A 28-year-old woman is brought to the hospital 18 hours after intentional ingestion of white powder (suspected arsenic trioxide) in a suicide attempt. She is now haemodynamically stable after aggressive fluid resuscitation. Serum arsenic level is 85 mcg/dL (normal < 15 mcg/dL). Urine arsenic is 120 mcg/L. She has mild peripheral neuropathy and skin changes (Mees' lines). What is the most appropriate next step in management?