Leaching of Oxyanionic Trace Contaminants from Metakaolin Geopolymers under Batch or Continuous Water Extraction

This study investigates the largely omitted but surprisingly extensive leaching of oxyanionic trace contaminants (vanadium, arsenic, and chromium) from metakaolin geopolymers. Understanding the leaching of these toxic metal-(loid)-s is increasingly important as geopolymers are considered for applications in which they are in contact with water, such as in wastewater treatment. The leaching was quantified from 1 to 4 mm sized crushed geopolymers with batch and continuous water extraction at a pH of 11-13 (i.e., natural pH of geopolymers). The results revealed that the batch extraction (at a liquid/solid ratio of 10) resulted in a substantial release of vanadium (190-4300 μg/L or 3.8-86 μg/g geopolymer), arsenic (39-460 μg/L or 0.78-9.2 μg/g), and chromium (1.3-130 μg/L or 0.026-2.6 μg/g). These values exceed many regulatory guidelines related to environmental safety. However, the continuous extraction with a flow-through setup indicated that the high initial leaching diminished to safe concentrations (<100 μg/L vanadium, < 10 μg/L arsenic, and <50 μg/L chromium) after reaching a cumulative liquid/solid ratio of approximately 11.2. In both batch and continuous leaching, the total released amount of the metal-(loid)-s followed the trend As (4.4-100%) > V (0.8-62.9%) > Cr (0.02-33.2%). As a conclusion, metakaolin geopolymers may require washing before their use when the leaching of oxyanionic metal-(loid) contamination is relevant. Additionally, the batch leaching results (at liquid/solid = 10) of metakaolin geopolymers should be considered with caution, as they represent the initially high release of oxyanions, and thus may create an overly adverse impression of the leaching behavior.