Charles R. O'Melia

Fundamentals of flocculation

Flocculation (aggregation) of particles is a vital stage in many solid‐liquid separation operations. The process usually involves some form of chemical destabilization and a step in which particles collide and form aggregates (flocs). Destabilization may involve simply overcoming any repulsive forces between particles, as with simple salts, “bridging” between particles by polymeric flocculants, or the precipitation of metal hydroxides leading to “sweep flocculation” as with aluminum and iron salts. Recent work on interparticle forces, the action of polymeric flocculants, and the nature of species produced by hydrolyzing metal salts is reviewed and the relevance to practical flocculation processes is discussed. The kinetics of particle aggregation, aggregate size distribution, and the morphology of aggregates have received a great deal of fundamental attention recently. Most of this work is concerned with rather ideal systems, but conclusions of much wider applicability can be drawn. In particular, the rel...

Natural organic matter and colloidal stability: Models and measurements

Laboratory and field observations by several investigators indicate that natural organic matter (NOM) affects and probably controls the colloidal stability of particles in aquatic systems. The enhanced stability of particles in aquatic systems containing NOM is a consistent observation without a clear cause. In this work, the potential importance of the macromolecular nature of NOM was investigated using model systems. A mathematical model for the adsorption of linear, flexible polyelectrolytes was used to examine the effects of molecular weight, pH, and ionic strength on the conformations of these surrogates for NOM at interfaces in natural waters. Laboratory experiments involving submicron hematite particles, two anionic polyelectrolytes, and an aquatic NOM were used to examine the effects of solution composition on colloidal stability. Together, the results of the mathematical simulations and the laboratory experiments indicate that electrostatic effects dominate particle—particle interactions, but that the macromolecular nature of NOM can have direct influence under certain conditions. At low ionic strength, anionic polyelectrolytes affect the coagulation of positively charged particles by altering net surface charge in a way similar to specifically adsorbing, multivalent, monomeric anions. At high ionic strength (I⩾ 0.1), the conformational characteristics of adsorbed polyelectrolytes at the solid/water interface directly affect coagulation by expanding the effective distance of electrostatic repulsion between approaching particles, as well as by altering net surface charge. Non-electrostatic steric repulsion plays little or no role in the stabilization of hematite particles by the organic macromolecules used in this work. Calcium acts to destabilize hematite particles in the presence of the organic macromolecules, perhaps through a combination of specific chemical and charge effects.

Effect of electrolyte type on the electrophoretic mobility of polystyrene latex colloids

Electrophoretic mobility studies of surfactant-free polystyrene latex particles in various types of inorganic electrolytes are reported. The particles carry sulfate functional groups and cover a wide range of surface charge. The electrophoretic mobility curves of all latices exhibit a pronounced maximum as a function of 1: 1 electrolyte (KCl) concentrations. With 2 : 1 and 3 : 1 electrolytes (CaCl* and LaCl, respectively), the electrophoretic mobility curves pass through a minimum followed by a maximum. The results are discussed in terms of models that have been previously proposed to explain these maxima in the mobility curves. Our results suggest that the increase of mobility with salt concentration may be attributed to the approach of co-ions close to the hydrophobic surface of the particles. It is also suggested that three competing processes at the interface determine the shape of the mobility curves.

Particle-particle interactions in aquatic systems

Abstract The significance of particles in the transport and fate of substances in aquatic environments is illustrated and present knowledge of the speciation and structure of interfaces in these environments is assessed. The importance of particle deposition and aggregation in ground water aquifers and in lakes is illustrated. Attention is focussed on the adsorption of natural organic macromolecules and polyelectrolytes at interfaces in natural systems and the impacts of these substances on colloidal stability and deposition in natural systems. Finally, the status of present understandings in this area is assessed and directions for future study are proposed.

Removal of naturally occurring compounds by coagulation and sedimentation

Naturally occurring compounds are defined as all nonsynthetic materials of constant composition. The emphasis in this review is placed on the removal of humic materials, other organic compounds, trace metals, and certain inorganic ligands. Removal of the alkaline earths is not reviewed. Associations between different compounds, or between compounds of interest and naturally occurring particulates, are important and complex. In this context, the formation and removal of such aggregates is reviewed. As a result, the removal of particles by coagulation and settling is briefly discussed. The removal of certain synthetic compounds is also reviewed, to the extent that removal of such compounds is dependent on the removal of naturally occurring compounds. Both inorganic and organic coagulants are considered.

Origins and effects of coagulation in lakes

A physical model for the transport and deposition of particles in lakes is formulated mathematically, solved numerically, compared with some field results available in the literature, and used to simulate the effects of coagulation and sedimentation in some limnetic systems. The results indicate that these processes exert significant and perhaps controlling effects on the transport and fate of particles and associated pollutants and nutrients in lakes.

Particle formation and growth in dilute aluminum(III) solutions. Characterization of particle size distributions at pH 5.5

Abstract Particle formation and growth over the 1–40 μm size range in dilute aluminum solutions (approx. 2 × 10 −4 M) have been studied using an electronic particle counter. Sulfate, fulvate and hydroxide ion accelerate the rate of particle formation and changes of the particle size distribution over time. Increasing ionic strength (inert electrolyte) produces similar but less dramatic effects. Combinations of sulfate and fulvic acid or sulfate and inert electrolyte further accelerate the rate of particle formation. Aluminum chloride solutions at moderate ionic strength are devoid of supramicron particles after several days. A conceptual pathway model is developed which suggests that two different solids are formed when aluminum is added to fulvic acid solutions: an aluminum-fulvate precipitate and Al(OH) 2 (s). The first solid dominates in fulvic acid solutions at pH ∼5.5.

Particle size and countingSome effects of particle size in separation processes involving colloids

The size of colloidal particles has long been considered to be a major factor in their mass transport in aquatic environments. In aggregation and deposition reactions, the mass transport rates of colloidal particles are expected to decrease with increasing particle size since thermal diffusion dominates this process. Particle size has also been considered to affect colloid chemical interactions, at least on theoretical grounds, with predicted rates of attachment in aggregation and deposition processes decreasing substantially as colloidal particle size is increased. Observations have failed to confirm this prediction. The situation becomes more complex and predictions more consistent with observations when reversible aggregation and deposition such as can occur in secondary minima are considered. This is done in this paper in simulations using a combination of Brownian dynamics and Monte Carlo (BDMC) techniques. Some experimental observations of virus transport in porous media are presented. The BDMC simulations provide a partial explanation for these results.

Peer reviewed: making watershed management work.

New York Citys watershed management strategy is unprecedented in its scope, scale, and cost.

Editorial. Water cleanup strategies.

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