Comparing arsenic(V) adsorption by two types of red soil weathered from granite and sandstone in Hunan, China

Abstract

Different soils have large differences in efficiencies and mechanisms in arsenic (As) adsorption, due to the dissimilarities of soil minerals, organic matter (OM) and physiochemical properties. To understand the differences of As adsorption resulting from these differences among soils, the red soil weathered from granite (RS-G) and sandstone (RS-S) were employed to comparatively study the minerals, OM, physicochemical properties, and the isotherms and kinetics of As(V) adsorption. The soil samples were collected at 0–20 cm depth on the top of the mother rocks in Dingziwan and Yuelu Mountain, Changsha, Hunan province in China. The soil minerals were characterized by X-ray powder diffraction (XRD). The specific surface area (SSA) of the soil samples was determined using a BET Analyzer with nitrogen and multi-point analysis after degassing overnight at 100 °C. The cation exchange capacity (CEC) was determined by the method of ammonium acetate centrifugal exchange, and the OM, pH, and point of zero charge (pHPZC) were measured using improved potassium dichromate volumetric weight method, titration, and potentiometric titration method, respectively. The As(V) in the solution was measured by graphite furnace atomic absorption spectrometry (GFAAS). RS-G contains more clay minerals and iron (hydr)oxides, and less quartz than RS-S by 25.31%, 111.69%, and − 22.39%, respectively. There are similar SSA, CEC, and OM content, but different pHpzc and OM sources between the two soil types. The adsorption experiments reveal (1) the As(V) removal efficiency drops rapidly as the initial As(V) concentration is greater than 4.0 mg/L for RS-G and 8.0 mg/L for RS-S, and that of RS-S decreases more slowly as the As(V) concentration increasing from 4.0 to 20.0 mg/L, (2) the As(V) removal efficiency decreases rapidly as the pH is larger than 8.0 for RS-G and 7.0 for RS-S, and (3) the time reaching equilibrium is 120 min for RS-G and 180 min for RS-S, and their kinetics show two and three stages adsorption processes, respectively. The higher mineral content and pHPZC are the main causes for RS-G showing higher As(V) removal efficiency, shorter time to reach equilibrium, and wider pH range for As(V) adsorption. The young OM in RS-G may be responsible for the rapid decrease of As(V) removal efficiency as the As(V) concentration larger than 4.0 mg/L. Comparatively, the RS-G is more efficient for As(V) adsorption, and is more potential for groundwater protection from As contamination.