Arsenic (As), molybdenum (Mo), antimony (Sn), and selenium (Se), are metalloid contaminants found at some mine sites that must be treated to meet strict environmental standards. In most cases, co-precipitation with ferric hydroxides can solve these issues. Se often requires additional treatment as only selenite (SeO₃=). is treated via co-precipitation and selenate (SeO₄=) is often present as the dominant fraction of the total concentration. Envirobay personnel have been involved in all stages of design for more than a dozen systems for treating metalloids and can provide a complete solution for their control.