HP inkjet inks are water-based and free of the heavy metals cadmium, mercury, lead, and chromium that are typically of most concern in the environment.
However, HP inks have not been evaluated for suitability in composting systems. Information from the West Virginia University Extension Service indicates that office paper generally is not suitable for garden mulch (http://www.wvu.edu/%7Eexten/infores/pubs/crops/msw10.pdf).
This is more likely due to chemical residues that may be present in office paper than it is to the chemicals in the ink or toner used for printing. Instead, newspaper is recommended for use as mulch in the garden. Newspaper is not bleached and would therefore contain less chemical residues than office paper. It also breaks down more readily than office paper. For office papers, it is likely that greater environmental benefit can be realized by recycling them for use in new paper products.