The adsorption of organic arsenate, monomethylarsonate (MMA), onto a calcium alginate encapsulated magnetic sorbent is studied in this paper. A novel alginate encalsulated magnetic sorbent was used in the experiments on adsorption isotherm, kinetics, and pH effect. It was found that the equilibrium sorption can be attained within 25 It. Solution pH plays a key role in the removal of MMA from the solution. A greater adsorption can be achieved at pH 4 and below. The maximum sorption capacity of MMA was 8.57 mg As/g, which is slightly higher than the reported adsorbents. The interaction characteristics between the organic arsenate and magnetic sorbent were elucidated by applying FT-IR and XPS analyses. It is shown that the -COOH and Fe-O groups in the sorbent are involved in the adsorption process. The appearance of As-CH3 and alkane C-H groups in the FT-IR spectrum reveals the binding of the organic arsenate to the sorbent. The XPS analysis indicates that reduction of organic arsenate to organic arsenite on the sorbent's surface happens through solid state redox reaction via charge transport from Fe(II) and C-O species in the sorbent. The XPS results also show the disappearance of C-OH and formation of As-O. It is deduced from the spectral results that mechanisms of organic arsenate adsorption involve C-OH, As-O, and Fe-O groups with the solid state redox process.