Synthesis of Nitrogen-Doped Sago Bark Biochar (Metroxylon sago) via Hydrothermal Method as a Congo Red Adsorbent H. B. F. Haryanto, Rodiansono, E. Mikrianto, D. A. P. Wardani, G. R. Lempang, Sunardi International Journal of Acta Material Vol. 3 No. 1 2026