Bip32 Keys encryption in Golang Nacl Secretbox - go

Question

I have created the Assymetric struct which generates a BIP39 and BIP32 Master public and private key. Now I want to use these keys and their derived children to encrypt files or strings depending upon the use case. Please correct me if I am wrong — the keys generated from Mnemonics BIP32 are Elliptic curve keys(secp256k1). I am trying to use the NACL Go implementation to encrypt text with these keys, but it only accepts keys which are 32 bytes but the keys that are being generated from Mnemonics are 256 Bytes. Please help. The bip32 implementation can be found here https://github.com/tyler-smith/go-bip32/blob/master/bip32.go#L59
After going through the implementation, I have found out that the resulting key is a combination of Key and chaincode, key.Key is a 32byte key that must be used for encryption. Just wanted to check if this is the right way to do it and guarantees security.
package encryption
import (
"github.com/tyler-smith/go-bip39"
"github.com/tyler-smith/go-bip32"
"crypto/ecdsa"
"crypto/elliptic"
"golang.org/x/crypto/nacl/secretbox"
"crypto/rand"
"encoding/hex"
"io"
"log"
)
type Encryption interface {
GenerateMnemonic() ( *bip32.Key, *bip32.Key)
}
type Assymetric struct{
RootPrivateKey *bip32.Key
RootPublicKeyKey *bip32.Key
RootMnemonic string
RootPassphrase string
}
func (c *Assymetric) GenerateMnemonic() (*bip32.Key, *bip32.Key){
entropy, _ := bip39.NewEntropy(256)
mnemonic, _ := bip39.NewMnemonic(entropy)
seed := bip39.NewSeed(mnemonic, c.RootPassphrase)
rootPrivateKey, _ := bip32.NewMasterKey(seed)
rootPublicKey := rootPrivateKey.PublicKey()
// Display mnemonic and keys
//c.RootMnemonic = mnemonic
c.RootPrivateKey = rootPrivateKey
c.RootPublicKeyKey = rootPublicKey
key, _ := rootPrivateKey.NewChildKey(0)
log.Printf("Private key 0 is %s", key)
log.Printf("Public key 0 is %s", key.PublicKey())
// You must use a different nonce for each message you encrypt with the
// same key. Since the nonce here is 192 bits long, a random value
// provides a sufficiently small probability of repeats.
log.Printf("This is the secret key %s", secretKey)
var nonce [24]byte
if _, err := io.ReadFull(rand.Reader, nonce[:]); err != nil {
panic(err)
}
log.Printf("This is the nonce %s", nonce)
var a [32]byte
copy(a[:], key.Key)
log.Printf("\nlen=%d cap=%d %v\n", len(a), cap(a), a)
encrypted := secretbox.Seal(nonce[:], []byte("hello world"), &nonce, &a)
log.Println(encrypted)
return rootPrivateKey, rootPublicKey