The influence of irrigation-induced water table fluctuation on iron redistribution and arsenic immobilization within the unsaturation zone

Highlights: • As and Fe accumulation within the unsaturation zone during irrigation is studied. • Oxygen is more likely exists in gaseous-phase at LWP but dissolved oxygen at HWP. • As and Fe immobilization ratio is slow at LWP but accelerated by OH at HWP. • Combined state of fixed As and Fe changed with the growth of Fe(III) minerals. Given the long-term potential risk of arsenic (As)-contaminated agricultural soil to public health, the redistribution of iron (Fe) and immobilization of As within the unsaturation zone during irrigation and consequent water table fluctuations were studied via a column experiment and corresponding geochemical modeling. Experimental results show that As and Fe accumulated significantly at the top of the column during irrigation. A tremendous increase in As and Fe accumulation rates exists after water table recovery. It was deduced that Fe(II) and As(III) were oxidized directly by O₂ at the period of low water table. But the production of hydroxyl radical (OH) was promoted at the period of high water table due to the oxidation of adsorbed Fe(II). The generated OH further accelerate the oxidation of Fe(II) and As(III). Moreover, the combination of As and Fe is more stronger at the top of the column due to the transformation of combined states of As from surface complexation into surface precipitation with the growth of Fe(III) minerals. This study details the processes and mechanisms of As and Fe immobilization within the unsaturation zone during different irrigation periods and accordingly provides some insights to mitigate As accumulation in topsoil.