A 28-year-old male labourer presents to the poison control centre 8 hours after accidental ingestion of a pesticide containing lead arsenate. He complains of severe abdominal pain, persistent vomiting, and bloody diarrhoea. Vital signs: BP 100/65 mmHg, HR 110/min, RR 24/min, temperature 37.2°C. Serum arsenic level is 45 µg/dL (normal < 10 µg/dL), and serum lead level is 68 µg/dL (normal < 40 µg/dL). Urine output is 0.3 mL/kg/hr. What is the most appropriate next step in management?