Arsenic Speciation by Sequential Extraction from As-Fe Precipitates Formed Under Different Coagulation Conditions

The As(V) extraction in the pH-dependent As(V)-Fe(III) precipitates and the corresponding interaction mechanism of As(V) with ferric iron coagulant were systematically investigated in this study. Generally, As(V) removal by coagulation was more susceptible to the influence of the solution pH than that of the coagulant dosage. There was a distinctively bell-shaped pattern for As(V) removal with increasing the solution pH_ini from 4.6 to 9.4 with varied mass ratios of Fe/As. Specifically, the removal efficiencies of As(V) were enhanced progressively with increasing pH from 4.6 to 6.2. However, As(V) removal declined appreciably as pH further increased to 9.4. The maximum uptake capacities of As(V) by the precipitates were 1.21, 1.10, and 0.95 mg As per mg Fe at pH_ini 6.2 with the Fe/As mass ratio of 0.6, 0.8, and 1.0, respectively. Approximately 99 % of sorbed amorphous hydrous and crystalline hydrous oxide-bound As(V) were extracted in bearing-As(V) precipitates at relevant pH_ini values (i.e., 5.0, 7.0, and 9.0), implying that the main mechanism governing As(V) removal process was forming the inner sphere complexes, which can cause much more powerful forces than chemical compounds. Moreover, it has been accounted well with the performances of floc coagulation for As(V) removal evidenced by the characterizations of the floc size distribution, the floc fractal dimension, and the Fourier transform infrared spectroscopy (FT-IR) spectra, respectively. Considering that As extraction can provide insights for understanding As speciation and mobility in settled precipitates, this study will definitely count much in predicting the long-term risks of As-Fe sediments to the natural environment.