Immobilization of Heavy Metals in Boroaluminosilicate Geopolymers

Boroaluminosilicate geopolymers were used for the immobilization of heavy metals. Then, their mechanical properties, phase composition, structure, and microstructure were investigated. The addition of borax and boric acid did not induce the formation of any crystalline phases. Boron was incorporated into the geopolymeric network and caused the formation of N-B-A-S-H (hydrated sodium boroaluminosilicate) gel. In the range of a B/Al molar ratio of 0.015-0.075, the compressive strength slightly increased (from 16.1 to 18.7 MPa), while at a ratio of 0.150, the compressive strength decreased (to 12 MPa). Heavy metals (lead and nickel) were added as nitrate salts. The loss of the strength of the geopolymers induced by heavy metals was limited by the presence of boron. However, it caused an increase in heavy metal leaching. Despite this, heavy metals were almost entirely immobilized (with immobilization rates of >99.8% in the case of lead and >99.99% in the case of nickel). The lower immobilization rate of lead was due to the formation of macroscopic crystalline inclusions of PbO center dot xH(2)O, which was vulnerable to leaching.

Automated summary

Boroaluminosilicate geopolymers were used to immobilize heavy metals, with the addition of boron enhancing compressive strength but also leading to increased heavy metal leaching. Despite this, heavy metals were almost entirely immobilized, with the exception of lead due to the formation of easily leachable PbO·xH(2)O macroscopic crystalline inclusions.