Arsenic is found in
wastewater from electronic manufactures making gallium arsenide wafers and
electronic devices. It also can be found in silicon semiconductor operations
that use high dose arsenic implants. Other sources of arsenic are ground water
in agricultural areas where arsenic was once used as an insecticide.
Arsenic can not be treated to regulated levels by pH adjustment and
precipitation. It is most often treated by precipitating ferric arsenate. In
this method, arsenic is oxidized by the addition of ferric ion and sodium
hypochlorite at a pH of 2.5 to 3.5. The pH is then raised to 7 to 8 and ferric
arsenate is precipitated along with excess ferric ion. This method has been
demonstrated to less than 0.1 ug/l. Caution must be taken to maintain the pH
between 7 and 8 or the arsenic concentration will rise.
Another method of treating arsenic is to precipitate it as the sulfide. Care
must be taken in using this method since the sulfide can reduce the arsenic to
AsH3 and form deadly arsine gas. If this method is used a second precipitation
with ferrous ion is required to remove the residual sulfide and aid in
flocculation and settling.
Low levels of arsenic a can be reduced to arsenic metal by the use of a reducing
agent. Again care must be taken to prevent the formation of arsine.
Arsenate can also be polished to low levels, less than 10 ug/l with activated
alumina or cation resin, which has been spent with ferric ion. Normal ion
exchange resins are ineffective at removing arsenic.