Effect of Citric Acid Modification of Aspen Wood on Sorption of Copper Ion

Abstract

Milled aspen wood was thermochemically modified with citric acid for the purpose of improving the copper (Cu²⁺) ion sorption capacity of the wood when tested in 24-hour equilibrium batch tests. The wood-citric acid adducts provided additional carboxyl groups to those in the native wood and substantially increased Cu²⁺ ion uptake of the modified wood compared with that of the unmodified wood. Sorption capacity (q_e) measured with an unbuffered standard solution increased to a maximum of 7.9 mg Cu²⁺ ion/g of wood (treated) from 1.3 mg Cu²⁺ ion/g wood (untreated). When measured with a buffered standard solution, the q_e increased to a maximum of 13.8 mg Cu²⁺ ion/g of wood (treated) from 4.1 mg Cu²⁺ ion/g wood (untreated). The treatment necessary for maximum q_e was 2 hours at 130°C. Modification treatments included three time periods (2, 4, 6 hours) and three temperature regimes (110, 120, 130°C). To further evaluate the efficacy of modification treatments, weight change after treatment was monitored as an indirect measure of bound citric acid. It was found that increases to the original mass of greater than about 30% were associated with no further increase or a decline in q_e. The contribution of citric acid ester linkages to increasing mass at longer reaction times was monitored with ATR/FTIR.

Key Words:

• Citric acid
• thermochemical reaction
• copper ion sorption
• aspen wood
• crosslinking reactions