Arsenic removal from drinking water produces a sludge that cannot be safely stored in water plants. The ferrihydrite technologies produce so called arsenic ferri-sludges belonging to dangerous waste category that may not be stable under long-term storage conditions, because the amorphous arsenic ferri-sludge is not a thermodynamically stable phase. The thermodynamically stable phases are e.g. goethite, hematite. The meta-stable amorphous phase converts to a more stable equilibrium phase, and when this conversion occurs, the surface area of initially ar-senic ferri-sludges will be greatly decreased. When the surface area de-crease of arsenic ferri-sludge occurs, arsenic is likely to be desorbed from the solid arsenic ferri-sludge phase back to the solution phase, to the ground water. The amount of arsenic ferri-sludges produced by drinking water technologies in the Northern Great Plain Region, Hungary are increasing dramatically because of currently very low (10µg/L) arsenic limit concentration of drinking water.

The present project has solved the problem of dangerous waste arsenic ferri-sludges and now it belongs to harmless waste category. Our study and findings have demonstrated that compounds, such as arsenate-phosphate-hydroxide-apatite Ca₁₀(AsO₄)_x(PO₄)_y(OH)₂, and arsenate-phosphate-carbonate-apatite Ca₁₀(AsO₄)_x(PO₄)_yCO₃ (x+y=6), are formed in the arsenic ferri-sludge phase via a two-step alkaline arsenic desorbing and apatite forming process. Furthermore, barium and radium content of drinking water has been decreased due to our procedure, surprisingly, and these heavy metals have incorporated the apatite-like compounds. The precipitation of harmless apatite-like compounds in ferri-sludge phase occurs quickly and spontaneously. The phase separation has been completed by use of anionic poly-electrolytes, such as functionalized poly-acrylic-amide-derivatives. According to our procedure neither solid phase nor solution phase are dangerous wastes anymore, both of these are harmless and allowed to deposit together with ordinary wastes. The apatite-like compounds, both arsenate-phosphate-hydroxide-apatite and arsenate-phosphate-carbonate-apatite, have an exceedingly low solubility under waste storage conditions and are thermodynamically extremely stable phases.

Financial support by the National Office for Research and Technology, Hungary (Contract No. BAROSS-2-2007-0023) is gratefully acknowledged.