Arsenic Concentration, Fraction, and Environmental Implication in Fe–Mn Nodules in the Karst Area of Guangxi

Abstract

We determined the concentrations, geochemical fractions, and potential environmental implications of arsenic (As) via pH-static extraction experiments, X-ray photoelectron spectroscopy (XPS), and sequential extraction. Compared with the corresponding soils, the enrichment factors followed the order As (4.27) > Fe (2.14) > P (1.71) > Mn (1.41) > Al (0.95) > Ti (0.44) > Si (0.39) > Mg (0.28) > K (0.13). As showed a higher enrichment factor than all other elements. Arsenic showed a high linear correlation with iron in the FMNs, which can be expressed as As = 18.68Fe − 175.89 (r2 = 0.97, p < 0.01), indicating that Fe plays an important role in the geochemical behavior of As. Most of the As occurred as As (V) (83.79%) in the Fe–Mn nodules (FMNs), and As (III) (16.21%) only occupied a small portion. The distribution of As in the geochemical fractions of the FMNs followed the order F5 (99.54%) > F3 (0.25%) > F4 (0.10%) > F2 (0.09%) > F1 (0.02%), indicating that the residual fraction (F5) of As is the most dominant component. The total release of As from the nodules was extremely low (<0.01%) under neutral pH conditions (pH 6.0–8.0), and As was adsorbed and stabilized by the FMNs under neutral pH conditions (pH 6.0–8.0). However, overacidification or alkalization of the soil environment will promote As release, with subsequent ecological hazards.

Arsenic in a Karstic Paddy Soil with a High Geochemical Background in Guangxi, China: Its Bioavailability and Controlling Factors

“…Previous studies have shown that the enrichment of soil As in karst areas is closely related to the extensive development of Fe-Mn nodules in the soil [29,33]; therefore, six typical karst areas enriched with rooted soil As and where Fe-Mn nodules had extensively In total, 305 paired soil-rice samples were obtained from the karst region in the study area in cooperation with the Guangxi Bureau of Geological and Mineral Exploration (Figure 1), and the samples were pre-processed in compliance with the Specification of Land Quality Geochemical Assessment [32]. Each rice sample consisted of an equal amount of rice grains from 3 sub-sampling points.…”

“…Previous studies have shown that the enrichment of soil As in karst areas is closely related to the extensive development of Fe-Mn nodules in the soil [29,33]; therefore, six typical karst areas enriched with rooted soil As and where Fe-Mn nodules had extensively developed were selected in the study area (Figure 2), and the screening of the samples of Fe-Mn nodules with different diameters was carried out (Figure 1).…”

Geochemical Behaviors of Heavy Metal(loid)s in Soil Ferromanganese Nodules in Typical Karst Areas in Southwest China

“…FMNs are widespread in the soils of karst areas in Guangxi province, showing relatively high proportions in the soils [19]. In addition, large amounts of FMNs were formed during the weathering processes of carbonate rocks to form soils, resulting in strong secondary enrichment of soil heavy metal(loid)s. In fact, previous studies have shown substantially higher heavy metal(loid)s contents in soil FMNs in karst areas than those in non-karst areas [20,21,24]. According to previous studies, the cadmium (Cd) content in soil FMNs in karst areas reached 299.00 mg kg −1 , which is substantially higher than that of the Chinese soil background of Cd (0.097 mg kg −1 ) [13,28].…”

“…Karst areas in Southwest China are among the most affected areas by soil heavy metal pollution in China, especially in Guangxi, Guizhou, Yunnan, and Sichuan provinces. According to the Chinese Soil Environmental Quality Standard [16], soil heavy metal contents exceedances in karst areas of Southwest China are closely related to geological processes, unlike other areas, where exogenous inputs are the main sources of soil pollution [17][18][19][20][21]. High geological background-induced soil contamination in karst areas has attracted increasing attention from researchers in recent years [22][23][24].…”

“…FMNs in karst areas are relatively common and have been increasingly investigated by numerous researchers in recent years [24][25][26]. Previous studies on FMNs in karst areas have mainly focused on the formation mechanisms of FMNs, isotopic fractionation effects, elemental differences in FMNs with different grain sizes, and mineral composition characteristics [13,[19][20][21]25,26]. However, the enrichment mechanisms of heavy metal(loid)s in FMNs are still unclear.…”