DSpace at VNU: Sources and leaching of manganese and iron in the Saigon River Basin, Vietnam

Sources and leaching of manganese and iron in the SaigonRiver Basin, Vietnam Nguyen Thi Van Ha, Satoshi Takizawa, Kumiko Oguma and Nguyen Van Phuoc ABSTRACT High concentrations of mangan

Sources and leaching of manganese and iron in the Saigon

River Basin, Vietnam

Nguyen Thi Van Ha, Satoshi Takizawa, Kumiko Oguma and

Nguyen Van Phuoc

ABSTRACT

High concentrations of manganese and iron in the Saigon River are major problems for the water

supply in Ho Chi Minh City (HCMC), Viet Nam To identify their sources and leaching processes, we

surveyed water quality along the Saigon River and ran batch leaching tests using soil and sediment

samples Two important leaching processes were identi fied: acidic leaching from acid sulfate soil

(ASS) in the middle reaches of the river, and Mn dissolution and Fe reduction from sediments in the

downstream reaches Low pH caused the concurrent release of Fe and Mn from the ASS In contrast,

anoxia caused the release of Fe but not Mn from the sediments, whereas low pH facilitated Mn

dissolution Sediments are a more important source of Mn because of their higher Mn contents

(10 times) and release rates (14 times) than those from ASS.

Nguyen Thi Van Ha (corresponding author) Nguyen Van Phuoc

Faculty of Environment,

Ho Chi Minh City University of Technology,

268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam

E-mail: ntvha2003@gmail.com

Satoshi Takizawa Kumiko Oguma Department of Urban Engineering, Graduate School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Key words|acid sulfate soil, iron leaching, manganese leaching, pH-Eh diagram, redox condition,

sediment

INTRODUCTION

Manganese and iron are common metals found in Earth’s crust

and in natural water Excessive exposure to Mn is associated

with adverse health effects and neurotoxicity, and retards the

intellectual development of children (Wasserman et al.)

The health-based Mn guideline value for drinking water is

rec-ommended at 0.4 mg/L (World Health Organization (WHO)

) and 0.3 mg/L (US EPA) In many countries, the

guideline values for Fe and Mn are set at lower concentrations

than the health-based guidelines The Vietnamese

environ-mental standards for Fe (QCVN 08:2009, A2) and for Mn

(TCVN 5942: 1995, A) are set at 1 mg/L and 0.1 mg/L,

respect-ively; and the US EPA’s levels are 0.3 mg/L and 0.05 mg/L,

respectively

Saigon River is the second most important source of water

for HCMC and Binh Phuoc Province However, it is facing

water quality problems of low pH, high turbidity, and high

con-centrations of Mn, ammonia and total coliform Tan Hiep water

treatment plant (WTP) reported that Fe and Mn concentrations

in the river water frequently exceed the Vietnamese standards

for water supply The monthly average concentration of Mn

was about 0.16 mg/L in the dry season from 2005 to 2008,

and in the rainy season it has reached 0.24 mg/L in the rainy

season, exceeding several times the US EPA health risk level

of 0.3 mg/L High Mn caused difficulties in operation of the WTPs (Kohl et al.) In rivers and lakes receiving industrial effluents, heavy metals and Mn are deposited in the sediments (Youger & Mitsch) The amounts of Fe and Mn leaching depend on their concentrations in the top sediment layer ( Guer-ios et al.), pH and redox condition (Eh) (Davison) There is limited information about sources of Mn and Fe in the Saigon River A better understanding of the Mn and Fe sources and their leaching processes is required to predict when and where its high concentrations will occur The objec-tive of this study was to identify the main sources and transport

of Mn and Fe entering the Saigon River to assist development

of river basin management strategies for controlling point and diffuse sources of Mn and Fe inputs into the river

MATERIALS AND METHODS Study sites

The Saigon River has a total length of about 280 km, its

rate is 85 m3/s The upstream Dau Tieng Reservoir is the

fourth biggest reservoir in Vietnam, with a storage capacity

of 1.48
109

m3 The Dau Tieng– Saigon River system is the

largest irrigation system in Vietnam and an important water

supply source At two water intakes in the middle section

(SG15 and SG16 inFigure 1), about 326,000 m3per day is

withdrawn for supply to HCMC and Binh Duong Province

The tidal effect sometimes went up to the location of SG9

(Figure 1) There is about 16,670 ha of ASS around the water

intakes in Cu Chi District in HCMC The Thi Tinh River

(ThT), the Rach Tra (RT) Canal and the Vam Thuat (VT)

Canal are the 3 main tributaries of the Saigon River The

ThT and VT receive wastewater from industrial zones in

Binh Duong Province and HCMC, respectively The RT

Canal collects most of the drainage from potential acid sulfate

soil (PASS) in the Saigon River basin and acidic drainage from

actual acid sulfate soil (AASS) in the Vam Co Dong River basin

Water sampling and analysis

On August 20th, 2006 we collected three surface water

samples (at 0.5 m) from the DT Reservoir (denoted as DT1,

DT9 and DT7) and 11 from the Saigon River (SG4 to SG22,

of which some points were skipped) On May 9th, 2008 seven river surface water samples (SG9 to SG20) and seven canal water samples (ThT, RM, RBC, RBB, RBL, RT, VT) were collected at the same depths We measured pH, turbidity and DO on-site by using a Horiba W-23XD probe, and measured total Mn and total Fe with a DR820 Colorimeter Soil and sediment sampling and analysis

Three undisturbed ASS samples were collected at locations S1

to S3 from two depths: 0–25 cm and 25–50 cm Fourteen sedi-ment samples were taken along the river and kept in plastic bags at 4W

C until the batch leaching tests were conducted Total Mn and Fe were measured following the US EPA method 3051 (US EPA) and expressed in mg/kg dry weight Batch leaching test

About 30 g of air-dried soil was mixed with 150 mL Milli-Q water or 150 mL acid solution at about pH 1.5

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(Mehlich 1 extraction solution: 0.025 N H2SO4and 0.05

N HCl) in serum bottles, shaken for 5 min on a

recipro-cating shaker at 180 oscillations per min, and then

incubated at room temperature in aerobic conditions

Lea-chates were sampled at 1, 18, 72 and 168 h after mixing to

measure pH and dissolved Mn and Fe by inductively

coupled plasma atomic emission spectrometer

(ICP-AES) Four sediment samples containing high amounts

of Mn and Fe (SG15, SG17, SG18 and SG19) were used

for batch leaching tests following the American Society

5 g of wet sediment was put into 120 mL serum bottles

with 100 mL synthetic Saigon River water (Naþ, 80.9 mg/L;

Mg2þ, 15.6 mg/L; Kþ, 17.2 mg/L; Ca2þ, 10.5 mg/L; Cl,

95.7 mg/L; NO3 , 3.5 mg/L; and SO4 –, 19.7 mg/L; pH 5.78;

potential (ORP), 216 mV) Duplicate samples were

pre-pared for aerobic (denoted as A) and anaerobic (denoted

as An) leachants with (denoted P) and without (no

annota-tion) pH amendment, to 4.29 Control samples contained

only synthetic Saigon River water The anaerobic bottles

were sealed with Teflon-coated rubber stoppers and

alu-minium caps, and then purged with pure N2gas for 5 min at

60 mL/min The aerobic bottles were capped with

oxygen-per-meable caps made of silicone foam rubber All bottles were

gently shaken by bio-shaker (BR-300LF) at 30W

C at a horizon-tal shaking rate of 30 rpm for 1 month Leachates were taken at

1, 18, 72, 168 and 720 h after mixing for the analysis of Mn, Fe

and Ca by ICP-AES and of SO4by ion chromatography The

initial andfinal leachates were measured for pH, electricity

conductivity (EC), dissolved oxygen (DO) and ORP, which

was converted to Eh later Data were plotted on the Eh–pH

dia-grams of the Fe-O-H or Mn-O-H-C system (M¼ 105mol/L,

298.15 K, and P(CO2)¼ 0.00035 atm) using FACTSAGE soft-ware, v 6.1

RESULTS AND DISCUSSION Water quality variation along the Saigon River Overall, pH, DO, Mn and Fe varied similarly in both river surveys in August 2006 and May 2008, although there were some differences at a few sampling locations The water was slightly acidic: pH varied from 5.2 to 6.2 (Figure 2(a)) and from 5.5 to 6.5 (Figure 2(b)), and was lower in the middle section (SG13 to SG16) in both

which increased the acidity in the middle section The low pH in the middle section and the anoxia in the down-stream section are important characteristics for water quality In the upstream and middle sections, it varied from 1.1 to 6.9 mg/L in 2006 and from 2.1 to 6.8 mg/L

in 2008 In the downstream section it was depleted owing to urban drainage from Thu Dau Mot Town and HCMC DO depletion was more distinct and Fe concen-tration was lower in August 2006 than in May 2008 This is because, in early rainy season in May, water con-tained low Fe in runoffs and therefore less Fe oxidation occurred Fe concentrations varied from 0.5 to 2.33 mg/L

in 2006 and from 0.65 to 1.67 mg/L in 2008 Concen-trations were higher in the middle section owing to the erosion of Fe-rich soil in the basin In the downstream sec-tion, because of the high pH ( 6), most of the ferrous iron was oxidized and precipitated, resulting in a decrease

of Fe concentration downstream The canal drainages

Figure 2 | Pro files of pH, DO, Mn and Fe in surface water of Saigon River.

(RM, RBC and RBL) and tributaries (ThT and RT) in the

middle section had low pH and high Fe concentrations,

indicating that acidic leachates from ASS contribute to

the Fe inputs and acidification of the Saigon River The

acidic drainage from ASS contained Al, Mn and Fe

(Green et al ) Mn concentrations in DT Reservoir

varied from 0.007 to 0.099 mg/L, lower than those in

the Saigon River due to less impact from ASS, which

means that DT Reservoir was not an important source

of Mn Iron was at a peak at SG18 in August 2006 due

to receiving more acidic drainages from Vam Thuat

Canal, and more turbulence which resuspended Fe

sedi-ments, than those occurring in the early rainy season in

2008 In contrast to Fe, Mn concentrations increased in

the downstream Dissolved Mn, which is oxidized very

slowly at pH 6, remained in the water, when dissolved

Fe was readily oxidized and precipitated

Mn and Fe releases from ASS

The pH of the soil leachates varied from 2.13 to 4.94

depending on soil type and acidity (Table 1) During

leach-ing tests, pH of S3 leachate remained stable in both soil

layers, while pH of S2 leachate increased slightly Periodic

inputs of such acidic water acidified Saigon River because

of its low buffering capacity

Mn and Fe are likely to be the major trace metals released

by the oxidation of ASS (Welch et al.) and in soil water

(Abesser et al ) The amounts of Mn leached varied among the soil types (Table 1): S3 (AASS) had the lowest

pH, 2.13, the highest dissolved Mn concentration in the lea-chates, 9.34 mg/L, and the highest Mn release percentage (30%) S1 had the highest pH and the lowest Mn leaching rate, 1.36 mg/L, or 3.5% Similar to Mn, total Fe contents did not differ between the two soil layers but differed greatly among soil types S3 had the highest Fe contents, of 29,800 mg/kg dry top-soil, and the highest Fe leaching rates of 925 mg/L S1 had the lowest Fe leaching rates, which were nearly negligible compared with their total contents (17.9 mg/L, or,0.1%) The similar leaching results between Mn and Fe implied that they were associated with each other in the soil samples The lower pH leachants increased metal leaching rates from PASS Acidic leachant (pH∼ 1.5) dra-matically increased Mn and Fe leached at S1 After a week

Mn and Fe concentrations in leachates increased about 10 times and 14 times, respectively

Total Fe contents in surface sediments did not vary signi fi-cantly along the Saigon River, 31,300–61,220 mg/kg, while total Mn contents varied greatly, 261–1,370 mg/kg Mn contents in the downstream sediments at SG18, SG19 and SG20 were.1,000 mg/kg, about 10 times higher than that

in ASS Sediment SG19 had a slower Mn release rate than SG17 and SG18, and the lowest concentration of released

Table 1 | pH and dissolved Mn and Fe concentrations of soil leachates and total Mn and Fe contents in top-soils (0 –25cm)

Mn concentrations in leachates (mg/L) Fe concentrations in leachates (mg/L) Site pH a Total Mn mg/kg

Water leachants b

Acidic leachants c Total Fe mg/kg

Water leachants b Acidic leachants c

Note: a pH of water leachants pH of acidic leachants was about 1.5 and did not change significantly; b soil: milli-Q water ¼ 1:5; c soil: Mehlich 1 solution ¼ 1:5.

Table 2 | Amount of Mn and Fe released from sediments leaching test

Mn released (mg/kg) Iron released (mg/kg) Moisture total Mn 1 h 720 h Total Fe 1 h 720 h

Sediment (%) mg/kg (A) (An) (A) (An) mg/kg (A) (An) (A) (An)

Mn (167–244 mg/kg), although it had the highest total Mn

content, i.e., Mn species in SG15, SG17 and SG18 sediments

were mostly in the readily leachable form, whereas Mn at

SG19 was more slowly leachable Sediment SG19 was

HCMC, whereas SG17 and SG18 were located in

agricul-tural areas and received drainage from ASS paddyfields in

Saigon and VCD River basin Unlike Mn, Fe was slow to

dis-solve into the leachants at low content pH adjustment was

too minor to bring any significant effects on releasing rate

The anaerobic condition had a significant influence on the

amount of Fe released (t-test, P, 0.001), but had no effect

on Mn leaching After 168 h, anaerobic leachates had pH

more than 6 and zero DO, which could facilitate the Fe

releasing process (Figure 3(b)) Sediments SG17 and SG18

had higher sulfate release rates (14.3–28.8 g/kg), indicating

deposition of SO4-rich particles derived from ASS at those

locations

The release of Mn from sediments was about 14 times that from the soils, indicating sediments are a more im-portant source of Mn in the Saigon River In contrast, the amounts of Fe leached from sediments varied from

21 mg/kg in aerobic tests to 124 mg/kg in anaerobic tests, far less than that from soils, 2,390 mg/kg This suggests that Fe was derived mainly from soil erosion and leaching from soil rather than from sediment

Effects of pH and Eh on Mn and Fe release from sediments

Figure 4 shows the effects of pH, Eh and concentrations, and distribution of Fe and Mn species in the final sedi-ment leachates Sedisedi-ments SG15, SG17 and SG18 were more acidic, explaining the lower pH and higher Eh of the aerobic samples than those of the control samples, whereas sediment SG19 showed a pH increase and an

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Eh decrease Aerobic treatment moved the pH–Eh

conditions along the border between Fe2O3 and Fe2þ,

i.e., moving to lower pH with higher Eh (Figure 4(a))

Hence, it did not promote dissolution of Fe In contrast,

anaerobic treatment raised pH and lowered Eh During

the course of the batch leaching tests, Eh was lowered

before pH was increased, so that the anaerobic condition

accelerated the dissolution of Fe from sediments The

aqu-eous concentation of Fe also affects its state; the Fe2þ

area gradually narrows as the aqueous Fe concentration

increases from 1010 to 102mol/L Fe tends to remain

in solid form in soil and sediment owing to the higher

aqueous Fe concentration in pore water, but, when soil

is eroded into river water or when sediment is

resus-pended, Fe can be dissolved if pH and Eh are lower in

the Fe2þ area of Figure 4(a)

Most Mn leaching data fell in the Mn2þarea (Figure 4(b)),

indicating that Mn in the sediments is readily soluble in

water, as proved by the batch leaching tests This contrasts

with Fe, which can be dissolved only if the sediments are

placed under strong anaerobic, i.e reducing, conditions

Around the area plotted with the data of thefinal leachate,

variation of Eh has less effect than pH The concentration

of aqueous Mn2þ and partial pressure of carbon dioxide

P(CO2) also affected the leaching of Mn (Figure 4(b)) In

because of higher Mn2þand P(CO2) in pore water, whereas

in the Saigon River Mn can be easily dissolved because of

lower aqueous Mn2þand P(CO2) than in the pore water of

the sediments

CONCLUSIONS

1 Two major sources of Mn and Fe inputs into the Saigon River were identified: (1) acidic leaching from ASS in the middle river section, and (2) dissolution from Mn-and Fe-enriched sediments downstream

2 Fe leaching from ASS was more critical than Mn leaching Low pH was a determinant cause of Mn and Fe leaching from ASS Reducing pH from 4 to 1.5 increased Mn leach-ing from PASS by 10 times and Fe leachleach-ing by 14 times

3 The acidic pH in river water, especially in the rainy season, facilitates Mn dissolution and its release from sediments, whereas anoxia and low pH facilitate Fe

pressure also contribute to Mn dissolution

4 ASS-derived sediments (SG17 and SG18) had faster and higher Mn and Fe leaching rates than the other sediments

5 Land management and improvement of water quality

in the rainy season to avoid the acidification and DO depletion will help to reduce Mn and Fe leaching into the river water

ACKNOWLEDGEMENTS The Japan Society on Promotion of Science and the HCMC Department of Science and Technology are greatly appreci-ated for funding to conduct this study

Figure 4 | Eh-pH diagram of Fe and Mn plotted with the data of the final sediment leachates (The open and solid symbols are aerobic and anaerobic samples and the asterisks are control samples Additional lines were drawn in (a) for aqueous Fe concentration from 1010to 102mol/L and in (b) for aqueous Mn ¼ 10 2 mol/L at P(CO 2 ) ¼ 0.00035 atm, and 0.0035 atm.).

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