<p>The coexistence of cadmium (Cd(II)) and arsenic (As(III)) has long been an environmental problem. Green and cost-effective biochar (BC) shows considerable potential for addressing environmental issues, including the concurrent elimination of cadmium (Cd(II)) and arsenic (As(III)) from water and soil after nano-sized goethite modification. However, the behavior of goethite-modified rice straw-derived biochar (GBC) during co-adsorption of Cd (II)) and As (III)) in the presence of competing ions and anoxic vs oxic environments is unclear yet. This experiment (GBC) was successfully synthesized to study co-adsorption and the effects of environmental factors on it. The adsorption kinetics and isotherms for the mixed adsorption of Cd(II) and As(III) onto GBC showed that the pseudo-2nd-order model (<i>R</i>² Cd(II) = 0.998, <i>R</i>² As(III) = 0.996) and the Langmuir model (<i>R</i>² Cd(II) = 0.982, <i>R</i>² As(III) = 0.997) were both correctly portrayed. The highest adsorption of As(III) was 87.38&#xa0;mg/g, and Cd(II) was 71.07&#xa0;mg/g in a single adsorption system, which is considerably more significant than the values of 68.6 and 48.38&#xa0;mg/g, correspondingly, in the co-adsorption system. The competitive adsorption of Cd(II) and As(III) on GBC was primarily driven by co-precipitation and ion exchange. Its efficacy in soil systems under aerobic and anaerobic situations remained undisturbed. At the same time, the anaerobic environment favors Cd adsorption, and the aerobic environment favors more As remediation in an aqueous system. The interactive ions Ca²⁺ and Mg²⁺ significantly enhanced the adsorption of As(III). On the other hand, phosphate and humic acid significantly promote Cd(II) adsorption. In summary, the different environmental&#xa0;conditions revealed by this study help a deeper understanding of the behaviors of As and Cd by GBC.</p>