Mixtures including wastes from the mussel shell processing industry: retention of arsenic, chromium and mercury

Three mixtures containing ashes from the mussel shell calcination industry, sewage sludge, wood ash and/or mussel shell were studied. Such wastes are difficult to recycle individually, but performing adequate mixtures could aid to solve this problem. Two mixtures included different percentages of mussel shell calcination ashes, sewage sludge and mussel shell, while the third was composed by mussel shell calcination ashes, sewage sludge and wood ash. Adsorption was very high for some pollutants: 98-99% for Hg, and 90-96% for As, while it was not higher than 32% for Cr. The pH values in the mixtures showed a clear diminution in the alkalinity of the shell ashes. Further, a drastic reduction of the extreme electric conductivity of these ashes occurred. The carbon/nitrogen ratio in the mixtures clearly improved when compared with the individual wastes. The nutrient content was similar in the three mixtures. The heavy metals concentrations were low in all three mixtures. Calcite was the most prevalent crystalline compound (≥83%) and aragonite was greater than 3.7%. In view of these results, using such mixtures would facilitate the treatment and recycling of shell calcination ashes, as well as other wastes. The mixtures could be useful in degraded environments, such as mine dumps, due to their nutrient content and the adsorption potential for Hg and As.