XAD-8 & XAD-4 Organic Matter Separation

Background

XAD-8

Composition: acrylic ester

Pore diameter: 250 angstroms

Specific surface area: 140 m2/g

Tendency: hydrophobic acid fraction (fulvic acid)

Aliphatic carboxylic acids of 5-9 carbons
One & two ring aromatic carboxylic acids
One & two ring phenols
Aquatic humic substances

XAD-4

Composition: styrene divinylbenzene

Pore diameter: 50 angstroms

Specific surface area: 750 m2/g

Tendency: transphilic acid fraction

Polyfunctional organic acids
Aliphatic acids with five or fewer carbons

low molecular weight solutes. This affinity is a function of the solute’s aqueous solubility and the resin’s usable surface area. As solubility increases the k’ factor decreases.

General Process Diagram

Note: NOM neutrals can be removed with CH3CN, but this extra step is rarely performed

Q = flow rate

BV = bed volume

= 12 ml max.

Vo = void volume =  x BV

 = porosity = 0.65

VE = breakthrough volume required for saturation

k’ = column capacity factor

IDcolumn (20 x ODbead)

Selection column

Sample volume and column size are chosen where the solute, NOM (with k’=50, per Leenheer, 1981), is 50% retained by column. Want to find the breakthrough volume at which 50% of the total mass of solute has been retained (VE(0.5r)).

VE(0.5r) = 2Vo(1+k’0.5r)

There are two options available depending on sample volume available. If sample volume is limited, set VE(0.5r) and solve for Vo and subsequent BV. If not, then set the BV and subsequent Vo and solve for VE(0.5r).

The reality of k’ is that it is not thermodynamically dependent. There is some adsorption capacity of the resin that allows for 50% breakthrough. If the capacity is exceeded, breakthrough will occur earlier. Per George Aiken, 15 mg/L DOC is the limit used before something needs to be changed (i.e. add more resin, dilute the sample).

The reality of achieving exactly 50% breakthrough at the end of the run is that this will not happen with variations in DOC content and flow. It would appear that the best solution would be to be conservative, using the low end of the flow range and/or using a little more resin. The k’ and relation to DOC absorption capacities, etc. is being explored to obtain a better understanding and make recommendations on flow and resin volume.

Solving for VE(0.5r)

XAD-8

BV = 8 ml (recommended)

Vo = 0.65(8 ml) = 5.2 ml

k’0.5r = 50

VE(0.5r) = 2(5.2 ml)[1+(50)] = 530.4 ml

XAD-4

BV = 6 ml (recommended)

Vo = 0.65(6 ml) = 3.9 ml

k’0.5r = 50

VE(0.5r) = 2(3.9 ml)[1+(50)] = 397.8 ml

Solving for Run Time (Time)

Recommended range of flow rates vary between resins. Notice that the run times are kept about the same for each resin. This is not required, but is recommended.

XAD-8

Q = 1.3 to 2.0 ml/min.

XAD-4

Q = 1.0 to 1.5 ml/min.

Humic Substances

10/06/18XAD - Background.doc.doc