The best way (I think) to treat 'have to' is as a semi-modal auxiliary followed by a bare infinitive, so that 'land' is DO of 'purify'. In any case, 'have' here does not represent possession as it can as a main verb; in fact, it means nothing at all standing alone without its particle 'to'. With 'to', 'have to' = 'must', approximately.
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