My research

SAPS’ INFLUENT AND EFFLUENT GEOCHEMICAL PARAMETERS RELATED TO AMD POLLUTION CONTROL: A CASE STUDY

Jayanta Bhattacharya1, Gil Jae Yim2, Sang Woo Ji 2, Hyeon Seok Lee 2 and Young Wook Cheong 2
1Department of Mining Engineering, Indian Institute of Technology,Kharagpur-721302,India
2Geological & Environmental Hazards Division, KIGAM, 30 Gajeong-Dong, Yuseong-Gu, Daejeon 305-350, Korea

Abstract
This paper presents a time-series study on the geochemical parameters of a Successive Alkalinity Producing System (SAPS) in an abandoned mine of South Korea. Two new performance parameters in terms of ratio are proposed: Feinfluent : Feeffluent and Sulfateinfluent : Sulfateeffluent. The ratios directly point to the state of performance of SAPS. After the 6 year study, iron reduction still remains acceptable but sulfate reduction becomes negligible.

Introduction
The aim of the study was to verify whether the empirical relationships of different wetland geochemical parameters are valid for the Successive Alkalinity Producing System (SAPS) under discussion, as well as to develop measures of long-term performance applicable to all SAPS or treatment wetlands. A SAPS is made up of: a) the entrainment to carry the mine water under a reduced state; b) constructed or naturally embanked wetland containing vertical flow reactor (VFR) and c) one or more aerobic wetlands. The VFR is the main reacting compartment where acidity is neutralized and sulfate ions are decomposed. The reactor contains submerged spent mushroom compost substrate (SMS) on top of a column with limestone particles on bottom. High alkalinity is generated to prevent Fe2+ ions from being converted to Fe3+ ions, with consequent precipitation in the VFR; as well as to decompose SO42- into S2- and promote the precipitation of sulfides that increase precipitation over and above hydroxides. Since sulfides in water generally have lower solubility than hydroxides over a wider range of pH, when both hydroxides and sulfides occurring togehter the total metals precipitation increases )e)Finally, an aerobic wetland allows the settlement of particles and favors further neutralization.
Figure 1 gives a complete scheme of SAPS.


Figure 1. The scheme of a SAPS.


Methods
Over a six year period, influent water to the SAPS and effluent water from the SAPS were studied.. Dissolved cations were determined using an inductively coupled plasma atomic emission spectrophotometer (ICP-AES; Jobin Yvon Co. 138 Ultrace) . Anions were determined using the ion chromatograph (IC; Dionex series 500DX).Table 1 and Table 2 provide the complete observed data of the influent and effluent connected to the SAPS, respectively. The data sets contain 15 sampling data of parameters like temperature, electrical conductivity (EC) and pH, and concentrations of dissolved oxygen (DO), Fe(III), Fe(II), Al, Mn and SO42- .
Table 1. Determined parameters of the influent during the period of study.
Date Temp
(° C ) EC
(mS/cm) pH DO
(mg/L) Fe(III)
(mg/L) Fe(II)
(mg/L) Fe(II)/Fe(III)
Al
(mg/L) Mn
(mg/L) Sulfate
(mg/L)
Aug-01 16.2 90 5.5 6.9 47.9 0.9 0.02 1.1 5.7 445.9
Nov-01 15.1 130 5.8 5.9 16.6 2.5 0.02 0.9 5 486.4
Mar-02 15 120 5.9 5.3 6 49.9 8.32 0.6 4.4 278.9
May-02 15.5 440 5.8 4.5 43.6 3.2 0.07 0.7 4.1 304.2
Sep-02 15.9 120 5.3 2.9 4.1 35.7 8.71 0.9 4.5 514
Nov-02 15.4 130 5.6 3.9 4.6 58.8 12.78 1.1 5.8 436
Mar-03 14 140 5.7 4.6 0.2 50.8 254 0.8 4.8 478.6
Jun-03 16.2 100 5.8 2.6 16.8 29.1 1.7321 0.9 4.9 395.8
Sep-03 16.1 120 4.6 2.4 52.5 17.4 0.33 11.3 6.3 548.6
Dec-03 14.9 90 5.7 3 16.3 50.8 3.11 1.3 11.4 503.6
Mar-04 14.8 110 5.8 3.2 16.3 55.3 3.39 1.1 11 478.4
Jun-04 16.3 378 6 3.7 56.3 1 0.018 1.7 8.1 737.8
Nov-04 14.8 100 5.8 6.2 3 49.2 16.4 4.9 0.5 308
Mar-05 14.6 130 5.8 3.8 2.4 57.6 24 1.8 0.8 443.3
Jul-06 16.9 100 6 3.31 12.3 24.4 1.98 0.01 4.25 436


Table 2. Determined parameters of the effluent during the period of study.
Date Temp
(° C) EC
(mS/cm) pH DO
(mg/L) Fe(II)
(mg/L) Fe(III)
(mg/L) Fe(II)/Fe(III)
(up to 4 decimals) Al
(mg/L) Mn
(mg/L) Sulfate
(mg/L)
Aug-01 24.6 0.09 6.3 3.7 0.4 0.6 0.67 0.4 6.5 181.2
Nov-01 12.3 0.13 6.6 13.1 0.6 0.4 1.5 0.4 5.7 461.1
Mar-02 10.4 0.13 7.1 10.9 0.3 0.1 3 0.3 3.8 389.6
May-02 17.9 3.45 6.6 3.5 0.1 0.7 0.14 0.2 4.2 284.5
Sep-02 22.4 0.14 6.7 5.5 0.1 0.6 0.17 0.9 5.8 554
Nov-02 15.5 0.13 6.7 10.7 0.5 0.3 1.67 0.4 4.4 440
Mar-03 14.1 0.13 6.6 12 0.2 0.2 1 0.6 3.5 487.3
Jun-03 20.5 0.13 6.7 2.6 0.2 0.5 0.4 0.5 4.5 339.8
Sep-03 20.2 0.1 6.5 4 0.1 0.7 0.14 0.5 4.9 467.8
Dec-03 9.3 0.08 6.6 5.6 0.3 0.1 3 0.9 2.2 497.6
Mar-04 8.5 0.11 6.6 4.4 0.5 0.4 1.25 0.9 0.8 499.6
Jun-04 20 0.18 6.9 3.9 0.2 0.7 0.29 0.8 1 283.8
Nov-04 13.6 3.61 6.4 6.3 0.3 0.1 3 1.5 0.2 365.7
Mar-05 8.9 0.01 6.8 4.7 0.1 0.1 1 1 0.8 437.5
Jul-06 23.7 0.17 7.34 3.66 0.3 0.1 3 0.02 2.88 344

Results

1. Iron loading and treatment efficiency
Ferrous iron in the influent occurs in the range of 0.9 to 57.6. The influent Fe2+ concentration is higher than Fe3+ in 10 out of 15 cases. Though Fe2+ concentration is expected to be always higher than Fe3+, the variation can be attributed to the lack of equilibrium in the running water. In the case of the effluent, Fe2+ is higher than Fe3+ in 7 cases but the concentrations of both Fe2+ and Fe3+ are in the range of 0.1-1 mg/L, much lower than in the influent The total reduction of Fe species is given in a performance ratio of Fe ( influent) dividedby Fe ( effluent).
The SAPS performance can be assessed by the ratio of total Feinfluent to total Feeffluent during the period of study: except for the spike observed in June’2004 as in fig.3 , the average values remain within a range of 50 to100. This indicates an efficient iron reduction.






Fig.2. Performance ratio ( Fe) of the SAPS over the period.





Fig.3 Sulfate reduction Performance ratio during the study period.
The same cannot be told of sulfate reduction. In fact, the sulfate reduction performance ratio (influent sulfate to effluent sulfate ratio) shows an average value of about 1, meaning that sulfate is discharged without much reduction. The ratio is high (2.5) in August 2001 but begins to decrease down to about 1 in March 2002 before stabilizing definitely. Neglecting the sulfate contribution from the sediment, this points to organic carbon consumption in the system, a phenomenon that can be called as carbon drought in SAPS.

2. Sulfate and Electrical Conductivity
Poor correlation between sulfate and conductivity was observed in both the cases of influent and effluent so that any trend could be recognized. The conductivity values are lower in the influent than in the effluent. Large differences in conductivity ranges was not observed: in influent between 90 and 140 mS/cm, and in effluent between 80 and 180 µS/cm .

3. pH and Aluminum
pH and aluminum values, though not very well correlated, in general show respectively increasing and decreasing trends in the influent and effluent.

4. Ferrous and Ferric Ions
In both the cases of influent and effluent water Fe2+ and Fe3+ exhibit a negative relationship, i.e. one increases at the expense of the other. The figs. 4 and 5 show the plots of Fe 2+ and Fe 3+ in both influent and effluent.This relationship is more definite in the influent water (r2 = 0.6285) than in the effluent water ( r2 = 0.053) as concentration decreases. The decrease in Fe2+ and Fe3+ concentration are supposed to alter the relative concentration of other cations and anions .





Fig.4. Ferrous and Ferric Ion relationship in the influent





Figure 5. Ferrous and Ferric Ion relationship in the effluent
5. Mn2+ and Fe2+ Ions
The Mn2+ and Fe2+ ions, in both influent and effluent, are very poorly correlated with each other. Manganese concentration is nearly independent on Fe2+ concentration in influent as well as effluent. The average Mn concentration in the influent is about 6 mg/L; Mn concentration in the effluent decreases to about 4 mg/L with the scatter suggesting re-solubilization from the sediment. This signifies that effective co-precipitation in the wetland is only about 33%.

Discussion
From the results of the study, it can be pointed out that:
1. The proposed performance ratios of iron removal (Feinfluent : Feeffluent ) and sulfate removal (Sulfateinfluent : Sulfateeffluent ) can be representative of SAPS performance under steady state operation that possibly sets in after a month.
2. Sulfate reduction to sulfide is stopped within a year possibly because of the decreasing concentration of dissolved organic carbon as the SMS is consummated. Over time, the mineralization of the AMD (the variation in concentration of acid and base) shows a decreasing trend, as indicated by lower values in conductivity.
3. Aluminum concentration, though poorly correlated with pH, decreases as pH values increase, a typical characteristic of AMD.
4. Fe2+ and Fe3+ concentrations exhibit a negative relationship. This is possibly because of reduction in concentrations of both in the effluent as well as by the increasing influence of other ions.
5. The expected positive linear relationship of Mn2+ and Fe2+ failed because of extremely poor correlation.

Conclusions
The study evaluates the parameters related to performance of SAPS. The salient findings of this work are as follows:
1. Sulfate reduction almost comes to a stop within a year of SAPS operation. As a result, hydroxide precipitation is likely to prevail in the wetland as against the desired combined precipitation of hydroxides and sulfides.
2. In both the cases of influent and effluent water, excluding two high values, the conductivity seems to be independent on the sulfate concentration.
3. pH and aluminum concentration show a poor negative correlation. Not only the value of aluminum decreases as pH increases but also the rate of decrease decreases with increasing pH.
4. In both influent and effluent Fe2+ and Fe3+ concentrations exhibit a negative relationship, i.e. one increases at the expense of the other.

Acknowledgement
Authors duly acknowledge the technical and financial support provided by the Korean Institute of Geosciences and Minerals (KIGAM) to conduct and report the study.

References
Barton C.D., Karathansis A.D. (1999). Renovation of a failed constructed wetland treating acid mine drainage. Environmental Geology 39(1), 39-49.
Berger A.C., Bethke C.M., Krumhansl J.L. (2000). A process model of natural attenuation in drainage from a historic mining district. Appl. Geochem. 15, 655-666.
Bigham J.M., Schwertmann U., Pfab G. (1996). Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage. Appl. Geochem. 11, 845-849.
Foos A. (1997). Geochemical modeling of coal mine drainage, Summit County, Ohio. Environmental Geology 31(3/4), 205-210.