Arsenic (As) is a carcinogenic metalloid commonly occurring in the environment stemming from both natural sources and human activities like mining. Given its high toxicity and widespread presence in global water systems, exploring effective treatment methods is crucial. Adsorption is recognized as a highly effective method for removing As from water. In the last decade, there has been a rising interest for using agricultural biomass residues as plentiful and economical resources for creating biomass and biochar-based adsorbents. In the current study, canola straw biochar (CSB) was treated with H3PO4 and FeCl3 for arsenite (As(III)) and arsenate (As(V)) adsorption. Two factors were investigated for adsorption enhancement including FeCl3 concentration and thermal treatment. Overall, an intermediate iron concentration and lower thermal treatment led to higher As adsorption capacities, with the optimized canola straw biochar (OCSB) being at 0.25 M FeCl3 and 70 °C. It was found that an As solution of pH 10 was optimal for both As(III) and As(V) adsorption and thermodynamic experiments showed that both processes were spontaneous while As(III) was an endothermic adsorption process and As(V) was an exothermic adsorption process. Kinetic data showed that As adsorption followed a pseudo-second-order model, while isotherm modeling resulted in the acceptance of a Freundlich model. Characterization analyses (BET, XRD, and Raman) suggested amorphous element structures and the deposition of α-Fe2O3 and α-FeOOH. In summary, the outcomes point to OCSB as a viable candidate for removing As from water making it a potentially effective adsorbent.