Flocculation In Natural And Engineered Environmental Systems - Chapter 9 pptx

Particulate material in aquatic systems covers a range of sizes greater than amillion-fold, from nanoscale colloidal particles to millimeter-sized flocs.1,9,11–14Particle size distributi

Saltwater Environments

9 Transport of Materials and

Chemicals by Nanoscale Colloids and Micro- to Macro-Scale Flocs in Marine, Freshwater, and Engineered Systems

Peter H Santschi, Adrian B Burd, Jean-Francois Gaillard, and Anne A Lazarides

CONTENTS

9.1 Introduction 191

9.2 The Structure and Properties of Fibrils 196

9.3 Mechanisms and Models of Colloidal Aggregation and Scavenging 198

9.4 Unresolved Questions 200

9.4.1 How Does the Presence of Metals Affect the Properties of Fibrils? 200

9.4.2 How Does the Presence or Absence of Fibrils Affect Particle Formation and Particle Aggregation Rates? 201

9.4.3 What Role Do Nanoscale Fibrils Play in Determining the Structure of Larger Scale Aggregates? 201

Acknowledgments 203

References 203

9.1 INTRODUCTION

Particles are the vehicles of vertical transport of material in aquatic systems Large, heterogeneous aggregates can sink through the water column at rates of 10 to 100 m per day carrying with them carbon, nutrients, and trace metals.1In the open ocean, sinking particles carry carbon (e.g., in the form of phytoplankton, detritus, and mucilage) from the surface waters to the sediments, thereby playing an important role in the global

1-56670-615-7/05/$0.00 +$1.50

carbon cycle.2These particles also carry nutrients, which help support food webs inthe mid-depths and benthos.3In estuarine and coastal systems, terrigenous particlessettle out of the water column removing clays and a large and variable amount oftrace elements In rivers, large quantities of suspended material are transported inthe form of nanoparticles.4 Nanoscale particles of Fe and Mn are also formed atoxic/anoxic transitions in aquatic systems.5–7Aggregation and subsequent settling ofparticulate material is a crucial step in many industrial processes such as those used

in water treatment plants.8This removal process, whose efficiency depends on thepresence of some principal components, that is, fibrillar microbial exudates, humic-type material, and mineral matter,9,10as well as environmental conditions, that is, pHand ionic strength, is depicted inFigure 9.1

Particulate material in aquatic systems covers a range of sizes greater than amillion-fold, from nanoscale colloidal particles to millimeter-sized flocs.1,9,11–14Particle size distributions in marine environments tend to follow a power-lawdistribution.15–19 Large particles (>100 µm) are relatively rare and represent the

dominant agent of sedimentation For example, for aggregates with equivalent ical diameters>1.5 mm, numbers of 4 to 40 aggregates/l, peaking at the euphotic

spher-zone and in mid-depth and near-bottom nepheloid layers, have been reported forthe Middle Atlantic Bight.20 Aggregate peak concentration regions coincided withstrong234Th deficiencies in the water column, demonstrating their high efficiency forscavenging particles and particle-reactive elements.20Sediment trap data and in situ

camera observations21–23indicate that marine particles settle as large, heterogeneousaggregates, such as marine snow (Figure 9.2).The sinking rate of an aggregate is afunction of its size, composition, and structure Dense, compact particles (e.g., fecalpellets) sink faster than larger, porous marine snow particles Differences in the tim-ing between peaks in surface particle concentrations and peaks detected by sedimenttraps throughout the water column indicate that these aggregates can have settlingvelocities of 50 to 100 m per day or more.24–26

Colloidal particles (operationally defined in environmental aquatic chemistry

as microparticles and macromolecules with sizes between about 1µm and 1 nm)

FIGURE 9.1 Diagram representing the major routes of the formation of large-scale aggregates

from the aggregation of fibrils and colloidal particles

Copyright 2005 by CRC Press

FIGURE 9.2 Marine snow Clear organic matrix that enmeshes fecal pellets and smaller

biomolecules.11

dominate the particle number density and surface area Ultrafiltration measurements27revealed typical concentrations of colloidal organic carbon (COC) in oceanic surfacewaters with sizes between about 1 nm (1 kDa) and<0.2 µm, of about 30 to 40 µM-C

(about 1 mg, organic matter/l), COC>3 kDa about 11 µM, and COC > 10 kDa about

3µM If marine colloids are present as spherical particles, the average molecular

weight of COC> 1 kDa in marine environments would be about 2 to 3 kDa This

should give an average particle number density in surface ocean water of 1014 to

1015nanoparticles per milliliter However, Wells and Goldberg28,29reported numberdensities of at most 109per milliliter of spherical nanoparticles they called “Koike”particles, a concentration that is similar to that in ground water where colloid con-centrations are in the range of a few micrograms per liter.30This large discrepancybetween expected and measured colloids concentration in marine environments indic-ates that (1) the majority of the colloidal fraction was undetected by Wells andGoldberg,12,28,29which is likely, since the colloids were not stained for transmissionelectron microscopy (TEM); (2) the assumption of spherical shape for calculating theaverage molecular weight is incorrect; this is likely, since many biomolecules are notspherical but fibrillar; (3) colloids are present as aggregates

Colloids are indeed present as aggregates, since recent observations of colloidalparticles using TEM31 and atomic force microscopy (AFM)14,32have revealed that

an important fraction of colloidal organic matter (COM) in aquatic systems is present

as nanoscale fibrils that also contain smaller molecules assembled like pearls on

a necklace(Figure 9.3) These fibrils are acid-polysaccharide rich, have diameters

of 1 to 3 nm and can be missed by standard fractionation techniques.14,31 Fibrilshave estimated molecular weights between 105and 106kDa and yet, because of theirshape, they are able to pass through a 10 kDa filter.14Wells and Goldberg12did not usestate-of-the-art preparatory and staining techniques for electron microscopy imagingand, therefore, were not able to document existing colloids in a representative manner.Santschi et al.,14Leppard et al.,33and Wilkinson et al.32used state-of-the-art electron

m 0 2.5 5.0 7.5

10.0

1200 (e)

FIGURE 9.3 Transmission Electron Microscopy (TEM) and Atomic Force Microscopy

(AFM) micrographs of nanoscale fibrils in aquatic systems (a) TEM whole mount men showing the interconnections between fibrils and nanoscale particles from the MiddleAtlantic Bight (courtesy of K Wilkinson; scale bar= 500 nm) (b) AFM image of fibrils

speci-and small nano-colloids from the Middle Atlantic Bight, with an architecture like pearls on

a necklace.14 (c) A specimen collected by centrifugation from a freshwater lake, Paul Lake(MI), imaged by TEM, showing fibrils rendered electron dense by the attachment of nanoscaleglobules of natural iron oxide (scale bar: 500 nm)6; (d) natural hydrous iron oxide aggregatesfound between 6.5 and 7.5 m in the water column of Paul Lake, where particulate Fe shows

a maximum, and below which [Fe2+] is increasing in concentration(scale bar = 1 µm).6

The TEM micrographs in (d) display intimate mixtures of organic fibrils naturally stained bynatural iron oxides The EPS spectrum shown in (e) of these mixtures shown in (d) displayssome Fe–Pb elemental association The Cu peak originates from the TEM grid

Copyright 2005 by CRC Press

and atomic force microscopy techniques to document the various forms, shapes,and architectures of marine and freshwater colloids from different environments.For the first time, polysaccharide-rich fibrils of recent (determined by radiocarbonanalysis; ref [14]) origin were documented to make up a significant fraction of allcolloidal sized nanoparticles (Figure 9.3).It is also important to realize that thesefibrillar extracellular polymeric substances (EPS) molecules are much more abundant

in the≤0.5 µm “dissolved” than in the ≥0.5 µm particulate fraction This is due to the

approximately two orders of magnitude higher concentration of DOC than POC in theocean, and the relative abundances of total and acid polysaccharides (APSs) that aresimilar in the two size fractions of organic carbon.34Being able to accurately detectthese nanoparticles is important because, although they are too small to settle out ofthe water column at appreciable rates, they do aggregate and are capable of formingthe matrix for the formation of larger aggregates that can settle faster.35,36However,

so far no quantitative estimate exists of their number concentration in marine systems.Transparent exopolymer particles (TEP, Figure 9.2 and Figure 9.3) form animportant component of aggregates in natural waters.37–41These particles are naturalexudates from marine algae and bacteria.42They consist of surface active polysac-charides rich in acidic functional groups43,44and are formed from the aggregation

of nanoscale fibrils.45,46Recent results, however, indicate that only a small fraction

of the total carbohydrate content of marine suspended and sinking matter consists

of surface-active acid polysaccharide compounds, with total uronic acids making upabout 7% (0.2% to 2% of POC), and total acid polysaccharides about 11% of the totalcarbohydrate, or about 1% of the POC content.34,47,48Thus, it appears that, much likesmall amounts of glue needed to hold man-made materials together, surface-activesubstances that provide the stickiness of the TEP do not have to be in high abundance

to be effective

TEPs have a high stickiness and their presence has been shown to stimulateaggregation amongst phytoplankton cells.43 As a matter of fact, times of highestparticulate organic carbon export from the ocean coincide with times of large phyto-plankton blooms, diatoms in particular,49 which are strong TEP producers as well

as providers of “mineral ballast,” enhancing density and settling velocity of ing particle aggregates This relationship was documented by a close relationshipbetween diatom pigments (fucoxanthin) and234Th-derived POC flux from the sur-face ocean,49,50 producing a higher efficiency of the “biological pump” (i.e., ratio

sink-of POC flux to primary production) In addition to phytoplankton species, bacteriaalso produce abundant acidic polysaccharide-rich compounds,31,42,51especially whenattached to particles as a “micro-biofilm.” Indeed, significant relationships betweenAPS concentrations and heterotrophic bacterial production (BP), and 234Th/POCratios and BP were recently demonstrated by Santschi et al.,47which strongly sug-gest microbial involvement through production of Th(IV)-binding APS compounds,while their enzymatic activities can produce smaller but more stable filter-passingTh(IV)-binding fragments

Macromolecular COM, a result of exopolymer formation by algae and bacteria,makes up 30% to 40% of conventionally defined dissolved organic matter.27,52–54The aggregation of fibrils and other biopolymers, with an architecture like pearls

on a necklace (Figure 9.3), into rapidly sinking marine snow provides an important

pathway for the removal of DOM and associated metals and radionuclides55,56fromsurface waters(Figure 9.1).

This important transport system is not, however, well understood A promisingresearch direction is suggested by potential gaps in conventional aggregation models.These models predict lower coagulation rates than those observed in nature It hasbeen suggested by Hill57that one could reconcile the model results with observations

if there existed a background distribution of particles, and by Alldredge and others thatthis background distribution can be accounted for by TEP(Figure 9.2andFigure 9.3)

Therefore, it would be important to characterize the hitherto neglected nanoscalecomponents of heterogeneous aquatic aggregates and integrate these componentsinto aggregation models, so that the models will be able to account for observedcoagulation rates

It is of great interest to aquatic scientists to better understand the processes bywhich components of these aggregates scavenge metals and pollutants and therebyendow the assembled aggregates with their pollutant-clearing properties Suspendedparticles can scavenge trace metals, providing an efficient mechanism for removingchemicals from solution.5,6,58–61Colloidal particles dominate the particulate surfacearea distribution, making them excellent at scavenging chemicals from the bulk water

In particular, metal oxides have been observed to coat fibrils (Figure 9.3c,d) So, tounderstand the removal of trace metals from solution requires understanding the prop-erties and dynamics of both the dissolved species and the properties of the particlesthat scavenge them

Extracellular polymeric substances (EPS) in specific marine or freshwater onments are known to initiate or modify precipitation of MnO2and FeOOH,62SiO2,63CaCO3,64 and uptake of different trace metals.56 Thus, the organic template can beimportant for mineral formation in the ocean These exopolymers are part of themarine DOC pool and have a modern radiocarbon age,14 as compared to the bulk

envir-of the DOC Microbially produced APS-rich compounds do not only have chelatingproperties for trace metals,31but also emulsifying properties through a protein tracecomponent, with the hydrophilic polysaccharide chains providing protective layersthat confer effective steric stabilization over time.65

In activated sludge flocs, EPS have been shown to be important for establishingthe floc pore structure,8whereby their relative composition can govern floc surfaceproperties and bioflocculation.66,67For example, the ratios of protein to total carbo-hydrates, hydrophobicity and surface charge are a function of EPS composition at thefloc/water interface, and thus are important parameters for predicting the extent ofbioflocculation.66–68Bacterial hydrophobicity appears to be a good overall parameterfor predicting the adhesion potential of their EPS to soil particles.69

9.2 THE STRUCTURE AND PROPERTIES OF FIBRILS

Aggregates in natural waters are composed of a disparate mixture of material: clayparticles, fulvics, fecal material, phytoplankton, extracellular polysaccharides, etc.1The essential ingredient of floc structure is a matrix composed mainly from struc-tural polysaccharides and peptidoglycans derived from cell exudates.31,70,71Thesemolecules form nanoscale fibrillar structures, which can be identified in a variety of

Copyright 2005 by CRC Press

aquatic environments.8,14,31,33,72 These polysaccharide-rich fibrils form 30% of theorganic material in freshwaters9,70and up to 60% in marine systems.14,73Fibrils aredistinct from terrestrially derived humic substances which account for the largest frac-tion (40% to 80%) of organic material in freshwater systems70and which typicallybehave as small nanoscale spherical particles.74–76

Early work on fibrils31 using transmission electron microscopy (TEM) showedthat, in the presence of phytoplankton and bacteria, a large fraction of autochthonousorganic material is composed of fibrillar particles rich in acid polysaccharides Thesefibrillar particles have been shown to stimulate aggregation (see ref [31] for a review)and to scavenge colloidal particles.10These fibrils have been found linked with ironparticles(Figure 9.3c,d)in both freshwater systems and batch reactors, leading to thesuggestion that fibrils can act as nucleation centers during oxidation reactions.6Properties of an aggregate, such as its settling speed, are dependent on its archi-tecture Aggregates typically possess a fractal structure.77–79For example, Alldredgeand Gotschalk,80demonstrated that marine snow aggregates settle with a velocity, v, proportional to d0.26rather than the Stokes relationship of d2, where d is the diameter

(Figure 9.4)

The relationship between mass (M) and size (L) of an aggregate is M = aL D,

where a is a constant and D is the fractal dimension of the aggregate Aggregates which preserve volume upon collision have D = 3; aggregates with D < 3 are more porous

and have a density which decreases as aggregate size increases.80Fractal dimensions

have been measured for aggregates in aquatic systems; in marine systems, D ranges

between 1.3 and 2.3.19,81–84In lacustrine systems, fractal dimensions range between1.19 and 1.69,81,85,86and in engineered systems from 1.4 to about 2.0 (see ref [87],and references therein) In general, for loose flocs, fractal dimensions are in the 1.7

to 1.8 range, and for more compact aggregates, they are of the order of 2.3 to 2.5.88,89After addition of small amounts (1 wt%) of cationic polymers, fractal dimensions

of aggregates in dewatered sludges from a waste water treatment plant decreasedfrom 2.2 to 1.75, amounting to a 2.5-fold decrease in density and a large increase inpermeability.90

Both fractal dimension and aggregate composition affect sinking rate Aggregateswith lower fractal dimensions are more porous and settle at slower rates than thosewith higher values Engel and Schartau91have shown that aggregates with a greaterproportion of TEP have lower sinking velocities and a less pronounced size-versusvelocity relationship indicating that the amount of TEP affects the architecture of theaggregate, possibly decreasing its fractal dimension It would therefore be important

to investigate the role of TEP in determining aggregate architecture, through structuraland modeling studies

9.3 MECHANISMS AND MODELS OF COLLOIDAL

AGGREGATION AND SCAVENGING

Scavenging of pollutants and trace metals depends upon the size spectrum of theparticulate material Large particles (e.g., greater than 50µm), although relatively

scarce, dominate the vertical flux because of their mass and large sinking velocity

On the other hand, colloidal particles dominate the particle number concentrationand adsorption kinetics Particle aggregation and disaggregation provide physicalmechanisms linking these two particle sizes — this is demonstrated in the BrownianPumping model92–96where trace metals are absorbed onto colloidal particles, whichsubsequently aggregate thereby incorporating the trace metals into larger particles.Scavenging and transport of materials, therefore, depend upon both the kinetics ofaggregation and adsorption, resulting in a particle concentration dependence of kineticconstants of metal transfer to particles with broken exponents.92,94,95

Two types of mechanism contribute to the formation of aggregates: particle sion and adhesion The classical theory of particle collisions is well developed, at leastfor particles of a simple shape.35,57,97,98The physical processes that bring particlestogether (Brownian motion, shear, differential sedimentation) are well described andhydrodynamic forces that can alter collision efficiencies can be taken into account.57,97Simple models assume that a single physical collision process operates in a givenparticle size range, but observations and more sophisticated models suggest thatthis is not the case.99–101However, on the whole, size distributions calculated fromaggregation models agree favorably with observed particle size distributions.102The probability that two particles will adhere once they have collided is less wellunderstood Traditionally, the DLVO (Derjaguin, Landau, Verwey, and Overbeek)theory has been used where the electrostatic and van der Waals forces between thetwo particles (and their environment) are evaluated to determine if the overall force isattractive or repulsive.103A coupling of statistical-based particle aggregation modelswith DLVO theory gives a good representation of the formation of aggregates com-prised of inorganic particles.103,104However, it has recently become apparent that such

colli-a model ccolli-annot fully describe colloidcolli-al intercolli-actions between colli-abiotic colli-and biotic loids in aquatic systems.105This is particularly important since biologically producedtransparent exopolymer particles (TEP) are thought to form the matrix around whichlarger aggregates form.43,45,71Indeed, steric forces may determine exopolymer inter-actions in seawater.106In addition, hydrophobic interactions and Brownian movementforces may also be important in particle adhesion involving bacterial exopolymers.107

col-Copyright 2005 by CRC Press

New experiments and models are needed to improve our understanding of exopolymerinteractions, and hence our ability to predict the stickiness of aquatic particles undervarious environmental conditions.

In many aggregation models, particularly those used to model aggregationbetween a broad range of heterogeneous particles, adhesion is usually describedusing a single, constant stickiness coefficient,α When α = 1, all collisions result

in attachment This produces aggregates that have open, highly porous structuresbecause small particles will have a low probability of diffusing to the central regions

of a larger aggregate before colliding with, and adhering to, some part of it Smallvalues of the stickiness coefficient result in more compact, less porous aggregates.Because of these structural differences, the value of the stickiness coefficient shouldaffect the sinking velocity of the particle since this depends on the particle’s excessdensity, and hence porosity Indeed, Engel108 has shown that increased TEP con-centrations enhance the stickiness coefficient during a diatom bloom In addition,Engel and Schartau91 have shown that particles with higher specific TEP contenthave lower settling velocities and a less pronounced variation of settling velocity withparticle size This indicates that the presence of biologically produced polymers canaffect the fundamental structure and physical properties of large-scale macroparticles,specifically their porosity or fractal dimension and settling velocity

Stickiness (α) is a function of many factors including pH, ionic strength, etc.

Using a combination of models and observational data, Mari and Burd41 estimatedthe stickiness between TEP particles as being 0.6, and that between TEP and non-TEPparticles as being lower at 0.3 Using radio-labeled colloidal organic matter, whichwas passed through silica columns, Quigley et al.55 determined a slightly higherstickiness factor of 0.88 for the polysaccharide enriched fraction (containing mostlyfibrils) vs 0.7 for the bulk fraction These estimates indicate that TEP concentration isimportant for determining the structure of aquatic particles; however, they are rarelyincluded explicitly in models

Simulations of particle aggregation in aquatic systems have usually been ted to considering aggregates composed of homogeneous primary particles, usuallyspheres In these simulations, all aggregates are assumed to have the same fractaldimension regardless of their size Aggregation dynamics proceeds by the standardSmoluchowsi model.97 These models have successfully incorporated particle sizesranging from 1 nm to 1µm and have been used to examine the scavenging of thorium

restric-from surface oceanic waters.96These models indicate the importance of particle size

in determining the adsorption rate of trace metals

In reality, environmental aggregates are highly heterogeneous.1,11The structureand physical properties of aggregates formed from monomers of different sizes differfrom those formed from monomers of a single size.109More sophisticated models thatcan include different particle types (e.g., phytoplankton and fecal pellets) have beendeveloped110,111and indicate the importance of particle aggregation for understandingthe vertical flux of material from the ocean surface

A different modeling approach has used combinations of small spherical particlesand polymer chains — bridging flocculation,112–114shown inFigure 9.5.The structure

of polymer chains varies with environmental conditions such as pH, and both tion kinetics and aggregate structure depend upon the concentration and conformation