Wednesday April Arsenic in Groundwater: Chemistry and How to Address It. David J Silverman P.E. New York Region Applications Engineer

Transcription

1 Wednesday April Arsenic in Groundwater: Chemistry and How to Address It David J Silverman P.E. New York Region Applications Engineer

2 Presentation Outline Arsenic- background information Arsenic occurrence Arsenic chemistry Arsenic standards for drinking water Approaches to mitigating arsenic Treatment options Project planning and execution

3 Arsenic- background information Chemical symbol As Nonmetal or metalloid in group V on the periodic table Often referred to as arsenic metal and for toxicologic purposes is classified as a metal Exists in several forms and has a long history of various uses, including insecticides, wood preservatives, herbicides, and even some medicinal uses. Responsible for many poisonings in people and animals

4 Arsenic- background information

5 Arsenic- Background Information Arsenic poisoning is caused by several different forms of the element The form may determine the toxicity. Arsenic is found as inorganic and organic forms with valences of +3 and +5. Arsenite (As +3) is more toxic than arsenate (As +5 ). The two forms of inorganic arsenic, reduced (trivalent As (III)) and oxidized (pentavalent As(V)), can be absorbed, and accumulated in tissues and body fluids. Toxicity varies with factors such as oxidation state of the arsenic, solubility, and duration of exposure. Non-cancer effects can include thickening and discoloration of the skin, stomach pain, nausea, vomiting; diarrhea; numbness in hands and feet; partial paralysis; and blindness. Arsenic has been linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate.

6 Arsenic occurrence Arsenic can be found in seawater (2-4 ppb), and in rivers (0.5-2 ppb). Half of the arsenic present is bound to particles. Geothermal springs have arsenic concentrations greater than 1000 ug/l Arsenic content (based on dry mass) of: Freshwater and ocean algae ppm of arsenic freshwater microphytes contain ppm marine molluscs 1-70 ppm marine crustaceans ppm fishes ppm In some marine organisms, such as algae and shrimp, arsenic can be found in organic compounds.

7 Arsenic occurrence Concentrations of naturally occurring arsenic in ground water vary regionally due to a combination of climate and geology. Although slightly less than half of 30,000 arsenic analyses of ground water in the United States were =< 1 µg/l, about 10% exceeded 10 µg/l. At a broad regional scale, arsenic concentrations exceeding 10 µg/l appear to be more frequently observed in the western United States than in the eastern half. Arsenic concentrations in ground water of the Appalachian Highlands and the Atlantic Plain generally are very low (=< 1 µg/l). Concentrations are somewhat greater in the Interior Plains and the Rocky Mountain System. Investigations of ground water in New England, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that arsenic concentrations exceeding 10 µg/l are more widespread and common than previously recognized.

8 Arsenic occurrence Arsenic release from iron oxide appears to be the most common cause of widespread arsenic concentrations exceeding 10 µg/l in ground water. This can occur in response to different geochemical conditions, including release of arsenic to ground water through reaction of iron oxide with either natural or anthropogenic (i.e., petroleum products) organic carbon. Iron oxide also can release arsenic to alkaline ground water, such as that found in some felsic volcanic rocks and alkaline aquifers of the western United States. Sulfide minerals are both a source and sink for arsenic. Geothermal water and high evaporation rates also are associated with arsenic concentrations >= 10 g/l in ground and surface water, particularly in the west.

9 Arsenic occurrence Arsenic concentrations in ground water of the Appalachian Highlands and the Atlantic Plain generally are very low.- the 75th percentile for both regions is =< 1 µg/l. Ground water in some bedrock units that underlie an area extending from Massachusetts into Maine contains high arsenic concentrations. Ground water in the coal-bearing portion of the Appalachian Highlands generally does not have high arsenic concentrations, although concentrations up to 180 µg/l have been reported in drainage from an anthracite coal mine. Arsenic concentrations in ground water and coal of the Warrior Basin of Alabama are high, most likely because of alteration of the coal by thermal water.

10 Arsenic occurrence In the New England crystalline bedrock aquifer, arsenic concentrations > 10 mg/l occur most frequently in wells located in specific metamorphic and igneous rock units. Correlated with rocks high in arsenic, high ph groundwater; Stream sediments show high arsenic levels in areas with rock formations high in arsenic Stream sediments show high arsenic in agricultural areas that use arsenic-based pesticides to grow apples, blueberries and potatoes, but groundwater wells high in arsenic show no correlation with agricultural activity. Other factors also affect arsenic concentration in well water, so it is highly variable and hard to predict.

11 Arsenic Chemistry In natural water arsenic participates in oxidation and reduction reactions, coagulation and adsorption. Adsorption of arsenic to fine particles in water and precipitation with aluminum or iron hydroxides causes arsenic to enter sediments. After some time arsenic may dissolve once again consequential to reduction reactions.

12 Solubility Elementary arsenic is fairly insoluble, whereas arsenic compounds may readily dissolve. Arsenic is mainly present in watery solutions as HAsO4 2- (aq) and H 2 AsO4 2- (aq), and most likely partially as H3AsO4 (aq), AsO43-(aq) or H2AsO3-(aq). Examples of solubility of arsenic compounds: arsenic(iii)hydride 700 mg/l, arsenic(iii)oxide 20 g/l, arsenic acid (H 3 AsO4) 170 g/l, and arsenic(iii)sulfide 0.5 mg/l.

13 Arsenic can move in aquifers Two categories of processes largely control arsenic mobility in aquifers: (1) adsorption and desorption reactions and (2) solid-phase precipitation and dissolution reactions. Attachment of arsenic to an iron oxide surface is an example of an adsorption reaction. The reverse of this reaction, arsenic becoming detached from such a surface, is an example of desorption. Solid-phase precipitation is the formation of a solid phase from components present in aqueous solution. Precipitation of the mineral calcite, from calcium and carbonate present in ground water, is an example of solid-phase precipitation.

14 Arsenic changes form in the aquifer Arsenic adsorption and desorption reactions are influenced by changes in ph, occurrence of redox (reduction/oxidation) reactions, presence of competing anions, and solid-phase structural changes at the atomic level. Solid-phase precipitation and dissolution reactions are controlled by solution chemistry, including ph, redox state, and chemical composition.

15 Redox reactions- arsenic Arsenic is a redox-sensitive element. This means that arsenic may gain or lose electrons in redox reactions. As a result, arsenic may be present in a variety of redox states. Arsenate and arsenite are the two forms of arsenic commonly found in ground water (Masscheleyn and others, 1991). Arsenate generally predominates under oxidizing conditions. Arsenite predominates when conditions become sufficiently reducing.

16 Arsenic speciation adsorption and desorption Under the ph conditions of most ground water, arsenate is present as the negatively charged oxyanions H2AsO4- or HAsO4 2-, whereas arsenite is present as the uncharged species H3AsO3 0. The ratio of arsenite to arsenate in groundwater is reported as roughly 70:30. The strength of adsorption and desorption reactions between these different arsenic species and solid-phase surfaces in aquifers varies, in part, because of these differences in charge. Differences in species charge affect the character of electrostatic interactions between species and surfaces.

17 Arsenic adsorbs to clay and rocks Arsenate and arsenite adsorb to surfaces of a variety of aquifer materials, including iron oxides, aluminum oxides, and clay minerals. Adsorption and desorption reactions between arsenate and iron-oxide surfaces are particularly important controlling reactions because iron oxides are widespread in the hydrogeologic environment as coatings on other solids, and because arsenate adsorbs strongly to ironoxide surfaces in acidic and near-neutral-ph water.

18 As ph increases, adsorption decreases Iron-oxide surfaces also adsorb arsenite, and both arsenate and arsenite adsorb to aluminum oxides and clay-mineral surfaces. Arsenite adsorption reactions appear generally to be weaker than arsenate adsorption to iron-oxide surfaces As with arsenate, adsorption of arsenite to iron-oxide surfaces tends to decrease as ph increases, at least between the range from ph 6 to ph 9

19 Arsenic concentrations vary with ph As a result of the ph dependence of arsenic adsorption, changes in groundwater ph can promote adsorption or desorption of arsenic. Because solid-phase diagenesis (waterrock interaction) typically consumes H+ the ph of ground water tends to increase over time, which increases arsenic concentrations.

20 Arsenic concentrations can increase over time Because iron-oxide surfaces can hold large amounts of adsorbed arsenate, geochemical evolution of ground water to high (alkaline) ph can induce desorption of arsenic sufficient to result in exceedances of the USEPA current MCL in some cases. Similarly, redox reactions can control aqueous arsenic concentrations by their effects on arsenic speciation, and hence, arsenic adsorption and desorption. For example, reduction of arsenate to arsenite can promote arsenic mobility because arsenite is generally less strongly adsorbed than is arsenate. Redox reactions involving either aqueous or adsorbed arsenic can affect arsenic mobility

21 Arsenite- not easy to remove Arsenite (As III) is non-ionic at neutral ph, high solubility, more toxic, and weakly adsorbed, especially at higher ph Arsenate (As V) is ionic (negatively charged) at neutral ph (more active), less soluble, more strongly adsorbed (more amenable to treatment/removal)

22 Competing ions can affect removal Arsenic adsorption also can be affected by the presence of competing ions. In particular, phosphate and arsenate have similar geochemical behavior, and as such, both compete for sorption sites. Oxyanions in addition to phosphate also may compete for sorption sites. For example, correlation of arsenate with oxyanions of molybdenum, selenium, and vanadium in ground water of the Southwestern United States may be evidence for competitive adsorption among those oxyanions.

23 Arsenic Regulations EPA has set the arsenic standard for drinking water at 10 ppb New York State regulations: Arsenic MCL is 10 ug/l (10 ppb) Arsenic compliance is based on analytical results at each sampling point For systems conducting sampling more frequently than annual, a violation occurs immediately when the running annual average exceeds the MCL For systems sampling annually, a change to quarterly sampling will be required if the annual sample exceeds the MCL. The system will not be considered out of compliance until at least four quarterly samples have been taken and the running annual average exceeds the MCL.

24 Arsenic Management Strategies If another (uncontaminated) water source is available, water blending may be the most economical option for reducing arsenic levels Drilling a new well is a more expensive option and does not guarantee arsenic free water Treatment should be investigated before investing in a new well It is possible that arsenic could be transported when anoxic water is pulled into an aquifer with rocks containing high levels of arsenic- this should be borne in mind

25 Arsenic treatment options Coagulation/filtration Oxidation Ion Exchange Reverse Osmosis Adsorption

26 Coagulation/Filtration Coagulation/filtration is really three processes: Precipitation- the formation of insoluble compounds Coprecipitation- the incorporation of arsenic compounds in growing metal hydroxide phase Adsorption- the binding of soluble arsenic to the external surfaces of the insoluble metal hydroxide

27 Coagulation/Filtration Ferric chloride (FeCl3) is an effective coagulant for arsenic removal Produces HCl which will lower ph Arsenic is removed by adsorption onto Fe(OH)3 floc Al(OH)3 also effective

28 Coagulation/filtration Surface charge of Fe(OH) becomes negatively charged as ph increases Arsenate (HAsO4 2- )is negatively charged, so will be more easily removed at lower ph Fe(OH) 3 and Al 2 O 3 have a high ph PZC (Point of Zero Charge), stays positive at higher ph to attract arsenate for adsorption Arsenic removal strongly correlated to iron removal when FeCl 3 is coagulant Weaker correlation to Al removal when alum is used as coagulant- possible sorption onto Al(OH) 3 which passes through filters

29 Coagulation/Filtration Coagulation/filtration is effective for removal of As (V), less so for As (III) (w/o oxidation) Coagulation/filtration is a good alternative for large systems (>1 MGD) Requires knowledge of coagulation May require ph adjustment, preoxidation Requires chemical feed system, appropriate tanks and filters (licensing) Requires backwashing, produces sludge difficult to dewater Requires process monitoring and adjustment

30 Coagulation/Filtration- Schematic Flocculation increases surface area for adsorption of arsenic Filters need to be backwashed periodically Filter backwash can be equalized, decanted to reduce volume

31 Oxidation Not a removal process for arsenic by itself, but a necessary first step to convert As(III) to As(V) As (V) is more easily removed Can co-remove other constituents such as iron, manganese with filters Most common oxidants are chlorine, potassium permanganate, H2O2, ozone, or manganese dioxide media

32 Ion Exchange Water is passed through a bed of resin beads that preferentially remove the contaminant When the resin is exhausted, (full of contaminant), it is regenerated, usually with salt water Bed life between regeneration is about ,000 bed volumes (BV), depending on application

33 Schematic - Ion Exchange Salt and raw water are mixed in brine tanks to make regenerant solution Brine can be recycled a limited number of times Raw water is pumped through ion exchange vessels for treatment Piping allows one vessel to continue to run while the other one is being regenerated

34 Ion Exchange- advantages and disadvantages Ion exchange can remove As (+V) from water. However, high levels of TDS, selenium, sulfate, fluoride and nitrate contained in water can affect the life of resin. Disposal of highly concentrated spent regenerant is a serious problem. As (III) is hardly removable by ion exchange method

35 Reverse Osmosis Membranes allow some constituents to pass while blocking others RO is a high pressure process which removes contaminants by chemical diffusion High pressure requires energy (pumping) Pretreatment required to prevent scaling, fouling Low recovery ratio (water is recirculated through membranes and brine is rejected) Larger membrane area required for cold water (low flux rates) Generates concentrated brine waste (10%-50% of flow) Does not remove As (III) without oxidation to As (V)

36 Reverse Osmosis- Cheap Home Systems $146 REVERSE OSMOSIS SYSTEM FACTS: Doesn t remove arsenic-5- but says on the box removes arsenic Produces a whopping 10 gallons per DAY Membrane will be destroyed by chlorine 78% of the water goes down the drain 900 gallon life, $50 replacement cartridge Requires water softener Requires ZERO iron and manganese in source water Comes with 31 cheap plastic Chinese parts that will break and you can never replace Owner s manual carefully worded by attorneys You get what you pay for- have a nice day

37 Reverse Osmosis- Cheap Home Systems Caveat Emptor

38 Reverse Osmosis- Schematic

39 Adsorption Adsorption involves removal of contaminants from water by putting it in contact with a material that has both micro and macro porous structure The adsorbate attaches to the surface of the adsorbent material and is held there by magnetic forces Common sorbents include activated carbon (GAC), granular ferric hydroxide/oxide (GFH/O), zeolites and IX resins

40 Granular Ferric Hydroxide Empty Bed Contact Time: 3 min Must be backwashed when pressure drop increases (can be automated) Removes both As(III) and As(V) 20,000 Bed Volume life May be more effective with pre-oxidation Requires support gravel- 10 gpm unit weighs 250 lbs. Requires ph below 9.0 Requires backwash water disposal

41 Granular Ferric Hydroxide- Schematic Effectively a reversal of the desorption process that takes place underground Raw water is passed through the filter ph adjustment may be necessary to keep ph <7, (6.5 optimum) When pressure loss builds, filter is backwashed

42 Granular Ferric Hydroxide- Control Valve

43 Titanium Based Media Empty Bed Contact Time (EBCT): min 40,000 Bed Volume Life Requires backwash water disposal Fe, Mn interfere with treatment Removes 50-75% of As (III) and As (V) SiO2=20 mg/l max. Calcium= 50 mg/l max. Requires ph of 6.5

44 Zirconium Based Media Empty Bed Contact Time: 90% removal in 10 seconds Surface area of 250 m2/g No backwash required- no waste Complete removal of both As(III) and As(V) without pre-oxidation or chemicals 10 gpm unit weighs 100 lbs. 10 gpm-600 gpm capacity 90,000 bed volume life Cartridges are disposable, non-hazardous ph range from

45 Zirconium Based Media - Schematic Raw Water Flow Meter Treated Water

46 Arsenic removal treatment- planning Characterize your water properly: Arsenic (III) and (V) concentrations ph, temperature, alkalinity Competing constituents: Fe, Mn, SiO2, PO4, V, hardness, calcium, etc. Evaluate your options Blending New Well Treatment Perform a technology evluation- no technology is a one-size fits all Work with your manufacturer s rep and engineer to design a system that works for you- cost, O&M considerations, value

47 201 Lincoln Blvd, Middlesex, New Jersey Tel David J. Silverman, P.E. PSI Process and Equipment (347)