As mentioned earlier, arsenic can be treated from water by using chemical precipitation, i.e. coagulation. The precipitation process is quite simple looking from the surface.
However, it is a complex process with several simultaneous mechanisms like forming precipitates and co-precipitates, adsorption processes to precipitated solid surfaces, and finally solid-liquid separation through settling or filtration (Pal, et al., 2007).
Removal processes utilizing chemical precipitation are effective but complicated, requiring different process equipment, chemical dosing, and pH control systems. Also, the treatment of arsenic-containing sludge can cause extra costs. Figure 5.2 is presented the typical process steps required in chemical arsenic precipitation.
Activation
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Figure 5.2. Arsenic removal by coagulation – simplified process example
One of the first implemented chemical arsenic precipitation processes was lime precipitation (lime softening). Both arsenates and arsenites chemically bond with hydrated lime to form precipitates:
2H3AsO4 + 3Ca(OH)2 (aq.) → Ca3(AsO4)2 (s) + 6H2O (5.4) H3AsO3 + Ca(OH)2 (aq.) → CaAsO2OH (s) + 2H2O (5.5) Lime softening processes are simple and quite effective for arsenic removal when operated at high pH. However, it should be noted that the lime softening process is not effective if operated at or below pH 10. According to Sullivan et al. (2010), arsenate Ca3(AsO4)2 is less soluble than arsenite in the form of CaHAsO3. This difference in solubilities showed that arsenates are more accessible to remove than arsenites with lime precipitation. Therefore, oxidation of As(III) to As(V) before lime precipitation increases the removal efficiency, especially in the cases where the dominant species in wastewater is As(III).
Lime precipitation processes generate considerable amounts of sludge, and its disposal is expensive. The USEPA Toxicity Characteristic Leaching Procedure (TCLP) describes whether the waste is hazardous or non-hazardous before disposal and if it is suitable for the landfill. When the precipitates coming from the lime precipitation process were studied with the TCLP test, the results showed that solids from the lime softening process are hazardous waste (EPA - United States Environmental protection agency, 2000).
Other common chemicals used for arsenic removal are alum Al2(SO4)3, ferric chloride FeCl3, and ferric sulphate Fe2(SO4)3. Adding iron or aluminium salts to the water converts soluble As(V) and As(III) species into insoluble precipitated species. According to Edwards (1994), one can identify three specific steps in the chemical precipitation/coagulation process with iron or aluminium reagents:
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1. Precipitation – Formation of insoluble arsenic species like Fe(AsO4) or Al(AsO4).
2. Co-precipitation – The inclusion of the soluble arsenic species into a growing ferric hydroxide phase.
3. Adsorption – Formation of surface complexes between soluble arsenic and the solid metal oxide, hydroxide or oxy-hydroxide surface sites.
Several parameters influence arsenic removal by coagulation; reagent type (Fe or Al) and its dosage, pH, initial arsenic concentration in the water, arsenic speciation (As(III) and As(V)), and other impurities in the water like trace metals, SO42-, PO43- and Cl-.
Ferric coagulants are generally more effective in removing arsenic than aluminium chemicals on a weight basis and more effective over a wider pH range. Besides precipitation of solid arsenic species, also adsorption to freshly precipitated surfaces plays an important role. When added to water, ferric salts form solid ferric hydroxide with a net positive surface charge, which is a function of pH. As the pH decreases, the number of positively charged sites on the ferric hydroxide particles increases. Arsenate as an oxyanion is negatively charged. Therefore, it will adsorb to the positively charged ferric hydroxide particles by surface complexation (Bratby, 2016). Therefore, arsenic removal with ferric ions is strongly dependent on the pH of the water. On range pH 4 - 9, arsenic removal decreases with increasing pH. In Figure 5.3 below is shown arsenic removal-%
as a function of equilibrium pH when initial arsenic concentration is relatively low. In this figure, the ferric chloride dosage with As(V) 100 µg/L initial concentration was 1 mg Fe(III)/L, with As(III) 50 µg/l initial concentration dosage was 3 mg/L accordingly (Meng, et al., 2000).
Figure 5.3. Removal of As(V) and As(III) in the 0.04 M KNO3 solution under N2 ( ) and air ( ) with ferric chloride as precipitation reagent (Meng, et al., 2000).
Song et al. (Song, et al., 2006) studied the treatment of concentrated arsenic waste waters (mine drainage waters) with ferric sulphate. From Figure 5.4 one can see that when waters
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having high arsenic concentrations are treated with ferric coagulation, better removal rates are obtained at a lower pH.
Figure 5.4. Arsenic removal-% as a function of pH. As(V) concentration 5.07 mg/l (Song, et al., 2006).
Figure 5.5 is shown the effect of two different ferric coagulant dosages on arsenic removal efficiency. Lower pH and higher ferric coagulant dose increased arsenic removal. There were no significant differences in ferric species; both sulphate and chloride-based coagulant performed in the same manner. Thus, reaching the drinking water maximum level of 10µgAs/L, but required significant overdosing of ferric coagulant.
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Figure 5.5. Removal of As(V) with different dosages of ferric sulphate (top) or ferric chloride (bottom). The initial arsenic concentration was 42 µg/l. Samples were filtered after coagulation with a 0.22 µm membrane (Han, et al., 2002).
When utilizing chemical precipitation processes for arsenic removal, or any other removal methods, consideration must be given to the disposal of waste generated by these processes. Waste streams from these processes may meet the criteria for hazardous waste.
This means that the leachate produced by the Toxicity Characteristic Leaching Procedure (TCLP) test of the arsenic-containing waste must contain 5.0 mg/L of arsenic or less for waste to be suitable for disposal in a landfill. If the arsenic-containing waste does not meet the TCLP requirement, the disposal has to be done according to hazardous waste material, increasing the cost of the whole treatment process considerably (Bratby, 2016).
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