Use of biomass for the removal of heavy metals at low concentrations from freshwater for Chilean Atlantic salmon farms

Abstract

In Chile and Norway, countries with the highest salmon production in the world, salmons have developed chronic diseases due to toxicity problems from free metallic ions. Aluminum (Al) and iron (Fe) were found to be present in freshwaters used by the Chilean salmon industry. In this work, different alternatives for Al and Fe removal were compared to achieve the required standards in salmon culture. Manganese (Mn) removal was also assessed since Fe and Mn removal can be accomplished in a single process.Since cellulose production (a principal economic activity in Chile) and the sawmill industry generate Pinus radiata bark as a waste product, this study also analyzed its application as an adsorbent of precipitate Al, Fe, and Mn in comparison with the traditional method of granular filtration. Al(OH)3 precipitation was achieved by pH exchange. For precipitation of Fe and Mn oxides, two alternatives were analyzed: (i) oxidation by the presence of dissolved oxygen and pH exchange and (ii) pH exchange by CO2 injection and oxidation produced by chemical filtration.Fe and Mn in solution, at low concentrations (less than 1mg/L) presented a maximum precipitation at pH equal to 8.7, different from the value they presented individually. The separation efficiency of the three processes: (a) oxidation and filtration in column packed with P. radiata bark; (b) oxidation and granular filtration (smaller particle size); (c) CO2 injection and chemical and granular filtration were of 93, 97 and 98%, for Fe and of 97, 99 and 29% for Mn, respectively. In all the studied alternatives, Fe concentrations less than 0.1mg/L, compatible with salmon life, were obtained; in contrast for Mn, it was only possible to reach an adequate concentration for salmon life with the granular filter for smaller particle sizes.The optimum pH of Al precipitation was 6.4 and the column filled with P. radiata bark achieved Al concentration values less than 0.01mg/L, limit value for salmon farms, obtaining removal efficiencies greater than 99.5%; in contrast, in the granular filter, the average obtained for cycle efficiency was 80.3%.Only the column filled with P. radiata bark achieves Al and Fe concentrations compatible with salmon life.