3. Materials and methods
3.1 Anion exchange resins
In the present work, starch and 2-naphthol uptake as well as NOM (fraction) adsorption from
“real” water samples, obtained from a demineralisation water supply company, was studied on four fresh AERs (Table 2). The resins were selected in order to cover a broad range in matrix material and functional groups. All four AERs hold a MP structure and were used in the hydroxide form, which is the application form in demineralisation plants of power stations after the cation exchange unit.
AERs characteristics were obtained from product data sheets of the manufacturers. The total volume (anion) capacity TVC (moleq/L) of an AER, which is the sum of functional groups present on the resin material per volume, was calculated from the experimental sulphate BTC (up to c/c0 = 1) by application of the integral mass balance equation for real BTCs (see Equation (2)). Sulphate BTCs were obtained from fixed-bed studies with 0.01 M Na2SO4 from KMF (neutral condition with pH 6) and 0.01 M H2SO4 from Merck (acidic condition with pH 1.9). The solutions were prepared with Millipore water (pH ≈ 6, conductivity < 1 µS/cm, and TOC < 50 µg/L) obtained from a Millipore ultrapure water system (Elix/Milli-Q Academics). The effluents were analysed by titration measurements with 0.02 M HCl or NaOH as titration solutions (both from Merck) and bromothymol blue as indicator (also from Merck). The detection limit was determined to be 0.0015 M. The evaluation of the TVC based on three column experiments for each resin. The relative standard deviation of the TVC values was determined to be about 2.9 %.
Significantly different TVC values were found under neutral and acidic conditions for the resins (see Table 2). AERs with tertiary amine functional groups need acidic conditions for their optimal function (see section 2.2). This is the reason for very low TVC values at neutral pH for weak and medium base AERs IRA96 and AP246. Also, for the strong base AERs IRA900 and A860 about 17 % lower capacities were found under neutral conditions compared to acidic ones. This indicates that the investigated strong base AERs hold not only quaternary amine functional groups, but also few tertiary ones.
Furthermore, four additionally fresh AERs (also in hydroxide form) were investigated under acidic pH conditions for NOM adsorption from the “real” water sample after cation exchange (Table 3).
Table 2. Properties of AERs investigated for starch and 2-naphthol adsorption as well as for NOM uptake from “real” water samples
Parameters
Resin type
MP weak and medium base AERs MP strong base AERs, type I
IRA96a) AP246b) IRA900a) A860c)
functional group tertiary amine tertiary/quaternary amine, type I
quaternary amine, type I
quaternary amine, type I matrix material polystyrene polyacrylic polystyrene polyacrylic
structure MP MP MP MP
a) Rohm and Haas, France S.A.S., Chauny Cedex, France; b) Bayer AG, Leverkusen, Germany; c) Purolite, Bala Cynwyd, USA; AERs = anion exchange resins; NOM = natural organic matter; MP = macroporous; TVC = total volume (anion exchange) capacity (median value ± measurement uncertainty after student t-distribution); ρB = bed density; ρP = particle density; εB = bed porosity; dP = particle diameter; rP = particle radius
MP64d) MP500d) MP600d) VPOC1071d)
functional group tertiary/quaternary matrix material polystyrene polystyrene polystyrene polyacrylic
structure MP MP MP gel uncertainty after student t-distribution); ρB = bed density; ρP = particle density; εB = bed porosity; dP = particle diameter; rP = particle radius
The cleaning procedure of the resins was optimised to minimise organic leachables from the AERs in batch and column experiments. Best results were found for the following procedure applied for 100 mL resin: washing with Millipore water, threefold shaking (200 rpm) for 1 h
with 500 mL 0.1 M NaOH, sequenced treatment in a soxhlet reactor first with 250 mL methanol and after that with 250 mL acetonitril (each for 24 h), washing with Millipore water and rinsing with 400 mL 1 M NaOH, 300 mL 1.4 M HCl and again twice with 400 mL 1 M NaOH (each step with 4 BV/h), and final washing with Millipore water. NaOH were obtained from VWR and HCl, methanol and acetonitril from Merck. All AERs were stored in Millipore water.
The success of the cleaning procedure by soxhlet-extraction was tested subsequently in batch experiments. For this, 50 mg untreated as well as soxhlet-treated resins were shaken (200 rpm) for 24 h with 100 mL Millipore water. The amount of organic leachables (analysed as DOC using a TOC analyzer multi N/C UV HS from Analytik Jena AG) in the decanted water solutions was measured. The DOC concentrations in the water samples for the experiments with untreated as well as soxhlet-treated resins are given in Table 4.
Table 4. DOC content in the water samples after batch experiment with untreated and soxhlet-treated AERs
Parameters
Resin type
MP weak base AER MP strong base AERs, type I
IRA96 IRA900 A860
Experiment with untreated resins
DOC (µg/L) 630.3 245.8 29.3
Experiment with soxhlet-treated resins
DOC (µg/L) 352.3 76.2 5.7
DOC = dissolved organic carbon; MP = macroporous; AER = anion exchange resin
As can be seen in Table 4, considerably lower DOC concentrations were found with soxhlet-treated resins. This is valid for all investigated AERs.
Several methods were compared to dose accurately small quantities of resin without loss of TVC as a consequence of drying processes. It was found that preparation methods for anion exchangers from other authors like vacuum filtration and storing in a desiccator for 24 h (Boyer et al., 2008) or desiccation in vacuum at 325 K for 24 h (Zhang et al., 2009), could not be applied for the AERs without a significant loss of TVC. Therefore, the Millipore water, in which the AERs were stored, was merely decanted before the resins were weighted (1.00 ± 0.02 g wet resin is equal to 1.11 ± 0.05 mL resin volume, which is herein after referred to as reactor volume VR).