If the Trump administration decides to impose sanctions in response Navalny's apparent poisoning, it would most likely do so under the 1991 Chemical and Biological Weapons Control and Warfare Elimination (CBW) Act, which mandates sanctions in response to chemical weapons attacks by foreign governments. The US in 2018-19 imposed several rounds of sanctions on Russia under the CBW Act in response to the March 2018 Salisbury (UK) Novichok nerve agent attack, culminating in a ban on US bank purchases of foreign currency-denominated sovereign bonds from Russia.
However, both a decision and any imposition of further economic sanctions are unlikely before the 3 November US election. CBW Act sanctions require an official determination that a foreign government has used a chemical agent as a weapon. In 2018, this determination was made five months after the Salisbury attack, with additional sanctions imposed a year later in August 2019 (nearly nine months after a subsequent determination that Russia had failed to meet remedial standards required by the CBW Act).
Congress is trying to accelerate the process this year. The bipartisan leaders of the US House of Representatives (lower house) Foreign Affairs Committee on 8 September required the administration to provide an assessment of the Navalny incident within 60 days (by 7 November). If a determination has not already been made by then, it would probably be required within a further 60 days of delivery of that report (by 6 January 2021), potentially triggering a further round of sanctions. Under the CBW Act, additional sanctions could include broad trade restrictions, terminating existing export licenses and waivers, further restrictions on bank transactions with the Russian government, suspending diplomatic relations, and blocking Russian air carriers from the US.
Of course, the administration could take other actions while a CBW Act review is pending. For example, the administration in April 2018 – in concert with European allies – expelled 60 diplomats and closed the Russian consulate in Seattle prior to reaching its official determination of a chemical attack. CBW Act sanctions, however, are unlikely before the election – and could get pushed into the next administration depending on how long it takes the US government to assess and reach a determination about the Navalny poisoning.