*2.9. Geochemical Analysis*

The calcium ion concentration of column e ffluent was measured using an Ion chromatograph (IC). Dionex ICS 5000+ Cation analysis was conducted using 20 mM methanesulfonic acid eluent starting concentration, on a Dionex CS12A column, using 112 mA suppressor output. The first 5 mL e ffluent from each column was discarded at this stage, since this may contain some unreacted substrates that had been held within the column outlet, outlet tubing had otherwise been drained after each treatment. Beyond this point, up to 20 mL the e ffluent was mixed for each column to obtain a representative average concentration of calcium ions for each column. The e ffluent beyond this point was not tested, since this may contain the new CM being injected at later stages in the treatment process as pore volume decreases. E ffluent samples were tested daily within two hours of collection to ensure accuracy of results.

### *2.10. Quantification of Calcium Carbonate Precipitate*

The calcium carbonate content of the sand columns was quantified using a calcimeter, in accordance with Eijkelkamp [32] and the method of Scheibler, to determine carbonate content based upon a volumetric method. Carbonates in the sample are converted to CO2 by adding HCl to the sample. The pressure of the CO2 released causes water in a burette to rise and this di fference in level from the start of the test enables calcium carbonate content by mass, w(CaCO3*)*, to be calculated according to the below equation obtained following calibration, whereby *V*1 is the volume of carbon dioxide produced by the reaction of the test portion.

$$w(\text{CaCO}\_3) = 0.0045 \times V\_1 \tag{5}$$

The result of the above divided by the mass of the sample tested gives the percentage of the sample which contains calcium carbonate. To obtain an estimate of the average calcium carbonate content of each column, as a percentage of the total dry mass, samples of between 4 g and 5 g were taken from the top, centre and base of each column for testing after oven drying at 105 ◦C. Tests were conducted at constant room temperature of 23 ◦C.
