Are there any soluble compounds capable of absorbing radio-frequency or long microwave radio-frequency electromagnetic radiation? I have noted that metals are often used to absorb these signals, but I cannot find any absorption spectra for radio/long microwave wavelengths.
Of course! Salt water is effective, because it is an ionic solution. Submarines and SCUBA divers cannot use radio-frequency (RF) communication over any distance because of that issue. Very high power, very-low-frequency (VLF) land-based transmitters can penetrate to perhaps 20 meters depth to subs, but salt water is essentially opaque to RF and microwaves.