An automated CHN Analyzer was compared with the Walkley-Black and Kjeldahl methods for organic carbon (C) and nitrogen (N). Four organic compounds, twenty nine plant materials and five soils were tested. The CHN Analyzer gave C and N values that were not significantly different (P<0.05) to the theoretical weight percents of the organic compounds. The Walkley Black method gave soil C values significantly lower (P<0.05) than those obtained with the CHN Analyzer. The Kjeldahl method gave soil N values significantly lower (P<0.05) than the CHN Analyzer on three of five soils tested. The discrepancies observed between methods appear to be due to different oxidation efficiencies. CHN Analyzer and Kjeldahl N analyses were not significantly different (P<0.

The fate of As in soils is regulated mostly by Its participation in sorption reactions and redox transformations. Few studies have f t examined the rate of arsenite and arsenate reduction or the cxtentjoiŵ hich these redox transformations may be affected by sorption rest-/ tions. The objective of this study was to examine changes in solutionconcentrations of H^AsOj and HjAsO<~_in two soils subjected to prolonged flooding. The soils, which differed in HjAsOj and HjAsOf sorption capacities, were flooded by suspending 1 g of soil in 25 mL of a solution containing 0.01 M CaClj and 1 g D-glucose kg" 1. The suspensions were amended with NaAsOi or Na 2 HAsO 4-7HiO and were incubated for 0.5 h to 20 d. Changes in solution chemistry (electrode potential [Eh], pH, and dissolved Fe, Mn, HjAsO!, and HiAsO 4~) were observed with time. Sorption processes controlled the dissolved concentrations of HjAsOj and HjAsOf during initial stages of flooding. When anaerobic conditions were achieved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorption of HjAsOS and HjAsOf. In NaAsOi-amended suspensions, desorbed HjAsOS disappeared from solution within 10 d. In NajHaSOr-amended suspensions, desorbed HjAsOr also disappeared within 10 d. Concurrent with the disappearance of HjAsO 4~ was the appearance of HjAsO!, indicating that HjAsOr was rapidly reduced to HjAsOS. First-order plots of HjAsO] and HiAsOi" disappearance had a linear relationship. Rates of desorption and disappearance of HjAsOS and HiAsOf were slower in the soil with higher adsorption capacity, suggesting that sorption processes may influence redox transformations of As oxyanlons.

Phosphate enhances the mobility of As in well‐aerated soils by competing for adsorption sites. Phosphate and As may also coexist in large concentrations in hydric soils, and the influence of P on As in anaerobic systems is largely unknown. To determine the effects of P on As dynamics during a soil flooding and aeration cycle, samples of two soils were amended with Na2HAsO4 and Na2HPO4 and incubated under a N2 atmosphere for 41 d, and then reaerated for 7 d. Subsamples were collected intermittently and dissolved As, Fe, Mn, Ca, S, P, and H3AsO3 concentrations were determined. Arsenic speciation in the soil solids was determined after 14 and 41 d of flooding and then after 13 h of aeration by X‐ray absorption near edge structure (XANES) spectroscopy. Arsenic sorption was small under anaerobic conditions, and H2PO−4 additions enhanced As(V) reduction rate in both soils and slightly suppressed As sorption in one soil. Arsenopyrite (FeAsS) was identified in the soil solids. Rapid and simultaneous As sorption and Fe precipitation occurred during the first 0.25 d of aeration, suggesting that As was retained on freshly precipitated Fe (hydr)oxides. Manganese precipitation and concomitant As sorption occurred after 1 d of aeration. Arsenopyrite was largely destroyed upon aeration but As(III) persisted. Thus, As is partitioned into the solid phase under both aerobic and anaerobic conditions, although more appreciably under the aerobic conditions of this study, and P has little influence on dissolved As during soil flooding–aeration cycles.

No abstract

Arsenic is known to be immobilized in soils through oxyanion sorption reactions involving metal (hydr)oxides. While the mechanisms involved in oxyanion sorption are relatively well characterized in aerobic systems, the response of (hydr)oxides to fluctuating soil redox conditions, and its impact on As sorption, is poorly understood. This study investigated the influence of soil flooding and drying on As sorption and Fe fractionation. Four soils exhibiting a range in As sorption capacity were incubated under flooded (anaerobic) conditions. After the flooding period, the pH and redox potentials of each soil suspension were measured and saturation indices were calculated for standard soil minerals. The suspensions were air dried at room temperature and the short‐range order Fe (oxalate‐extractable) and crystalline Fe (dithionite‐extractable) fractions determined. Separate flooded‐dried samples were resuspended in 0.1 M NaCl for AsO4 sorption measurements. Prolonged soil flooding resulted in a decrease in soil redox potential and an increase in dissolved Al, Fe, Mn, and Si. Drying the previously flooded soils resulted in an increase in As sorption. Selective extraction indicated that the flooding‐drying treatments increased the short‐range order Fe fraction while depleting the free oxide fraction. These changes in Fe mineralogy most likely increased the surface area and number of potential As sorption sites.