Abstract:

In order to improve the water solubility of chitosan and its chelating ability to metal ions, a Ca²⁺ ion treatment agent for drilling wastewater of shale gas field was successfully prepared by grafting acrylic acid (AA) onto water soluble chitosan (KJT-1) with high deacetylation degree (98%) using the potassium persulfate (KPS) as initiator. The synthesis reaction conditions (temperature, reaction time, initiator and acrylic acid dosage) were optimized using single factor experimental method with Ca²⁺ removal rate in wastewater as the optimization target, the synthesized product of the optimal synthesis reaction conditions was structurally characterized using Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) and evaluated for the effect on the removal of multivalent ions from simulated drilling wastewater. The results showed that the optimal synthetic reaction conditions for acrylic acid graft-modified water-soluble chitosan were chitosan∶potassium persulfate = 1∶1.2 (mass ratio), chitosan∶acrylic acid = 1∶25 (mass ratio), reaction time of 2 h, and reaction temperature of 70 ℃, the synthetic product (HCCLJ) under these conditions was consistent with the design of the molecular structure of the target product. When the amount of HCCLJ was added at 600 mg/L, the Ca²⁺ ion content in simulated water can be reduced from 1101 mg/L to 194 mg/L, and the calcium ion removal rate was 82.4%. The removal effect of different HCCLJ dosages on other multivalent ions was in the following order: total iron (Fe²⁺/Fe³⁺) > Ba²⁺ > Mg²⁺ > Al³⁺. HCCLJ can not only remove Ca²⁺ ion, but also have a certain removal effect on other multivalent ions, which has a multi-effect function, and lays a foundation for the research and development of multivalent ions removal agent for oilfield drilling wastewater with a broad application prospect.

Key words: chitosan, graft modification, Ca²⁺ ion treatment agent, multivalent ions