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Importance of Evaluating
Phosphate Levels in Tubewells
in High Arsenic Areas of Asia
Tom Mahin1,2, Tommy Ngai2, Susan Murcott1, Mohon Mondal3 1 - Massachusetts Institute of Technology (MIT)
2 - Centre for Affordable Water & Sanitation Technology (CAWST)
3 - LEDARS – Bangladesh
Presented at 33rd WEDC International Conference Accra, Ghana
2008
Arsenic in Asia
From: World Bank “Study: Arsenic Contamination of
Groundwater in South and East Asian Countries” 2005
Impact of Phosphates on
Arsenic Removal by Adsorption Systems
Numerous studies have shown that phosphates (PO4) have a significant impact on arsenic removals by some of the most common and cost effective arsenic treatment (iron-based) systems.
Because PO4 has a similar chemical structure to arsenate, they compete with arsenic for adsorption sites on iron oxides.
PO4 are relatively common in tubewells in high arsenic areas of Asia.
Origin of Phosphates in Ground Water
Phosphates are believed to result from the decomposition of organic matter. Also similar to the release of arsenic, phosphates can be released from sediments in aquifers by:
(1) dissolution of iron oxides under anoxic conditions (absence of dissolved oxygen), (2) desorption of phosphates from iron oxides iron (such as high pH conditions).
Fertilizers & latrines can also contribute phosphates to shallow groundwaters.
Phosphates Often are Elevated When
Arsenic is at High Levels (red line is 50 ug/L)
Pakistan India (West Bengal)
China - Inner Mongolia
Above from McArthur et al., Applied Geochemistry
Bangladesh
From KTH – Sweden by Jonsson & Lundell
Above from Nickson et al., Applied Geochemistry
From KTH Sweden by Fei Shi (2004)
Bangladesh Phosphate
Results (DPHE/BGS)
• 0.3 mg/L P (median)
for 3,530 samples
• But when As > 10 ppb
P averaged 1.3 mg/L
(median - 1 mg/L)
West Bengal
(north of Barasat in southern West Bengal)
From: McArthur et al., “Natural organic matter in sedimentary basins and its
relation to arsenic in anoxic ground water: the example of West Bengal and its
worldwide implications” Applied Geochemistry 19 (2004) 1255–1293
PO4 ranged from
(1–6.5 mg/L).
China - Inner Mongolia (Huhhot Basin)
> 50 ppb As Wells Have Elevated Phosphates
50 ug/L - As
Wells > 100 m ( ) have higher arsenic, phosphate & pH levels From : “Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the
Huhhot Basin, Inner Mongolia” Smedley et al, Appied Geochemistry (18) 2003
Example of How Phosphate, Iron & Arsenic Levels
Vary by Individual Wells (Bangladesh - Chandpur)
From: Targeting safe aquifers in regions with arsenic-rich groundwater in Bangladesh
Case study in Matlab Upazila - Jonsson and Lundell (2004) Swedish University of
Agricultural Sciences
For 2 different wells and iron levels are very similar
but phosphates differ by a factor of 10
Iron to Phosphate Ratios
The ratio of iron to phosphates (Fe/PO4-P) is important because higher iron levels increase and higher phosphates reduce arsenic % removals. So higher Fe/PO4-P ratios are associated with > arsenic % removals.
Phosphate results are reported as either PO4-P or P, both are the same. However results reported as PO4 are about 3 times higher than as P.
.
Red River Delta Vietnam
Parameter Average
(n=74) Median
Arsenic*
(range 9-382 ug/L)
115 ug/L 98 ug/L
PO4-P* 0.75 mg/L 0.56 mg/L
Fe* 13. 7 mg/L 13 mg/L
Fe/PO4-P ratio
(avg./avg.)
18
* Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters –
Comparative Field Study, Model Calculations, and Health Benefits”– E, S & T
Conclusion: PO4-P levels high but Fe also high, sand
filters can remove arsenic for some wells in Vietnam
High Arsenic Area of Cambodia (Kandal Province)
PO4-P (mg/L)* As (ug/L)* Fe (mg/L)*
Average - 0.66
Range:<0.2–3.14
Average – 233
Range:1 -1340
Average – 2.8
Range<0.05-16
Calculated Average Fe/PO4-P Ratio 4
* - Data from Swiss Institute of Aquatic Science and
Technology, Eawag, as published in Environ. Sci. Technol.
2007, 41
Conclusion – Generally high
PO4-P levels & moderate Fe
levels, supplemental iron
needs to be used not just filters
The Impact of Phosphates on % Removal
of Arsenic by Sand Filters
.
Pink circle = very high phosphates (P > 2.5 mg/L)
Adapted from graph by Berg et al., “Household Sand filters for
Arsenic Removal – Technical Report” – EAWAG 2004
* Note as iron levels increase, the
% As removals increase even with
very high phosphates
Fe/PO4-P Ratios and % As Removal
Raw Data from: Berg et al. “Arsenic Removal from Groundwater by Household Sand Filters – Comparative Field
Study, Model Calculations, and Health Benefits”– E S & T, Ratios by Mahin
Arsenic
(As) well ug/L
As - filtered
water
% As
removed
Iron (well) mg/L
PO4-P
mg/L
Fe/PO4-P
ratio
223 21 91% 11 0.05 220
137 49 64% 11 2.8 4
117 45 62% 10 2.6 4
70 9 87% 7 0.2 34
55 44 20% 6 3.7 2
Bangladesh
Vietnam
171 25 85% with nails 8 + iron
from nails 2.3 3 without nails
17 with nails
Conclusions
Even where PO4 levels are generally
lower, individual wells may have high PO4.
Fe/PO4 ratios should be considered when
evaluating treatment performance results &
when preparing sampling plans.
MIT/CAWST/LEDARS pilot tested Kanchan
Arsenic Filter (KAF) in areas of Bangladesh
with high arsenic/phosphate levels. The KAF
uses nails that rust adding iron & resulting in
high Fe/PO4 & higher As removals.
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