Arsenate Adsorption from Aqueous Solution onto Fe(III)-Modified Crop Straw Biochars

Biochars generated from peanut straw (PS) and rice straw (RS) were modified using Fe(III), which were then evaluated for As(V) adsorption in batch experimental systems. Results indicated that noncrystalline Fe hydroxides formed on the biochars during modification, reducing the negative charge on the biochar surface, and consequently the electrostatic repulsion to As(V) adsorption. Adsorption capacities of unmodified biochars for As(V) were low, but Fe(III) modification considerably enhanced As(V) adsorption by the biochars. The Langmuir isotherm fit the adsorption data well and could be used to describe As(V) adsorption by the Fe(III)-modified biochars. Maximum adsorption capacities for As(V), calculated using the Langmuir equation, were 33.7 g/kg for Fe(III)-modified PS biochar and 27.0 g/kg for Fe(III)-modified RS biochar at pH 5.0. As(V) adsorption capacities of both Fe(III)-modified biochars were much greater than that for goethite (13.1 g/kg) at pH 5.0. As(V) was mainly adsorbed by the Fe(III)-modified biochars through the formation of inner-sphere complexes on the surfaces of the biochars. Adsorption of As(V) by the Fe(III)-modified biochars increased as the solution pH fell, meaning that they could be efficient adsorbents to remove As(V) from acidic water. Fe(III)-modified PS biochar was more efficient than Fe(III)-modified RS biochar for adsorption of As(V). This is the first report of Fe(III)-modified crop straw biochars as adsorbents for As(V). The findings obtained in the present study are of practical significance in developing new low-cost adsorbents to remove arsenate and other anion pollutants from water.