet the sorbate is not covalently bonded to the sediment). Each sediment-contaminant combination exhibited a fixed maximum irreversible adsorption, , which could be filled in one or several steps and which is related to common molecular properties and sediment organic carbon content (OC). For most compounds, (μg/g) ≈ 103.8OC. Furthermore, the OC-normalized partition constant for this irreversible compartment is es sentially constant for the compounds and sediments studied with = 105.53±0.48 mL/g. After about 1−3 days of contact time, all laboratory adsorption and desorption data could be modeled using a single isotherm equation, based upon commonly measured chemical and sediment parameters. The isotherm equation consists of two terms, a linear term to represent reversible sorption and a Langmuirian-type term to represent irreversible sorption. This combined isotherm is used to interpret numerous published field studies. The potential impact of this model on sediment quality criteria (SQC) and remediation are discussed.

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A new method is described for studying, reproducibly, the kinetics of crystallization of minerals under conditions of constant solution composition even at very low supersaturations. For calcium phosphates the method provides direct evidence for octacalcium phosphate as the precursor to hydroxyapatite precipitation at physiological pH.