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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Higher environmental standards have made the removal of arsenic from water an important problem for environmental engineering. Iron oxide is a particularly interesting sorbent to consider for this application. Its magnetic properties allow relatively routine dispersal and recovery of the adsorbent into and from groundwater or industrial processing facilities; in addition, iron oxide has strong and specific interactions with both As(III) and As(V). Finally, this material can be produced with nanoscale dimensions, which enhance both its capacity and removal. The objective of this study is to evaluate the potential arsenic adsorption by nanoscale iron oxides, specifically magnetite (Fe 3 O 4 ) nanoparticles. We focus on the effect of Fe 3 O 4 particle size on the adsorption and desorption behavior of As(III) and As(V). The results show that the nanoparticle size has a dramatic effect on the adsorption and desorption of arsenic. As particle size is decreased from 300 to 12 nm the adsorption capacities for both As(III) and As(V) increase nearly 200 times. Interestingly, such an increase is more than expected from simple considerations of surface area and suggests that nanoscale iron oxide materials sorb arsenic through different means than bulk systems. The desorption process, however, exhibits some hysteresis with the effect becoming more pronounced with small nanoparticles. This hysteresis most likely results from a higher arsenic affinity for Fe 3 O 4 nanoparticles. This work suggests that Fe 3 O 4 nanocrystals and magnetic separations offer a promising method for arsenic removal. r

Several unique features of sorption irreversibility have been investigated in this paper. Adsorption has been found to be biphasic with about 30−50% of the adsorbed mass residing in the irreversibly sorbed compartment, until this compartment is filled, and the rest of the mass resides in the labile compartment. Naphthalene in the reversible compartment follows a linear adsorption isotherm with a normal organic carbon-based partition coefficient. A finite fixed total compartment size is observed for the irreversible fraction, (μg/g), on both natural and surrogate solids. In multiple batch adsorption/desorption experiments, the maximum concentrations that resist desorption are ≈ 10 μg/g for naphthalene on Lula sediment and ≈ 0.36 μg/g for 2,2‘,5,5‘-tetrachlorobiphenyl (2,2‘,5,5‘-CB) on both Lula and surrogate solids. The concentration in the irreversibly sorbed compartment varied with the initial naphthalene concentration available for adsorption. In addition, the amount in the irreversibly sorbed compartment increases linearly with the number of adsorption steps until the maximum concentration is reached. After the maximum concentration of the irreversibly sorbed compart ment is satisfied, the adsorption/desorption of naphthalene and 2,2‘,5,5‘-CB becomes reversible. The irreversibly sorbed compartment appears to be at equilibrium with the aqueous phase when the labile naphthalene or 2,2‘,5,5‘-CB is removed, but the equilibrium concentration is much lower than would be predicted with conventional hydrophobic partitioning theory. The aqueous phase concentration in equilibrium with the irreversibly sorbed compartment is about 2−5 μg/L for naphthalene and 0.05−0.8 μg/L for 2,2‘,5,5‘-CB. Similar adsorption/desorption phenomena are observed with both a natural sediment and a well-characterized sorrogate solid.