It seems to me that in slight paraphrase the central result of the article


* Marco Porta, Liran Shaul, Amnon Yekutieli, *On the Homology of Completion and Torsion* ([arXiv:1010.4386](http://arxiv.org/abs/1010.4386))


(theorems 6.11 and 6.12) means that for $\mathfrak{a} \subset A$ a suitably nice ideal inside a commutative ring $A$, then the total derived functors of 


1) adic completion of modules at $\mathfrak{a}$ 


and 


2) of taking $\mathfrak{a}$-torsion submodules 


form an adjoint pair of (co-)reflections of homotopy theories (i.e. an adjoint pair of idempotent $\infty$-(co-)monads on the $\infty$-category of chain complexes of $A$-modules).


I am wondering if an analogous result would not also hold for spectra in the case that $\mathfrak{a} = (p)$ is a prime. If so that would yield a nice enhancement of the [story of the arithmetic fracture square](http://ncatlab.org/nlab/show/fracture+theorem).


Is forming $p$-completion of spectra adjoint to forming universal $\mathbb{Z}[p^{-1}]$-acyclic spectra (hence adic completion to $\mathbb{Q}$-acyclification), maybe at least on suitably small spectra? 


And how about lifting either statement to commutative monoids, i.e. to dg-algebras and further to $E\_\infty$-rings, is anything known?



AdicCompletion$\dashv$Torsion adjunction on spectra?
If I understand what you are asking, then yes. p-completion of p-local spectra is $X \mapsto F(M, X)$, where $M=$ fiber of $S\to S\mathbb{Q}$, while the p-torsion approximation is $X\mapsto X\wedge M$. 


The same story holds for any "smashing" localization. **Added.** A "smashing localization" $L$ gives a map of spectra $\eta\colon S\to T:=LS$ such that $T\wedge \eta$ is an equivalence. Consider the cofiber sequence
$$
M\xrightarrow{\epsilon} S\xrightarrow{\eta} T.
$$
Then we obtain a couple of idempotent monads $T\wedge-$ and $F(M,-)$ on spectra, and a couple of idempotent comonads $M\wedge-$ and $F(T,-)$ on spectra. Clearly, these come as two adjoint pairs of functors on spectra.


Then $T\wedge -$ is just the original smashing localization $L$, and $F(M,-)$ is a "cosmashing localization". The other two functors are the corresponding acyclizations.


In the case of
$$
M=\Sigma^{-1} S\mathbb{Q}\_p/\mathbb{Z}\_p \to S \to S\mathbb{Z}[\tfrac{1}{p}]=T
$$
we get the situation you described, where $M\wedge -$ is the $p$-torsion-approximation idempotent-comonad, and $F(M,-)$ is the $p$-completion idempotent-monad. (Note: as a functor from spectra to spectra, $p$-torsion approximation $M\wedge-$ is a left adjoint, but is also right adjoint to the inclusion functor of $p$-torsion spectra into all spectra.)


