Most batteries—regardless of type—contain toxic chemicals. Think cadmium, lead, lithium, or sulfuric acid. If your old batteries end up in a landfill, pollutants like these can leak out and contaminate groundwater, damage fragile ecosystems, and potentially make their way into the food chain.
From what I’ve heard, the real primary reason is fire risk. This is obviously not 100% true, but landfills should be isolated from the surrounding environment especially when it comes to fluids/etc that could leak into groundwater. There are a lot of processes they should already be following to keep that from happening.
Recycle your old batteries https://www.consumerreports.org/home-garden/recycling/yes-you-need-to-recycle-your-old-batteries-a5385943645/
From what I’ve heard, the real primary reason is fire risk. This is obviously not 100% true, but landfills should be isolated from the surrounding environment especially when it comes to fluids/etc that could leak into groundwater. There are a lot of processes they should already be following to keep that from happening.