Watermelon rind, usually discarded as waste, has been shown by researchers in Pakistan to be capable of cheaply and efficiently removing arsenic from groundwater.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/watermelon-rind-cheaply-filters-arsenic-groundwater
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/environment-news-network
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/water-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/water-pollution-control
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/arsenic
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/groundwater
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pakistan
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/groundwater-quality
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/south-asia