Effects of carbonate on the electrolytic removal of ammonia and
urea from urine with thermally prepared IrO2 electrodes
Véronique Amstutz, Alexandros Katsaounis, Agnieszka Kapalka, Christos Comninellis, Kai M. Udert
Supplementary information
1) Calculation of the total charge passed, Q
𝑄=
𝐼∙𝜏
𝑉
Q is the charge passed through both electrolyte-electrode interfaces [Ah∙L-1], I is the current [A], τ is the
electrolysis time [h] and V is the volume of the solution [L].
2) Recipes for synthetic fresh and stored urine
The compounds have to be added in the order they are presented in the table, and every salt has to be well
dissolved before the addition of the next one.
Table 1 Synthetic fresh urine. Ac: acetate.
Compound
Concentration [g L-1]
Urea
16.0
NaAc anhydrous
10.25
Na2SO4 anhydrous
2.30
NH4Cl
1.80
NaH2PO4 anhydrous
2.90
KCl
4.20
MgCl2
0.370
CaCl2
0.510
NaOH
0.120
Table 2 Synthetic stored urine. Ac: acetate.
Compounds
Concentration [g L-1]
Na2SO4 anhydrous
NaH2PO4 anhydrous
2.30
NaCl
3.60
KCl
4.20
NH4Ac
9.60
25% NH4OH solution [mL·L-1]
13.0
NH4HCO3
21.40
2.10
1
3) Calculation of the initial and the overall current efficiencies, and of the conversion ratio of species i
The initial current efficiency for a reaction j is defined as
𝜂𝑗𝑖𝑛𝑖𝑡𝑖𝑎𝑙 = (
𝑑𝑛𝑖
)
⋅ 𝑧 ⋅ 𝐹 ⋅ 100%
𝑑𝑄 𝑄⟶0
The overall current efficiency for a reaction j is defined as:
𝜂𝑗𝑜𝑣𝑒𝑟𝑎𝑙𝑙 =
The term (
𝑑𝑛𝑖
)
𝑑𝑄 𝑄⟶0
Δ𝑛 ⋅ 𝑧 ⋅ 𝐹
⋅ 100%
𝐼⋅𝑡
[mol C-1] is the change of the amount of species i (ni in [mol]) as function of the charge
passed Q [C], when Q approaches time t = 0. We used the slope between the measurement points at 0h and 6h to
calculate this term. z is the number of exchanged electrons per mole of degraded species [mol mol-1], F is the
Faraday constant [F = 96,485 C mol-1], ∆n is the amount of product that has reacted [mol] during the complete
electrolysis experiment, I is the imposed current [A] and t is the total electrolysis time [s].
The conversion ratio of species i is defined as
Δ𝑛𝑖
𝑛𝑖𝑖𝑛𝑖𝑡𝑖𝑎𝑙
⋅ 100% where Δ𝑛𝑖 is the amount of species i that has
reacted [mol], and 𝑛𝑖𝑖𝑛𝑖𝑡𝑖𝑎𝑙 is the initial amount of species i present in the solution at the beginning of the
electrolysis experiment [mol].
4) Fit equations
Some of the curves shown in Figures 1 and 2 were fitted with the empirical curves given below using Igor Pro
software. The types of curves were chosen based on experiences with previous studies:
Figure 1c – CNtot curve:
CNtot = A+ Bexp(C ×Q)
with A = 7327.3, B = 959.15 and C = 0.29951
Figure 1d – Cactive chlorine curve:
Cactive chlorine = A +
B- A
C
1+ ( ) D
Q
with A = 0, B = 35.45, C = 10.17 and D = 1
Figure 2a – CNtot and CNH4 curves:
CNH 4 = A + BeCQ + De EQ
with A = 8680.1, B = -491.46, C = 0.012771, D = -481.69 and E = 0.0092258
2
CNH4 = A + BeCQ + De EQ
with A = 8298.1, B = -514.44, C = 0.012741, D = -504.64 and E = 0.009203
Figure 2b – Cactive chlorine curve:
Cactive chlorine = A + BeCQ
with A = 1.5775, B = -1.6188 and C = 0.19654
Figure 2c – CNtot and CNH4 curves :
TN = A+ BeCQ + De EQ
with A = 4591.1, B = 2486.9 and C = 0.16502
CNH 4 = A + BeCQ
with A = 3753.7, B = 2818.7 and C = 0.12226
Figure 2d – Cactive chlorine curve:
Cactive chlorine = A +
B- A
C
1+ ( ) D
Q
with A = 0, B = 106.35, C = 36.27 and D = 1
3