NDLI: Arsenic Chemistry in Soils: An Overview of Thermodynamic Predictions and Field Observations

Content Provider	Springer Nature Link
Author	Sadiq, Muhammad
Copyright Year	1997
Abstract	Published information, both theoretical and experimental, on As chemical behavior in soils isreviewed. Because of many emission sources, As is ubiquitous. Thermodynamic calculationsrevealed that As(V) species HAsO 4 2- >H$_{2}$AsO 4 - at pH 7) are more abundant in soilsolutions that are oxidized more than pe+pH>9. Arsenic is expected to be in As(III) formHAsO 2 0 =H$_{3}$AsO 3 0 >AsO 2 - =H$_{2}$AsO 3 0 at pH 7) inrelatively anoxic soil solutions with pe+pH<7.Adsorption on soil colloids is an important As scavenging mechanism. The adsorptioncapacity and behavior of these colloids (clay, oxides or hydroxides surfaces of Al, Fe and Mn,calcium carbonates, and/or organic matter) are dependent on ever-changing factors, such ashydration, soil pH, specific adsorption, changes in cation coordination, isomorphous replacement,crystallinity, etc. Because of the altering tendencies of soil colloids properties, adsorption of Ashas become a complex, empirical, ambiguous, and often a self contradicting process in soils. Ingeneral, Fe oxides/hydroxides are the most commonly involved in the adsorption of As in bothacidic and alkaline soils. The surfaces of Al oxides/hydroxides and clay may play a role in Asadsorption, but only in acidic soils. The carbonate minerals are expected to adsorb As incalcareous soils. The role of Mn oxides and biogenic particles in the As adsorption in soilsappears to be limited to acidic soils. Kinetically, As adsorption may reach over 90% completion interms of hours.Precipitation of a solid phase is another mechanism of As removal from soil solutions.Thermodynamic calculations showed that in the acidic oxic and suboxic soils, Fe-arsenate(Fe$_{3}$(AsO$_{4}$)$_{2}$) may control As solubility, whereas in the anoxic soils, sulfides of As(III) maycontrol the concentrations of the dissolved As in soil solutions. In alkaline acidic oxic and suboxicsoils, precipitation of both Fe- and Ca-arsenate may limit As concentrations in soil solutions.Field observations suggest that direct precipitation of discrete As solid phases may not occur,except in contaminated soils. Chemisorption of As oxyanions on soil colloid surfaces, especiallythose of Fe oxide/hydroxides and carbonates, is believed to a common mechanisms for As solidphase formation in soils. It is suggested that As oxyanions gradually concentrate on colloidsurfaces to a level high enough to precipitate a discrete or mixed As solid phase.Arsenic volatilization is another As scavenging mechanism operating in soils. Many soilorganisms are capable of converting arsenate and arsenite to several reduced forms, largelymethylated arsines which are volatile. These organisms may generate different or similarbiochemical products. Methylation and volatilization of As can be affected by several biotic (suchas type of organisms, ability of organism for methylation, etc.) and abiotic factors (soil pH,temperature, redox conditions, methyl donor, presence of other ions, etc.) factors. Information onthe rate of As biotransformations in soils is limited. In comparison to the biologically assistedvolatilization, the chemical volatilization of As in soils is negligible.
Starting Page	117
Ending Page	136
Page Count	20
File Format	PDF
ISSN	00496979
Journal	Water, Air, and Soil Pollution
Volume Number	93
Issue Number	1-4
e-ISSN	15732932
Language	English
Publisher	Kluwer Academic Publishers
Publisher Date	1997-01-01
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Hydrogeology Atmospheric Protection/Air Quality Control/Air Pollution
Content Type	Text
Resource Type	Article
Subject	Ecological Modeling Environmental Chemistry Pollution Environmental Engineering Water Science and Technology

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