Denitrification with isotopes

[Seven]

Dear users,
 I try to do the de-nitrification function by Phreeqc, and use the isotopes, 18O and 15N. The code I used is following the answer that other users have asked. But I'm not sure if there is any mistake, or this code can't be used in the case which contained the isotopes.
 The following is the information of the code:
http://phreeqcusers.org/index.php/topic,465.msg1412.html#msg1412 .



Title      TW-1      
            
SOLUTION 1            
units             
pH      6.87       
temp      25.4       
S(6)      227    as SO4-2   
N(5)      1004.93     as NO3-   
N(-3)      1294.35     as NH4+   
DOC      0.4
O(0)      0.31
[18O]      19.0    0.1   
[15N]      6.6          

SOLUTION_MASTER_SPECIES
#
#element   species   alk   gfw_formula   element_gfw
#
Doc   Doc   0.0   Doc   30

SOLUTION_SPECIES
Doc = Doc
   log_k 0
2 NO3- + 12 H+ + 10 e- = N2 + 6 H2O
   #-log_k   207.08
        -log_k        0
H+ = H+
      -gamma 9.0 0
e- = e-
H2O = H2O



RATES
  Nitrification
-start
10 Amm = MOL("AmmH+")
#20 if (Amm <= 0) then goto 200
30 p_Oxg = SR("O2(g)")
40 moles = 3.10e-10*Amm/(100.10e-6+Amm)*(p_Oxg/(15.10e-6+p_Oxg))*TIME
200 SAVE moles
-end
  Denitrification
-start
10 NO3 = MOL("NO3-")
#20 if (NO3 <= 0) then goto 200
25 p_Oxg = SR("O2(g)")
30 moles = 2.22e-9*NO3*(1 - p_Oxg/(15.10e-6+p_Oxg))*TIME
200 SAVE moles
-end
END
KINETICS  1-19
#-cvode
Nitrification
    -formula Amm -1.0 NH3 1.0
    -m 4.60
    -tol 1e-8
-step_divide 1
-runge_kutta 3
Denitrification
    -formula N2 -1 CH2O +1 Ntg 1
    -m 24.4
    -tol 1e-8
-step_divide 1
-runge_kutta 3
END

[dlparkhurst]

The approach to isotopes codified in the iso.dat database is that two isotopes of the same element act as separate components in a geochemical system. Thus, there are definitions for N, which represents 14N, and for [15N]. The definitions are almost identical, but if there is fractionation information, then the log Ks for [15N] will differ slightly from the log Ks for N (14N)

To model fractionation of nitrogen isotopes in nitrification and denitrification, it is necessary to define rates for [15N] in addition to rates for N. So you need two more rates: 15N nitrification and 15N denitrification. If the rates are identical between 14N and 15N, then there is no fractionation. If the rate for one isotope, say 14N, is slightly faster, then the product will be enriched in 14N.